313 lines
10 KiB
Plaintext
313 lines
10 KiB
Plaintext
_Macro Debugger_
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================
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Keywords: _syntax_, _syntax-browser_, _hygiene_,
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_referential transparency_, _lexical context_,
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_macro_, _expansion_, _macro-debugger_, _macro-stepper_
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OUTLINE
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=======
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* Introduction
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* APIs
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- expand.ss
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- syntax-browser.ss
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* Using the macro stepper
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* Using the syntax browser
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* Enabling the macro stepper in other languages
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* Notes for DrScheme language implementors
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Introduction
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============
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The macro-debugger collection contains two tools: a stepper for macro
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expansion and a standalone syntax browser. The macro stepper shows the
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programmer the expansion of a program as a sequence of rewriting
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steps, using the syntax browser to display the individual terms. The
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syntax browser uses colors and a properties panel to show the term's
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syntax properties, such as lexical binding information and source
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location.
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_stepper.ss_
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============
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> (require (lib "stepper.ss" "macro-debugger"))
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This module provides a single procedure:
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> (expand/step syntax-or-sexpr)
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Expands the syntