#lang scribble/doc @(require "mz.ss" racket/class (for-syntax racket/base) (for-label racket/trait)) @(begin (define-syntax sees (syntax-rules () [(_) ""] [(_ s) (elem " and " (secref s))] [(_ s ... s0) (elem (elem ", " (secref s)) ... ", and " (secref s0))])) (define-syntax (defclassforms stx) (syntax-case stx (*) [(_ [* (form ...) (also ...)]) #'(defform* (form ...) "See " (scheme class*) (sees also ...) "; use" " outside the body of a " (scheme class*) " form is a syntax error.")] [(_ [form (also ...)]) #'(defclassforms [* (form) (also ...)])] [(_ form ...) #'(begin (defclassforms form) ...)])) (define-syntax (defstarshorthands stx) (syntax-case stx () [(_ form) (with-syntax ([name (string->symbol (let ([s (symbol->string (syntax-e #'form))]) (substring s 0 (sub1 (string-length s)))))] [tmpl (let ([s #'(... (thing (id expr) ...))]) (datum->syntax s (cons (datum->syntax #'form (syntax-e #'form) (car (syntax-e s))) (cdr (syntax-e s))) s))]) #'(... (defform tmpl "Shorthand for " (scheme (begin (#,(scheme name) id) ... (define id _expr) ...)) ".")))] [(_ form ...) #'(begin (defstarshorthands form) ...)])) (define-syntax (defdefshorthands stx) (syntax-case stx () [(_ form) (with-syntax ([name (string->symbol (let ([s (symbol->string (syntax-e #'form))]) (string-append "define/" s)))]) (with-syntax ([tmpl1 (let ([s #'(define id expr)]) (datum->syntax s (cons (datum->syntax #'form (syntax-e #'name) (car (syntax-e s))) (cdr (syntax-e s))) s))]) #'(... (defform* [tmpl1 (#,(scheme name) (id . formals) body ...+)] "Shorthand for " (scheme (begin (#,(scheme form) id) (define id expr))) " or " (scheme (begin (#,(scheme form) id) (define (id . formals) body ...+)))))))] [(_ form ...) #'(begin (defdefshorthands form) ...)])) (define class-eval (make-base-eval)) ) @(interaction-eval #:eval class-eval (require racket/class)) @title[#:tag "mzlib:class" #:style 'toc]{Classes and Objects} @guideintro["classes"]{classes and objects} @note-lib[racket/class #:use-sources (racket/private/class-internal)] A @deftech{class} specifies @itemize[ @item{a collection of fields;} @item{a collection of methods;} @item{initial value expressions for the fields; and} @item{initialization variables that are bound to initialization arguments.} ] In the context of the class system, an @defterm{object} is a collection of bindings for fields that are instantiated according to a class description. The class system allows a program to define a new class (a @deftech{derived class}) in terms of an existing class (the @deftech{superclass}) using inheritance, overriding, and augmenting: @itemize[ @item{@deftech{inheritance}: An object of a derived class supports methods and instantiates fields declared by the derived class's superclass, as well as methods and fields declared in the derived class expression.} @item{@deftech{overriding}: Some methods declared in a superclass can be replaced in the derived class. References to the overridden method in the superclass use the implementation in the derived class.} @item{@deftech{augmenting}: Some methods declared in a superclass can be merely extended in the derived class. The superclass method specifically delegates to the augmenting method in the derived class.} ] An @deftech{interface} is a collection of method names to be implemented by a class, combined with a derivation requirement. A class @deftech{implements} an interface when it @itemize[ @item{declares (or inherits) a public method for each variable in the interface;} @item{is derived from the class required by the interface, if any; and} @item{specifically declares its intention to implement the interface.} ] A class can implement any number of interfaces. A derived class automatically implements any interface that its superclass implements. Each class also implements an implicitly-defined interface that is associated with the class. The implicitly-defined interface contains all of the class's public method names, and it requires that all other implementations of the interface are derived from the class. A new interface can @deftech{extend} one or more interfaces with additional method names; each class that implements the extended interface also implements the original interfaces. The derivation requirements of the original interface must be consistent, and the extended interface inherits the most specific derivation requirement from the original interfaces. Classes, objects, and interfaces are all values. However, a class or interface is not an object (i.e., there are no ``meta-classes'' or ``meta-interfaces''). @local-table-of-contents[] @; ------------------------------------------------------------------------ @section[#:tag "createinterface"]{Creating Interfaces} @guideintro["classes"]{classes, objects, and interfaces} @defform[(interface (super-interface-expr ...) id ...)]{ Produces an interface. The @scheme[id]s must be mutually distinct. Each @scheme[super-interface-expr] is evaluated (in order) when the @scheme[interface] expression is evaluated. The result of each @scheme[super-interface-expr] must be an interface value, otherwise the @exnraise[exn:fail:object]. The interfaces returned by the @scheme[super-interface-expr]s are the new interface's superinterfaces, which are all extended by the new interface. Any class that implements the new interface also implements all of the superinterfaces. The result of an @scheme[interface] expression is an interface that includes all of the specified @scheme[id]s, plus all identifiers from the superinterfaces. Duplicate identifier names among the superinterfaces are ignored, but if a superinterface contains one of the @scheme[id]s in the @scheme[interface] expression, the @exnraise[exn:fail:object]. If no @scheme[super-interface-expr]s are provided, then the derivation requirement of the resulting interface is trivial: any class that implements the interface must be derived from @scheme[object%]. Otherwise, the implementation requirement of the resulting interface is the most specific requirement from its superinterfaces. If the superinterfaces specify inconsistent derivation requirements, the @exnraise[exn:fail:object]. @defexamples[ #:eval class-eval (define file-interface (interface () open close read-byte write-byte)) (define directory-interface (interface (file-interface) file-list parent-directory)) ]} @defform[(interface* (super-interface-expr ...) ([property-expr val-expr] ...) id ...)]{ Like @scheme[interface], but also associates to the interface the structure-type properties produced by the @scheme[property-expr]s with the corresponding @scheme[val-expr]s. Whenever the resulting interface (or a sub-interface derived from it) is explicitly implemented by a class through the @scheme[class*] form, each property is attached with its value to a structure type that instantiated by instances of the class. Specifically, the property is attached to a structure type with zero immediate fields, which is extended to produce the internal structure type for instances of the class (so that no information about fields is accessible to the structure type property's guard, if any). @defexamples[ #:eval class-eval (define i (interface* () ([prop:custom-write (lambda (obj port mode) (void))]) method1 method2 method3)) ]} @; ------------------------------------------------------------------------ @section[#:tag "createclass"]{Creating Classes} @guideintro["classes"]{classes and objects} @defthing[object% class?]{ A built-in class that has no methods fields, implements only its own interface @scheme[(class->interface object%)], and is transparent (i.e,. its inspector is @scheme[#f], so all immediate instances are @scheme[equal?]). All other classes are derived from @scheme[object%].} @defform/subs[ #:literals (inspect init init-field field inherit-field init-rest init-rest public pubment public-final override override-final overment augment augride augment-final private inherit inherit/super inherit/inner rename-super rename-inner begin lambda case-lambda let-values letrec-values define-values #%plain-lambda) (class* superclass-expr (interface-expr ...) class-clause ...) ([class-clause (inspect inspector-expr) (init init-decl ...) (init-field init-decl ...) (field field-decl ...) (inherit-field maybe-renamed ...) (init-rest id) (init-rest) (public maybe-renamed ...) (pubment maybe-renamed ...) (public-final maybe-renamed ...) (override maybe-renamed ...) (overment maybe-renamed ...) (override-final maybe-renamed ...) (augment maybe-renamed ...) (augride maybe-renamed ...) (augment-final maybe-renamed ...) (private id ...) (inherit maybe-renamed ...) (inherit/super maybe-renamed ...) (inherit/inner maybe-renamed ...) (rename-super renamed ...) (rename-inner renamed ...) method-definition definition expr (begin class-clause ...)] [init-decl id (maybe-renamed) (maybe-renamed default-value-expr)] [field-decl (maybe-renamed default-value-expr)] [maybe-renamed id renamed] [renamed (internal-id external-id)] [method-definition (define-values (id) method-procedure)] [method-procedure (lambda kw-formals expr ...+) (case-lambda (formals expr ...+) ...) (#%plain-lambda formals expr ...+) (let-values (((id) method-procedure) ...) method-procedure) (letrec-values (((id) method-procedure) ...) method-procedure) (let-values (((id) method-procedure) ...+) id) (letrec-values (((id) method-procedure) ...+) id)])]{ Produces a class value. The @scheme[superclass-expr] expression is evaluated when the @scheme[class*] expression is evaluated. The result must be a class value (possibly @scheme[object%]), otherwise the @exnraise[exn:fail:object]. The result of the @scheme[superclass-expr] expression is the new class's superclass. The @scheme[interface-expr] expressions are also evaluated when the @scheme[class*] expression is evaluated, after @scheme[superclass-expr] is evaluated. The result of each @scheme[interface-expr] must be an interface value, otherwise the @exnraise[exn:fail:object]. The interfaces returned by the @scheme[interface-expr]s are all implemented by the class. For each identifier in each interface, the class (or one of its ancestors) must declare a public method with the same name, otherwise the @exnraise[exn:fail:object]. The class's superclass must satisfy the implementation requirement of each interface, otherwise the @exnraise[exn:fail:object]. An @scheme[inspect] @scheme[class-clause] selects an inspector (see @secref["inspectors"]) for the class extension. The @scheme[inspector-expr] must evaluate to an inspector or @scheme[#f] when the @scheme[class*] form is evaluated. Just as for structure types, an inspector controls access to the class's fields, including private fields, and also affects comparisons using @scheme[equal?]. If no @scheme[inspect] clause is provided, access to the class is controlled by the parent of the current inspector (see @secref["inspectors"]). A syntax error is reported if more than one @scheme[inspect] clause is specified. The other @scheme[class-clause]s define initialization arguments, public and private fields, and public and private methods. For each @scheme[id] or @scheme[maybe-renamed] in a @scheme[public], @scheme[override], @scheme[augment], @scheme[pubment], @scheme[overment], @scheme[augride], @scheme[public-final], @scheme[override-final], @scheme[augment-final], or @scheme[private] clause, there must be one @scheme[method-definition]. All other definition @scheme[class-clause]s create private fields. All remaining @scheme[expr]s are initialization expressions to be evaluated when the class is instantiated (see @secref["objcreation"]). The result of a @scheme[class*] expression is a new class, derived from the specified superclass and implementing the specified interfaces. Instances of the class are created with the @scheme[instantiate] form or @scheme[make-object] procedure, as described in @secref["objcreation"]. Each @scheme[class-clause] is (partially) macro-expanded to reveal its shapes. If a @scheme[class-clause] is a @scheme[begin] expression, its sub-expressions are lifted out of the @scheme[begin] and treated as @scheme[class-clause]s, in the same way that @scheme[begin] is flattened for top-level and embedded definitions. Within a @scheme[class*] form for instances of the new class, @scheme[this] is bound to the object itself; @scheme[this%] is bound to the class of the object; @scheme[super-instantiate], @scheme[super-make-object], and @scheme[super-new] are bound to forms to initialize fields in the superclass (see @secref["objcreation"]); @scheme[super] is available for calling superclass methods (see @secref["clmethoddefs"]); and @scheme[inner] is available for calling subclass augmentations of methods (see @secref["clmethoddefs"]).} @defform[(class superclass-expr class-clause ...)]{ Like @scheme[class*], but omits the @scheme[interface-expr]s, for the case that none are needed. @defexamples[ #:eval class-eval (define book-class (class object% (field (pages 5)) (define/public (letters) (* pages 500)) (super-new))) ]} @defidform[this]{ @index['("self")]{Within} a @scheme[class*] form, @scheme[this] refers to the current object (i.e., the object being initialized or whose method was called). Use outside the body of a @scheme[class*] form is a syntax error. @defexamples[ #:eval class-eval (define (describe obj) (printf "Hello ~a\n" obj)) (define table (class object% (define/public (describe-self) (describe this)) (super-new))) (send (new table) describe-self) ]} @defidform[this%]{ Within a @scheme[class*] form, @scheme[this%] refers to the class of the current object (i.e., the object being initialized or whose method was called). Use outside the body of a @scheme[class*] form is a syntax error. @defexamples[ #:eval class-eval (define account% (class object% (super-new) (init-field balance) (define/public (add n) (new this% [balance (+ n balance)])))) (define savings% (class account% (super-new) (inherit-field balance) (define interest 0.04) (define/public (add-interest) (send this add (* interest balance))))) (let* ([acct (new savings% [balance 500])] [acct (send acct add 500)] [acct (send acct add-interest)]) (printf "Current balance: ~a\n" (get-field balance acct))) ]} @defclassforms[ [(inspect inspector-expr) ()] [(init init-decl ...) ("clinitvars")] [(init-field init-decl ...) ("clinitvars" "clfields")] [(field field-decl ...) ("clfields")] [(inherit-field maybe-renamed ...) ("clfields")] [* ((init-rest id) (init-rest)) ("clinitvars")] [(public maybe-renamed ...) ("clmethoddefs")] [(pubment maybe-renamed ...) ("clmethoddefs")] [(public-final maybe-renamed ...) ("clmethoddefs")] [(override maybe-renamed ...) ("clmethoddefs")] [(overment maybe-renamed ...) ("clmethoddefs")] [(override-final maybe-renamed ...) ("clmethoddefs")] [(augment maybe-renamed ...) ("clmethoddefs")] [(augride maybe-renamed ...) ("clmethoddefs")] [(augment-final maybe-renamed ...) ("clmethoddefs")] [(private id ...) ("clmethoddefs")] [(inherit maybe-renamed ...) ("classinherit")] [(inherit/super maybe-renamed ...) ("classinherit")] [(inherit/inner maybe-renamed ...) ("classinherit")] [(rename-super renamed ...) ("classinherit")] [(rename-inner renamed ...) ("classinherit")] ] @defstarshorthands[ public* pubment* public-final* override* overment* override-final* augment* augride* augment-final* private* ] @defdefshorthands[ public pubment public-final override overment override-final augment augride augment-final private ] @defform[ (class/derived original-datum (name-id super-expr (interface-expr ...) deserialize-id-expr) class-clause ...) ]{ Like @scheme[class*], but includes a sub-expression to use used as the source for all syntax errors within the class definition. For example, @scheme[define-serializable-class] expands to @scheme[class/derived] so that error in the body of the class are reported in terms of @scheme[define-serializable-class] instead of @scheme[class]. The @scheme[original-datum] is the original expression to use for reporting errors. The @scheme[name-id] is used to name the resulting class; if it is @scheme[#f], the class name is inferred. The @scheme[super-expr], @scheme[interface-expr]s, and @scheme[class-clause]s are as for @scheme[class*]. If the @scheme[deserialize-id-expr] is not literally @scheme[#f], then a serializable class is generated, and the result is two values instead of one: the class and a deserialize-info structure produced by @scheme[make-deserialize-info]. The @scheme[deserialize-id-expr] should produce a value suitable as the second argument to @scheme[make-serialize-info], and it should refer to an export whose value is the deserialize-info structure. Future optional forms may be added to the sequence that currently ends with @scheme[deserialize-id-expr].} @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection[#:tag "clinitvars"]{Initialization Variables} A class's initialization variables, declared with @scheme[init], @scheme[init-field], and @scheme[init-rest], are instantiated for each object of a class. Initialization variables can be used in the initial value expressions of fields, default value expressions for initialization arguments, and in initialization expressions. Only initialization variables declared with @scheme[init-field] can be accessed from methods; accessing any other initialization variable from a method is a syntax error. The values bound to initialization variables are @itemize[ @item{the arguments provided with @scheme[instantiate] or passed to @scheme[make-object], if the object is created as a direct instance of the class; or,} @item{the arguments passed to the superclass initialization form or procedure, if the object is created as an instance of a derived class.} ] If an initialization argument is not provided for an initialization variable that has an associated @scheme[_default-value-expr], then the @scheme[_default-value-expr] expression is evaluated to obtain a value for the variable. A @scheme[_default-value-expr] is only evaluated when an argument is not provided for its variable. The environment of @scheme[_default-value-expr] includes all of the initialization variables, all of the fields, and all of the methods of the class. If multiple @scheme[_default-value-expr]s are evaluated, they are evaluated from left to right. Object creation and field initialization are described in detail in @secref["objcreation"]. If an initialization variable has no @scheme[_default-value-expr], then the object creation or superclass initialization call must supply an argument for the variable, otherwise the @exnraise[exn:fail:object]. Initialization arguments can be provided by name or by position. The external name of an initialization variable can be used with @scheme[instantiate] or with the superclass initialization form. Those forms also accept by-position arguments. The @scheme[make-object] procedure and the superclass initialization procedure accept only by-position arguments. Arguments provided by position are converted into by-name arguments using the order of @scheme[init] and @scheme[init-field] clauses and the order of variables within each clause. When a @scheme[instantiate] form provides both by-position and by-name arguments, the converted arguments are placed before by-name arguments. (The order can be significant; see also @secref["objcreation"].) Unless a class contains an @scheme[init-rest] clause, when the number of by-position arguments exceeds the number of declared initialization variables, the order of variables in the superclass (and so on, up the superclass chain) determines the by-name conversion. If a class expression contains an @scheme[init-rest] clause, there must be only one, and it must be last. If it declares a variable, then the variable receives extra by-position initialization arguments as a list (similar to a dotted ``rest argument'' in a procedure). An @scheme[init-rest] variable can receive by-position initialization arguments that are left over from a by-name conversion for a derived class. When a derived class's superclass initialization provides even more by-position arguments, they are prefixed onto the by-position arguments accumulated so far. If too few or too many by-position initialization arguments are provided to an object creation or superclass initialization, then the @exnraise[exn:fail:object]. Similarly, if extra by-position arguments are provided to a class with an @scheme[init-rest] clause, the @exnraise[exn:fail:object]. Unused (by-name) arguments are to be propagated to the superclass, as described in @secref["objcreation"]. Multiple initialization arguments can use the same name if the class derivation contains multiple declarations (in different classes) of initialization variables with the name. See @secref["objcreation"] for further details. See also @secref["extnames"] for information about internal and external names. @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection[#:tag "clfields"]{Fields} Each @scheme[field], @scheme[init-field], and non-method @scheme[define-values] clause in a class declares one or more new fields for the class. Fields declared with @scheme[field] or @scheme[init-field] are public. Public fields can be accessed and mutated by subclasses using @scheme[inherit-field]. Public fields are also accessible outside the class via @scheme[class-field-accessor] and mutable via @scheme[class-field-mutator] (see @secref["ivaraccess"]). Fields declared with @scheme[define-values] are accessible only within the class. A field declared with @scheme[init-field] is both a public field and an initialization variable. See @secref["clinitvars"] for information about initialization variables. An @scheme[inherit-field] declaration makes a public field defined by a superclass directly accessible in the class expression. If the indicated field is not defined in the superclass, the @exnraise[exn:fail:object] when the class expression is evaluated. Every field in a superclass is present in a derived class, even if it is not declared with @scheme[inherit-field] in the derived class. The @scheme[inherit-field] clause does not control inheritance, but merely controls lexical scope within a class expression. When an object is first created, all of its fields have the @|undefined-const| value (see @secref["void"]). The fields of a class are initialized at the same time that the class's initialization expressions are evaluated; see @secref["objcreation"] for more information. See also @secref["extnames"] for information about internal and external names. @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection[#:tag "clmethods"]{Methods} @subsubsection[#:tag "clmethoddefs"]{Method Definitions} Each @scheme[public], @scheme[override], @scheme[augment], @scheme[pubment], @scheme[overment], @scheme[augride], @scheme[public-final], @scheme[override-final], @scheme[augment-final], and @scheme[private] clause in a class declares one or more method names. Each method name must have a corresponding @scheme[method-definition]. The order of @scheme[public], @|etc| clauses and their corresponding definitions (among themselves, and with respect to other clauses in the class) does not matter. As shown in the grammar for @scheme[class*], a method definition is syntactically restricted to certain procedure forms, as defined by the grammar for @scheme[method-procedure]; in the last two forms of @scheme[method-procedure], the body @scheme[id] must be one of the @scheme[id]s bound by @scheme[let-values] or @scheme[letrec-values]. A @scheme[method-procedure] expression is not evaluated directly. Instead, for each method, a class-specific method procedure is created; it takes an initial object argument, in addition to the arguments the procedure would accept if the @scheme[method-procedure] expression were evaluated directly. The body of the procedure is transformed to access methods and fields through the object argument. A method declared with @scheme[public], @scheme[pubment], or @scheme[public-final] introduces a new method into a class. The method must not be present already in the superclass, otherwise the @exnraise[exn:fail:object] when the class expression is evaluated. A method declared with @scheme[public] can be overridden in a subclass that uses @scheme[override], @scheme[overment], or @scheme[override-final]. A method declared with @scheme[pubment] can be augmented in a subclass that uses @scheme[augment], @scheme[augride], or @scheme[augment-final]. A method declared with @scheme[public-final] cannot be overridden or augmented in a subclass. A method declared with @scheme[override], @scheme[overment], or @scheme[override-final] overrides a definition already present in the superclass. If the method is not already present, the @exnraise[exn:fail:object] when the class expression is evaluated. A method declared with @scheme[override] can be overridden again in a subclass that uses @scheme[override], @scheme[overment], or @scheme[override-final]. A method declared with @scheme[overment] can be augmented in a subclass that uses @scheme[augment], @scheme[augride], or @scheme[augment-final]. A method declared with @scheme[override-final] cannot be overridden further or augmented in a subclass. A method declared with @scheme[augment], @scheme[augride], or @scheme[augment-final] augments a definition already present in the superclass. If the method is not already present, the @exnraise[exn:fail:object] when the class expression is evaluated. A method declared with @scheme[augment] can be augmented further in a subclass that uses @scheme[augment], @scheme[augride], or @scheme[augment-final]. A method declared with @scheme[augride] can be overridden in a subclass that uses @scheme[override], @scheme[overment], or @scheme[override-final]. (Such an override merely replaces the augmentation, not the method that is augmented.) A method declared with @scheme[augment-final] cannot be overridden or augmented further in a subclass. A method declared with @scheme[private] is not accessible outside the class expression, cannot be overridden, and never overrides a method in the superclass. When a method is declared with @scheme[override], @scheme[overment], or @scheme[override-final], then the superclass implementation of the method can be called using @scheme[super] form. When a method is declared with @scheme[pubment], @scheme[augment], or @scheme[overment], then a subclass augmenting method can be called using the @scheme[inner] form. The only difference between @scheme[public-final] and @scheme[pubment] without a corresponding @scheme[inner] is that @scheme[public-final] prevents the declaration of augmenting methods that would be ignored. @defform*[[(super id arg ...) (super id arg ... . arg-list-expr)]]{ Always accesses the superclass method, independent of whether the method is overridden again in subclasses. Using the @scheme[super] form outside of @scheme[class*] is an syntax error. Each @scheme[arg] is as for @scheme[#%app]: either @scheme[_arg-expr] or @scheme[_keyword _arg-expr]. The second form is analogous to using @scheme[apply] with a procedure; the @scheme[arg-list-expr] must not be a parenthesized expression.} @defform*[[(inner default-expr id arg ...) (inner default-expr id arg ... . arg-list-expr)]]{ If the object's class does not supply an augmenting method, then @scheme[default-expr] is evaluated, and the @scheme[arg] expressions are not evaluated. Otherwise, the augmenting method is called with the @scheme[arg] results as arguments, and @scheme[default-expr] is not evaluated. If no @scheme[inner] call is evaluated for a particular method, then augmenting methods supplied by subclasses are never used. Using the @scheme[inner] form outside of @scheme[class*] is an syntax error. The second form is analogous to using @scheme[apply] with a procedure; the @scheme[arg-list-expr] must not be a parenthesized expression.} @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsubsection[#:tag "classinherit"]{Inherited and Superclass Methods} Each @scheme[inherit], @scheme[inherit/super], @scheme[inherit/inner], @scheme[rename-super], and @scheme[rename-inner] clause declares one or more methods that are defined in the class, but must be present in the superclass. The @scheme[rename-super] and @scheme[rename-inner] declarations are rarely used, since @scheme[inherit/super] and @scheme[inherit/inner] provide the same access. Also, superclass and augmenting methods are typically accessed through @scheme[super] and @scheme[inner] in a class that also declares the methods, instead of through @scheme[inherit/super], @scheme[inherit/inner], @scheme[rename-super], or @scheme[rename-inner]. Method names declared with @scheme[inherit], @scheme[inherit/super], or @scheme[inherit/inner] access overriding declarations, if any, at run time. Method names declared with @scheme[inherit/super] can also be used with the @scheme[super] form to access the superclass implementation, and method names declared with @scheme[inherit/inner] can also be used with the @scheme[inner] form to access an augmenting method, if any. Method names declared with @scheme[rename-super] always access the superclass's implementation at run-time. Methods declared with @scheme[rename-inner] access a subclass's augmenting method, if any, and must be called with the form @schemeblock[ (_id (lambda () _default-expr) _arg ...) ] so that a @scheme[default-expr] is available to evaluate when no augmenting method is available. In such a form, @scheme[lambda] is a literal identifier to separate the @scheme[default-expr] from the @scheme[arg]. When an augmenting method is available, it receives the results of the @scheme[arg] expressions as arguments. Methods that are present in the superclass but not declared with @scheme[inherit], @scheme[inherit/super], or @scheme[inherit/inner] or @scheme[rename-super] are not directly accessible in the class (through they can be called with @scheme[send]). Every public method in a superclass is present in a derived class, even if it is not declared with @scheme[inherit] in the derived class; the @scheme[inherit] clause does not control inheritance, but merely controls lexical scope within a class expression. If a method declared with @scheme[inherit], @scheme[inherit/super], @scheme[inherit/inner], @scheme[rename-super], or @scheme[rename-inner] is not present in the superclass, the @exnraise[exn:fail:object] when the class expression is evaluated. @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsubsection[#:tag "extnames"]{Internal and External Names} Each method declared with @scheme[public], @scheme[override], @scheme[augment], @scheme[pubment], @scheme[overment], @scheme[augride], @scheme[public-final], @scheme[override-final], @scheme[augment-final], @scheme[inherit], @scheme[inherit/super], @scheme[inherit/inner], @scheme[rename-super], and @scheme[rename-inner] can have separate internal and external names when @scheme[(internal-id external-id)] is used for declaring the method. The internal name is used to access the method directly within the class expression (including within @scheme[super] or @scheme[inner] forms), while the external name is used with @scheme[send] and @scheme[generic] (see @secref["ivaraccess"]). If a single @scheme[id] is provided for a method declaration, the identifier is used for both the internal and external names. Method inheritance, overriding, and augmentation are based external names, only. Separate internal and external names are required for @scheme[rename-super] and @scheme[rename-inner] (for historical reasons, mainly). Each @scheme[init], @scheme[init-field], @scheme[field], or @scheme[inherit-field] variable similarly has an internal and an external name. The internal name is used within the class to access the variable, while the external name is used outside the class when providing initialization arguments (e.g., to @scheme[instantiate]), inheriting a field, or accessing a field externally (e.g., with @scheme[class-field-accessor]). As for methods, when inheriting a field with @scheme[inherit-field], the external name is matched to an external field name in the superclass, while the internal name is bound in the @scheme[class] expression. A single identifier can be used as an internal identifier and an external identifier, and it is possible to use the same identifier as internal and external identifiers for different bindings. Furthermore, within a single class, a single name can be used as an external method name, an external field name, and an external initialization argument name. Overall, each internal identifier must be distinct from all other internal identifiers, each external method name must be distinct from all other method names, each external field name must be distinct from all other field names, and each initialization argument name must be distinct from all other initialization argument names By default, external names have no lexical scope, which means, for example, that an external method name matches the same syntactic symbol in all uses of @scheme[send]. The @scheme[define-local-member-name] and @scheme[define-member-name] forms introduce scoped external names. When a @scheme[class] expression is compiled, identifiers used in place of external names must be symbolically distinct (when the corresponding external names are required to be distinct), otherwise a syntax error is reported. When no external name is bound by @scheme[define-member-name], then the actual external names are guaranteed to be distinct when @scheme[class] expression is evaluated. When any external name is bound by @scheme[define-member-name], the @exnraise[exn:fail:object] by @scheme[class] if the actual external names are not distinct. @defform[(define-local-member-name id ...)]{ Unless it appears as the top-level definition, binds each @scheme[id] so that, within the scope of the definition, each use of each @scheme[id] as an external name is resolved to a hidden name generated by the @scheme[define-local-member-name] declaration. Thus, methods, fields, and initialization arguments declared with such external-name @scheme[id]s are accessible only in the scope of the @scheme[define-local-member-name] declaration. As a top-level definition, @scheme[define-local-member-name] binds @scheme[id] to its symbolic form. The binding introduced by @scheme[define-local-member-name] is a syntax binding that can be exported and imported with @scheme[module]s. Each execution of a @scheme[define-local-member-name] declaration generates a distinct hidden name (except as a top-level definitions). The @scheme[interface->method-names] procedure does not expose hidden names. @defexamples[ #:eval class-eval (define-values (r o) (let () (define-local-member-name m) (define c% (class object% (define/public (m) 10) (super-new))) (define o (new c%)) (values (send o m) o))) r (send o m) ]} @defform[(define-member-name id key-expr)]{ Maps a single external name to an external name that is determined by an expression. The value of @scheme[key-expr] must be the result of either a @scheme[member-name-key] expression or a @scheme[generate-member-key] call.} @defform[(member-name-key identifier)]{ Produces a representation of the external name for @scheme[id] in the environment of the @scheme[member-name-key] expression.} @defproc[(generate-member-key) member-name-key?]{ Produces a hidden name, just like the binding for @scheme[define-local-member-name].} @defproc[(member-name-key? [v any/c]) boolean?]{ Returns @scheme[#t] for values produced by @scheme[member-name-key] and @scheme[generate-member-key], @scheme[#f] otherwise.} @defproc[(member-name-key=? [a-key member-name-key?] [b-key member-name-key?]) boolean?]{ Produces @scheme[#t] if member-name keys @scheme[a-key] and @scheme[b-key] represent the same external name, @scheme[#f] otherwise.} @defproc[(member-name-key-hash-code [a-key member-name-key?]) integer?]{ Produces an integer hash code consistent with @scheme[member-name-key=?] comparisons, analogous to @scheme[equal-hash-code].} @defexamples[ #:eval class-eval (define (make-c% key) (define-member-name m key) (class object% (define/public (m) 10) (super-new))) (send (new (make-c% (member-name-key m))) m) (send (new (make-c% (member-name-key p))) m) (send (new (make-c% (member-name-key p))) p) ] @defs+int[ #:eval class-eval [(define (fresh-c%) (let ([key (generate-member-key)]) (values (make-c% key) key))) (define-values (fc% key) (fresh-c%))] (send (new fc%) m) (let () (define-member-name p key) (send (new fc%) p)) ] @; ------------------------------------------------------------------------ @section[#:tag "objcreation"]{Creating Objects} The @scheme[make-object] procedure creates a new object with by-position initialization arguments, the @scheme[new] form creates a new object with by-name initialization arguments, and the @scheme[instantiate] form creates a new object with both by-position and by-name initialization arguments. All fields in the newly created object are initially bound to the special @|undefined-const| value (see @secref["void"]). Initialization variables with default value expressions (and no provided value) are also initialized to @|undefined-const|. After argument values are assigned to initialization variables, expressions in @scheme[field] clauses, @scheme[init-field] clauses with no provided argument, @scheme[init] clauses with no provided argument, private field definitions, and other expressions are evaluated. Those expressions are evaluated as they appear in the class expression, from left to right. Sometime during the evaluation of the expressions, superclass-declared initializations must be executed once by using the @scheme[super-make-object] procedure, @scheme[super-new] form, or @scheme[super-instantiate] form. By-name initialization arguments to a class that have no matching initialization variable are implicitly added as by-name arguments to a @scheme[super-make-object], @scheme[super-new], or @scheme[super-instantiate] invocation, after the explicit arguments. If multiple initialization arguments are provided for the same name, the first (if any) is used, and the unused arguments are propagated to the superclass. (Note that converted by-position arguments are always placed before explicit by-name arguments.) The initialization procedure for the @scheme[object%] class accepts zero initialization arguments; if it receives any by-name initialization arguments, then @exnraise[exn:fail:object]. If the end of initialization is reached for any class in the hierarchy without invoking the superclass's initialization, the @exnraise[exn:fail:object]. Also, if superclass initialization is invoked more than once, the @exnraise[exn:fail:object]. Fields inherited from a superclass are not initialized until the superclass's initialization procedure is invoked. In contrast, all methods are available for an object as soon as the object is created; the overriding of methods is not affect by initialization (unlike objects in C++). @defproc[(make-object [class class?] [init-v any/c] ...) object?]{ Creates an instance of @scheme[class]. The @scheme[init-v]s are passed as initialization arguments, bound to the initialization variables of @scheme[class] for the newly created object as described in @secref["clinitvars"]. If @scheme[class] is not a class, the @exnraise[exn:fail:contract].} @defform[(new class-expr (id by-name-expr) ...)]{ Creates an instance of the value of @scheme[class-expr] (which must be a class), and the value of each @scheme[by-name-expr] is provided as a by-name argument for the corresponding @scheme[id].} @defform[(instantiate class-expr (by-pos-expr ...) (id by-name-expr) ...)]{ Creates an instance of the value of @scheme[class-expr] (which must be a class), and the values of the @scheme[by-pos-expr]s are provided as by-position initialization arguments. In addition, the value of each @scheme[by-name-expr] is provided as a by-name argument for the corresponding @scheme[id].} @defidform[super-make-object]{ Produces a procedure that takes by-position arguments an invokes superclass initialization. See @secref["objcreation"] for more information.} @defform[(super-instantiate (by-pos-expr ...) (id by-expr ...) ...)]{ Invokes superclass initialization with the specified by-position and by-name arguments. See @secref["objcreation"] for more information.} @defform[(super-new (id by-name-expr ...) ...)]{ Invokes superclass initialization with the specified by-name arguments. See @secref["objcreation"] for more information.} @; ------------------------------------------------------------------------ @section[#:tag "ivaraccess"]{Field and Method Access} In expressions within a class definition, the initialization variables, fields, and methods of the class all part of the environment. Within a method body, only the fields and other methods of the class can be referenced; a reference to any other class-introduced identifier is a syntax error. Elsewhere within the class, all class-introduced identifiers are available, and fields and initialization variables can be mutated with @scheme[set!]. @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection[#:tag "methodcalls"]{Methods} Method names within a class can only be used in the procedure position of an application expression; any other use is a syntax error. To allow methods to be applied to lists of arguments, a method application can have the following form: @specsubform[ (method-id arg ... . arg-list-expr) ] This form calls the method in a way analogous to @scheme[(apply _method-id _arg ... _arg-list-expr)]. The @scheme[arg-list-expr] must not be a parenthesized expression. Methods are called from outside a class with the @scheme[send] and @scheme[send/apply] forms. @defform*[[(send obj-expr method-id arg ...) (send obj-expr method-id arg ... . arg-list-expr)]]{ Evaluates @scheme[obj-expr] to obtain an object, and calls the method with (external) name @scheme[method-id] on the object, providing the @scheme[arg] results as arguments. Each @scheme[arg] is as for @scheme[#%app]: either @scheme[_arg-expr] or @scheme[_keyword _arg-expr]. In the second form, @scheme[arg-list-expr] cannot be a parenthesized expression. If @scheme[obj-expr] does not produce an object, the @exnraise[exn:fail:contract]. If the object has no public method named @scheme[method-id], the @exnraise[exn:fail:object].} @defform[(send/apply obj-expr method-id arg ... arg-list-expr)]{ Like the dotted form of @scheme[send], but @scheme[arg-list-expr] can be any expression.} @defform/subs[(send* obj-expr msg ...) ([msg (method-id arg ...) (method-id arg ... . arg-list-expr)])]{ Calls multiple methods (in order) of the same object. Each @scheme[msg] corresponds to a use of @scheme[send]. For example, @schemeblock[ (send* edit (begin-edit-sequence) (insert "Hello") (insert #\newline) (end-edit-sequence)) ] is the same as @schemeblock[ (let ([o edit]) (send o begin-edit-sequence) (send o insert "Hello") (send o insert #\newline) (send o end-edit-sequence)) ]} @defform[(with-method ((id (obj-expr method-id)) ...) body ...+)]{ Extracts methods from an object and binds a local name that can be applied directly (in the same way as declared methods within a class) for each method. The each @scheme[obj-expr] must produce an object, which must have a public method named by the corresponding @scheme[method-id]. The corresponding @scheme[id] is bound so that it can be applied directly (see @secref["methodcalls"]). Example: @schemeblock[ (let ([s (new stack%)]) (with-method ([push (s push!)] [pop (s pop!)]) (push 10) (push 9) (pop))) ] is the same as @schemeblock[ (let ([s (new stack%)]) (send s push! 10) (send s push! 9) (send s pop!)) ]} @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection{Fields} @defform[(get-field id obj-expr)]{ Extracts the field with (external) name @scheme[id] from the value of @scheme[obj-expr]. If @scheme[obj-expr] does not produce an object, the @exnraise[exn:fail:contract]. If the object has no @scheme[id] method, the @exnraise[exn:fail:object].} @defform[(set-field! id obj-expr expr)]{ Sets the field with (external) name @scheme[id] from the value of @scheme[obj-expr] to the value of @scheme[expr]. If @scheme[obj-expr] does not produce an object, the @exnraise[exn:fail:contract]. If the object has no @scheme[id] method, the @exnraise[exn:fail:object].} @defform[(field-bound? id obj-expr)]{ Produces @scheme[#t] if the object result of @scheme[obj-expr] has an field with (external) name @scheme[id], @scheme[#f] otherwise. If @scheme[obj-expr] does not produce an object, the @exnraise[exn:fail:contract].} @defform[(class-field-accessor class-expr field-id)]{ Returns an accessor procedure that takes an instance of the class produced by @scheme[class-expr] and returns the value of the object's field with (external) name @scheme[field-id]. If @scheme[class-expr] does not produce a class, the @exnraise[exn:fail:contract]. If the class has no @scheme[field-id] field, the @exnraise[exn:fail:object].} @defform[(class-field-mutator class-expr field-id)]{ Returns a mutator procedure that takes an instance of the class produced by @scheme[class-expr] and a value, and sets the value of the object's field with (external) name @scheme[field-id] to the given value. The result is @|void-const|. If @scheme[class-expr] does not produce a class, the @exnraise[exn:fail:contract]. If the class has no @scheme[field-id] field, the @exnraise[exn:fail:object].} @; - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - @subsection{Generics} A @deftech{generic} can be used instead of a method name to avoid the cost of relocating a method by name within a class. @defform[(generic class-or-interface-expr id)]{ Produces a generic that works on instances of the class or interface produced by @scheme[class-or-interface-expr] (or an instance of a class/interface derived from @scheme[class-or-interface]) to call the method with (external) name @scheme[id]. If @scheme[class-or-interface-expr] does not produce a class or interface, the @exnraise[exn:fail:contract]. If the resulting class or interface does not contain a method named @scheme[id], the @exnraise[exn:fail:object].} @defform*[[(send-generic obj-expr generic-expr arg ...) (send-generic obj-expr generic-expr arg ... . arg-list-expr)]]{ Calls a method of the object produced by @scheme[obj-expr] as indicated by the generic produced by @scheme[generic-expr]. Each @scheme[arg] is as for @scheme[#%app]: either @scheme[_arg-expr] or @scheme[_keyword _arg-expr]. The second form is analogous to calling a procedure with @scheme[apply], where @scheme[arg-list-expr] is not a parenthesized expression. If @scheme[obj-expr] does not produce a object, or if @scheme[generic-expr] does not produce a generic, the @exnraise[exn:fail:contract]. If the result of @scheme[obj-expr] is not an instance of the class or interface encapsulated by the result of @scheme[generic-expr], the @exnraise[exn:fail:object].} @defproc[(make-generic [type (or/c class? interface?)] [method-name symbol?]) generic?]{ Like the @scheme[generic] form, but as a procedure that accepts a symbolic method name.} @; ------------------------------------------------------------------------ @section[#:tag "mixins"]{Mixins} @defform[(mixin (interface-expr ...) (interface-expr ...) class-clause ...)]{ Produces a @deftech{mixin}, which is a procedure that encapsulates a class extension, leaving the superclass unspecified. Each time that a mixin is applied to a specific superclass, it produces a new derived class using the encapsulated extension. The given class must implement interfaces produced by the first set of @scheme[interface-expr]s. The result of the procedure is a subclass of the given class that implements the interfaces produced by the second set of @scheme[interface-expr]s. The @scheme[class-clause]s are as for @scheme[class*], to define the class extension encapsulated by the mixin. Evaluation of a @scheme[mixin] form checks that the @scheme[class-clause]s are consistent with both sets of @scheme[interface-expr]s.} @; ------------------------------------------------------------------------ @section[#:tag "trait"]{Traits} @note-lib-only[racket/trait] A @deftech{trait} is a collection of methods that can be converted to a @tech{mixin} and then applied to a @tech{class}. Before a trait is converted to a mixin, the methods of a trait can be individually renamed, and multiple traits can be merged to form a new trait. @defform/subs[#:literals (public pubment public-final override override-final overment augment augride augment-final private inherit inherit/super inherit/inner rename-super field inherit-field) (trait trait-clause ...) ([trait-clause (public maybe-renamed ...) (pubment maybe-renamed ...) (public-final maybe-renamed ...) (override maybe-renamed ...) (overment maybe-renamed ...) (override-final maybe-renamed ...) (augment maybe-renamed ...) (augride maybe-renamed ...) (augment-final maybe-renamed ...) (inherit maybe-renamed ...) (inherit/super maybe-renamed ...) (inherit/inner maybe-renamed ...) method-definition (field field-declaration ...) (inherit-field maybe-renamed ...)])]{ Creates a @tech{trait}. The body of a @scheme[trait] form is similar to the body of a @scheme[class*] form, but restricted to non-private method definitions. In particular, the grammar of @scheme[maybe-renamed], @scheme[method-definition], and @scheme[field-declaration] are the same as for @scheme[class*], and every @scheme[method-definition] must have a corresponding declaration (one of @scheme[public], @scheme[override], etc.). As in @scheme[class], uses of method names in direct calls, @scheme[super] calls, and @scheme[inner] calls depend on bringing method names into scope via @scheme[inherit], @scheme[inherit/super], @scheme[inherit/inner], and other method declarations in the same trait; an exception, compared to @scheme[class] is that @scheme[overment] binds a method name only in the corresponding method, and not in other methods of the same trait. Finally, macros such as @scheme[public*] and @scheme[define/public] work in @scheme[trait] as in @scheme[class]. External identifiers in @scheme[trait], @scheme[trait-exclude], @scheme[trait-exclude-field], @scheme[trait-alias], @scheme[trait-rename], and @scheme[trait-rename-field] forms are subject to binding via @scheme[define-member-name] and @scheme[define-local-member-name]. Although @scheme[private] methods or fields are not allowed in a @scheme[trait] form, they can be simulated by using a @scheme[public] or @scheme[field] declaration and a name whose scope is limited to the @scheme[trait] form.} @defproc[(trait? [v any/c]) boolean?]{ Returns @scheme[#t] if @scheme[v] is a trait, @scheme[#f] otherwise.} @defproc[(trait->mixin [tr trait?]) (class? . -> . class?)]{ Converts a @tech{trait} to a @tech{mixin}, which can be applied to a @tech{class} to produce a new @tech{class}. An expression of the form @schemeblock[ (trait->mixin (trait _trait-clause ...)) ] is equivalent to @schemeblock[ (lambda (%) (class % _trait-clause ... (super-new))) ] Normally, however, a trait's methods are changed and combined with other traits before converting to a mixin.} @defproc[(trait-sum [tr trait?] ...+) trait?]{ Produces a @tech{trait} that combines all of the methods of the given @scheme[tr]s. For example, @schemeblock[ (define t1 (trait (define/public (m1) 1))) (define t2 (trait (define/public (m2) 2))) (define t3 (trait-sum t1 t2)) ] creates a trait @scheme[t3] that is equivalent to @schemeblock[ (trait (define/public (m1) 1) (define/public (m2) 2)) ] but @scheme[t1] and @scheme[t2] can still be used individually or combined with other traits. When traits are combined with @scheme[trait-sum], the combination drops @scheme[inherit], @scheme[inherit/super], @scheme[inherit/inner], and @scheme[inherit-field] declarations when a definition is supplied for the same method or field name by another trait. The @scheme[trait-sum] operation fails (the @exnraise[exn:fail:contract]) if any of the traits to combine define a method or field with the same name, or if an @scheme[inherit/super] or @scheme[inherit/inner] declaration to be dropped is inconsistent with the supplied definition. In other words, declaring a method with @scheme[inherit], @scheme[inherit/super], or @scheme[inherit/inner], does not count as defining the method; at the same time, for example, a trait that contains an @scheme[inherit/super] declaration for a method @scheme[m] cannot be combined with a trait that defines @scheme[m] as @scheme[augment], since no class could satisfy the requirements of both @scheme[augment] and @scheme[inherit/super] when the trait is later converted to a mixin and applied to a class.} @defform[(trait-exclude trait-expr id)]{ Produces a new @tech{trait} that is like the @tech{trait} result of @scheme[trait-expr], but with the definition of a method named by @scheme[id] removed; as the method definition is removed, either a @scheme[inherit], @scheme[inherit/super], or @scheme[inherit/inner] declaration is added: @itemize[ @item{A method declared with @scheme[public], @scheme[pubment], or @scheme[public-final] is replaced with a @scheme[inherit] declaration.} @item{A method declared with @scheme[override] or @scheme[override-final] is replaced with a @scheme[inherit/super] declaration.} @item{A method declared with @scheme[augment], @scheme[augride], or @scheme[augment-final] is replaced with a @scheme[inherit/inner] declaration.} @item{A method declared with @scheme[overment] is not replaced with any @scheme[inherit] declaration.} ] If the trait produced by @scheme[trait-expr] has no method definition for @scheme[id], the @exnraise[exn:fail:contract].} @defform[(trait-exclude-field trait-expr id)]{ Produces a new @tech{trait} that is like the @tech{trait} result of @scheme[trait-expr], but with the definition of a field named by @scheme[id] removed; as the field definition is removed, an @scheme[inherit-field] declaration is added.} @defform[(trait-alias trait-expr id new-id)]{ Produces a new @tech{trait} that is like the @tech{trait} result of @scheme[trait-expr], but the definition and declaration of the method named by @scheme[id] is duplicated with the name @scheme[new-id]. The consistency requirements for the resulting trait are the same as for @scheme[trait-sum], otherwise the @exnraise[exn:fail:contract]. This operation does not rename any other use of @scheme[id], such as in method calls (even method calls to @scheme[identifier] in the cloned definition for @scheme[new-id]).} @defform[(trait-rename trait-expr id new-id)]{ Produces a new @tech{trait} that is like the @tech{trait} result of @scheme[trait-expr], but all definitions and references to methods named @scheme[id] are replaced by definitions and references to methods named by @scheme[new-id]. The consistency requirements for the resulting trait is the same as for @scheme[trait-sum], otherwise the @exnraise[exn:fail:contract].} @defform[(trait-rename-field trait-expr id new-id)]{ Produces a new @tech{trait} that is like the @tech{trait} result of @scheme[trait-expr], but all definitions and references to fields named @scheme[id] are replaced by definitions and references to fields named by @scheme[new-id]. The consistency requirements for the resulting trait is the same as for @scheme[trait-sum], otherwise the @exnraise[exn:fail:contract].} @; ------------------------------------------------------------------------ @section{Object and Class Contracts} @defform/subs[ #:literals (field init init-field inherit inherit-field super inner override augment augride) (class/c member-spec ...) ([member-spec method-spec (field field-spec ...) (init field-spec ...) (init-field field-spec ...) (inherit method-spec ...) (inherit-field field-spec ...) (super method-spec ...) (inner method-spec ...) (override method-spec ...) (augment method-spec ...) (augride method-spec ...)] [method-spec method-id (method-id method-contract)] [field-spec field-id (field-id contract-expr)])]{ Produces a contract for a class. There are two major categories of contracts listed in a @scheme[class/c] form: external and internal contracts. External contracts govern behavior when an object is instantiated from a class or when methods or fields are accessed via an object of that class. Internal contracts govern behavior when method or fields are accessed within the class hierarchy. This separation allows for stronger contracts for class clients and weaker contracts for subclasses. Method contracts must contain an additional initial argument which corresponds to the implicit @scheme[this] parameter of the method. This allows for contracts which discuss the state of the object when the method is called (or, for dependent contracts, in other parts of the contract). Alternative contract forms, such as @scheme[->m], are provided as a shorthand for writing method contracts. The external contracts are as follows: @itemize[ @item{A method contract without a tag describes the behavior of the implementation of @scheme[method-id] on method sends to an object of the contracted class. This contract will continue to be checked in subclasses until the contracted class's implementation is no longer the entry point for dynamic dispatch.} @item{A field contract, tagged with @scheme[field], describes the behavior of the value contained in that field when accessed via an object of that class. Since fields may be mutated, these contracts are checked on any external access and/or mutation of the field.} @item{An initialization argument contract, tagged with @scheme[init], describes the expected behavior of the value paired with that name during class instantiation. The same name can be provided more than once, in which case the first such contract in the @scheme[class/c] form is applied to the first value tagged with that name in the list of initialization arguments, and so on.} @item{The contracts listed in an @scheme[init-field] section are treated as if each contract appeared in an @scheme[init] section and a @scheme[field] section.} ] The internal contracts are as follows: @itemize[ @item{A method contract, tagged with @scheme[inherit], describes the behavior of the method when invoked directly (i.e., via @scheme[inherit]) in any subclass of the contracted class. This contract, like external method contracts, applies until the contracted class's method implementation is no longer the entry point for dynamic dispatch.} @item{A field contract, tagged with @scheme[inherit-field], describes the behavior of the value contained in that field when accessed directly (i.e., via @scheme[inherit-field]) in any subclass of the contracted class. Since fields may be mutated, these contracts are checked on any access and/or mutation of the field that occurs in such subclasses.} @item{A method contract, tagged with @scheme[super], describes the behavior of @scheme[method-id] when called by the @scheme[super] form in a subclass. This contract only affects @scheme[super] calls in subclasses which call the contract class's implementation of @scheme[method-id].} @item{A method contract, tagged with @scheme[inner], describes the behavior the class expects of an augmenting method in a subclass. This contract affects any implementations of @scheme[method-id] in subclasses which can be called via @scheme[inner] from the contracted class. This means a subclass which implements @scheme[method-id] via @scheme[augment] or @scheme[overment] stop future subclasses from being affected by the contract, since further extension cannot be reached via the contracted class.} @item{A method contract, tagged with @scheme[override], describes the behavior expected by the contracted class for @scheme[method-id] when called directly (i.e. by the application @scheme[(method-id ...)]). This form can only be used if overriding the method in subclasses will change the entry point to the dynamic dispatch chain (i.e., the method has never been augmentable).} @item{A method contract, tagged with either @scheme[augment] or @scheme[augride], describes the behavior provided by the contracted class for @scheme[method-id] when called directly from subclasses. These forms can only be used if the method has previously been augmentable, which means that no augmenting or overriding implementation will change the entry point to the dynamic dispatch chain. @scheme[augment] is used when subclasses can augment the method, and @scheme[augride] is used when subclasses can override the current augmentation.} ]} @defform[(->m dom ... range)]{ Similar to @scheme[->], except that the domain of the resulting contract contains one more element than the stated domain, where the first (implicit) argument is contracted with @scheme[any/c]. This contract is useful for writing simpler method contracts when no properties of @scheme[this] need to be checked.} @defform[(->*m (mandatory-dom ...) (optional-dom ...) rest range)]{ Similar to @scheme[->*], except that the mandatory domain of the resulting contract contains one more element than the stated domain, where the first (implicit) argument is contracted with @scheme[any/c]. This contract is useful for writing simpler method contracts when no properties of @scheme[this] need to be checked.} @defform[(case->m (-> dom ... rest range) ...)]{ Similar to @scheme[case->], except that the mandatory domain of each case of the resulting contract contains one more element than the stated domain, where the first (implicit) argument is contracted with @scheme[any/c]. This contract is useful for writing simpler method contracts when no properties of @scheme[this] need to be checked.} @defform[(->dm (mandatory-dependent-dom ...) (optional-dependent-dom ...) dependent-rest pre-cond dep-range)]{ Similar to @scheme[->d], except that the mandatory domain of the resulting contract contains one more element than the stated domain, where the first (implicit) argument is contracted with @scheme[any/c]. In addition, @scheme[this] is appropriately bound in the body of the contract. This contract is useful for writing simpler method contracts when no properties of @scheme[this] need to be checked.} @defform/subs[ #:literals (field) (object/c member-spec ...) ([member-spec method-spec (field field-spec ...)] [method-spec method-id (method-id method-contract)] [field-spec field-id (field-id contract-expr)])]{ Produces a contract for an object. Unlike the older form @scheme[object-contract], but like @scheme[class/c], arbitrary contract expressions are allowed. Also, method contracts for @scheme[object/c] follow those for @scheme[class/c]. An object wrapped with @scheme[object/c] behaves as if its class had been wrapped with the equivalent @scheme[class/c] contract. } @defform/subs[ #:literals (field -> ->* ->d) (object-contract member-spec ...) ([member-spec (method-id method-contract) (field field-id contract-expr)] [method-contract (-> dom ... range) (->* (mandatory-dom ...) (optional-dom ...) rest range) (->d (mandatory-dependent-dom ...) (optional-dependent-dom ...) dependent-rest pre-cond dep-range)] [dom dom-expr (code:line keyword dom-expr)] [range range-expr (values range-expr ...) any] [mandatory-dom dom-expr (code:line keyword dom-expr)] [optional-dom dom-expr (code:line keyword dom-expr)] [rest (code:line) (code:line #:rest rest-expr)] [mandatory-dependent-dom [id dom-expr] (code:line keyword [id dom-expr])] [optional-dependent-dom [id dom-expr] (code:line keyword [id dom-expr])] [dependent-rest (code:line) (code:line #:rest id rest-expr)] [pre-cond (code:line) (code:line #:pre-cond boolean-expr)] [dep-range any (code:line [id range-expr] post-cond) (code:line (values [id range-expr] ...) post-cond)] [post-cond (code:line) (code:line #:post-cond boolean-expr)] )]{ Produces a contract for an object. Each of the contracts for a method has the same semantics as the corresponding function contract, but the syntax of the method contract must be written directly in the body of the object-contract---much like the way that methods in class definitions use the same syntax as regular function definitions, but cannot be arbitrary procedures. Unlike the method contracts for @scheme[class/c], the implicit @scheme[this] argument is not part of the contract. To allow for the use of @scheme[this] in dependent contracts, @scheme[->d] contracts implicitly bind @scheme[this] to the object itself.} @defthing[mixin-contract contract?]{ A @tech{function contract} that recognizes mixins. It guarantees that the input to the function is a class and the result of the function is a subclass of the input.} @defproc[(make-mixin-contract [type (or/c class? interface?)] ...) contract?]{ Produces a @tech{function contract} that guarantees the input to the function is a class that implements/subclasses each @scheme[type], and that the result of the function is a subclass of the input.} @defproc[(is-a?/c [type (or/c class? interface?)]) flat-contract?]{ Accepts a class or interface and returns a flat contract that recognizes objects that instantiate the class/interface.} @defproc[(implementation?/c [interface interface?]) flat-contract?]{ Returns a flat contract that recognizes classes that implement @scheme[interface].} @defproc[(subclass?/c [class class?]) flat-contract?]{ Returns a flat-contract that recognizes classes that are subclasses of @scheme[class].} @; ------------------------------------------------------------------------ @section[#:tag "objectequality"]{Object Equality and Hashing} But default, objects that are instances of different classes or that are instances of a non-transparent class are @scheme[equal?] only if they are @scheme[eq?]. Like transparent structures, two objects that are instances of the same transparent class (i.e., every superclass of the class has @scheme[#f] as its inspector) are @scheme[equal?] when their field values are @scheme[equal?]. To customize the way that a class instance is compared to other instances by @scheme[equal?], implement the @scheme[equal<%>] interface. @definterface[equal<%> ()]{ The @scheme[equal<%>] interface includes three methods, which are analogous to the functions provided for a structure type with @scheme[prop:equal+hash]: @itemize[ @item{@scheme[equal-to?] --- Takes two arguments. The first argument is an object that is an instance of the same class (or a subclass that does not re-declare its implementation of @scheme[equal<%>]) and that is being compared to the target object. The second argument is a @scheme[equal?]-like procedure of two arguments that should be used for recursive equality testing. The result should be a true value if the object and the first argument of the method are equal, @scheme[#f] otherwise.} @item{@scheme[equal-hash-code-of] --- Takes one argument, which is a procedure of one argument that should be used for recursive hash-code computation. The result should be an exact integer representing the target object's hash code.} @item{@scheme[equal-secondary-hash-code-of] --- Takes one argument, which is a procedure of one argument that should be used for recursive hash-code computation. The result should be an exact integer representing the target object's secondary hash code.} ] The @scheme[equal<%>] interface is unusual in that declaring the implementation of the interface is different from inheriting the interface. Two objects can be equal only if they are instances of classes whose most specific ancestor to explicitly implement @scheme[equal<%>] is the same ancestor. See @scheme[prop:equal+hash] for more information on equality comparisons and hash codes. The @scheme[equal<%>] interface is implemented with @scheme[interface*] and @scheme[prop:equal+hash].} @; ------------------------------------------------------------------------ @section[#:tag "objectserialize"]{Object Serialization} @defform[ (define-serializable-class* class-id superclass-expr (interface-expr ...) class-clause ...) ]{ Binds @scheme[class-id] to a class, where @scheme[superclass-expr], the @scheme[interface-expr]s, and the @scheme[class-clause]s are as in @scheme[class*]. This forms can only be used at the top level, either within a module or outside. The @scheme[class-id] identifier is bound to the new class, and @schemeidfont{deserialize-info:}@scheme[class-id] is also defined; if the definition is within a module, then the latter is provided from the module. Serialization for the class works in one of two ways: @itemize[ @item{If the class implements the built-in interface @scheme[externalizable<%>], then an object is serialized by calling its @scheme[externalize] method; the result can be anything that is serializable (but, obviously, should not be the object itself). Deserialization creates an instance of the class with no initialization arguments, and then calls the object's @scheme[internalize] method with the result of @scheme[externalize] (or, more precisely, a deserialized version of the serialized result of a previous call). To support this form of serialization, the class must be instantiable with no initialization arguments. Furthermore, cycles involving only instances of the class (and other such classes) cannot be serialized.} @item{If the class does not implement @scheme[externalizable<%>], then every superclass of the class must be either serializable or transparent (i.e,. have @scheme[#f] as its inspector). Serialization and deserialization are fully automatic, and may involve cycles of instances. To support cycles of instances, deserialization may create an instance of the call with all fields as the undefined value, and then mutate the object to set the field values. Serialization support does not otherwise make an object's fields mutable.} ] In the second case, a serializable subclass can implement @scheme[externalizable<%>], in which case the @scheme[externalize] method is responsible for all serialization (i.e., automatic serialization is lost for instances of the subclass). In the first case, all serializable subclasses implement @scheme[externalizable<%>], since a subclass implements all of the interfaces of its parent class. In either case, if an object is an immediate instance of a subclass (that is not itself serializable), the object is serialized as if it was an immediate instance of the serializable class. In particular, overriding declarations of the @scheme[externalize] method are ignored for instances of non-serializable subclasses.} @defform[ (define-serializable-class class-id superclass-expr class-clause ...) ]{ Like @scheme[define-serializable-class*], but with not interface expressions (analogous to @scheme[class]).} @definterface[externalizable<%> ()]{ The @scheme[externalizable<%>] interface includes only the @scheme[externalize] and @scheme[internalize] methods. See @scheme[define-serializable-class*] for more information.} @; ------------------------------------------------------------------------ @section[#:tag "objectprinting"]{Object Printing} To customize the way that a class instance is printed by @racket[print], @scheme[write] and @scheme[display], implement the @scheme[printable<%>] interface. @defthing[printable<%> interface?]{ The @scheme[printable<%>] interface includes only the @scheme[custom-print], @scheme[custom-write], and @scheme[custom-display] methods. The @scheme[custom-print] method accepts two arguments: the destination port and the current @scheme[quaisquote] depth as an exact nonnegative integer. The @scheme[custom-write] and @scheme[custom-display] methods each accepts a single argument, which is the destination port to @scheme[write] or @scheme[display] the object. Calls to the @racket[custom-print], @scheme[custom-write], or @scheme[custom-display] methods are like calls to a procedure attached to a structure type through the @scheme[prop:custom-write] property. In particular, recursive printing can trigger an escape from the call. See @scheme[prop:custom-write] for more information. The @scheme[printable<%>] interface is implemented with @scheme[interface*] and @scheme[prop:custom-write].} @defthing[writable<%> interface?]{ Like @scheme[printable<%>], but includes only the @scheme[custom-write] and @scheme[custom-print] methods. A @racket[print] request is directed to @scheme[custom-write].} @; ------------------------------------------------------------------------ @section[#:tag "objectutils"]{Object, Class, and Interface Utilities} @defproc[(object? [v any/c]) boolean?]{ Returns @scheme[#t] if @scheme[v] is an object, @scheme[#f] otherwise.} @defproc[(class? [v any/c]) boolean?]{ Returns @scheme[#t] if @scheme[v] is a class, @scheme[#f] otherwise.} @defproc[(interface? [v any/c]) boolean?]{ Returns @scheme[#t] if @scheme[v] is an interface, @scheme[#f] otherwise.} @defproc[(generic? [v any/c]) boolean?]{ Returns @scheme[#t] if @scheme[v] is a @tech{generic}, @scheme[#f] otherwise.} @defproc[(object=? [a object?] [b object?]) boolean?]{ Determines if two objects are the same object, or not; this procedure uses @scheme[eq?], but also works properly with contracts.} @defproc[(object->vector [object object?] [opaque-v any/c #f]) vector?]{ Returns a vector representing @scheme[object] that shows its inspectable fields, analogous to @scheme[struct->vector].} @defproc[(class->interface [class class?]) interface?]{ Returns the interface implicitly defined by @scheme[class].} @defproc[(object-interface [object object?]) interface?]{ Returns the interface implicitly defined by the class of @scheme[object].} @defproc[(is-a? [v any/c] [type (or/c interface? class?)]) boolean?]{ Returns @scheme[#t] if @scheme[v] is an instance of a class @scheme[type] or a class that implements an interface @scheme[type], @scheme[#f] otherwise.} @defproc[(subclass? [v any/c] [class class?]) boolean?]{ Returns @scheme[#t] if @scheme[v] is a class derived from (or equal to) @scheme[class], @scheme[#f] otherwise.} @defproc[(implementation? [v any/c] [interface interface?]) boolean?]{ Returns @scheme[#t] if @scheme[v] is a class that implements @scheme[interface], @scheme[#f] otherwise.} @defproc[(interface-extension? [v any/c] [interface interface?]) boolean?]{ Returns @scheme[#t] if @scheme[v] is an interface that extends @scheme[interface], @scheme[#f] otherwise.} @defproc[(method-in-interface? [sym symbol?] [interface interface?]) boolean?]{ Returns @scheme[#t] if @scheme[interface] (or any of its ancestor interfaces) includes a member with the name @scheme[sym], @scheme[#f] otherwise.} @defproc[(interface->method-names [interface interface?]) (listof symbol?)]{ Returns a list of symbols for the method names in @scheme[interface], including methods inherited from superinterfaces, but not including methods whose names are local (i.e., declared with @scheme[define-local-member-names]).} @defproc[(object-method-arity-includes? [object object?] [sym symbol?] [cnt exact-nonnegative-integer?]) boolean?]{ Returns @scheme[#t] if @scheme[object] has a method named @scheme[sym] that accepts @scheme[cnt] arguments, @scheme[#f] otherwise.} @defproc[(field-names [object object?]) (listof symbol?)]{ Returns a list of all of the names of the fields bound in @scheme[object], including fields inherited from superinterfaces, but not including fields whose names are local (i.e., declared with @scheme[define-local-member-names]).} @defproc[(object-info [object any/c]) (values (or/c class? #f) boolean?)]{ Returns two values, analogous to the return values of @scheme[struct-info]: @itemize[ @item{@scheme[class]: a class or @scheme[#f]; the result is @scheme[#f] if the current inspector does not control any class for which the @scheme[object] is an instance.} @item{@scheme[skipped?]: @scheme[#f] if the first result corresponds to the most specific class of @scheme[object], @scheme[#t] otherwise.} ]} @defproc[(class-info [class class?]) (values symbol? exact-nonnegative-integer? (listof symbol?) (any/c exact-nonnegative-integer? . -> . any/c) (any/c exact-nonnegative-integer? any/c . -> . any/c) (or/c class? #f) boolean?)]{ Returns seven values, analogous to the return values of @scheme[struct-type-info]: @itemize[ @item{@scheme[name]: the class's name as a symbol;} @item{@scheme[field-cnt]: the number of fields (public and private) defined by the class;} @item{@scheme[field-name-list]: a list of symbols corresponding to the class's public fields; this list can be larger than @scheme[field-k] because it includes inherited fields;} @item{@scheme[field-accessor]: an accessor procedure for obtaining field values in instances of the class; the accessor takes an instance and a field index between @scheme[0] (inclusive) and @scheme[field-cnt] (exclusive);} @item{@scheme[field-mutator]: a mutator procedure for modifying field values in instances of the class; the mutator takes an instance, a field index between @scheme[0] (inclusive) and @scheme[field-cnt] (exclusive), and a new field value;} @item{@scheme[super-class]: a class for the most specific ancestor of the given class that is controlled by the current inspector, or @scheme[#f] if no ancestor is controlled by the current inspector;} @item{@scheme[skipped?]: @scheme[#f] if the sixth result is the most specific ancestor class, @scheme[#t] otherwise.} ]} @defstruct[(exn:fail:object exn:fail) ()]{ Raised for @scheme[class]-related failures, such as attempting to call a method that is not supplied by an object. } @; ---------------------------------------------------------------------- @include-section["surrogate.scrbl"] @close-eval[class-eval]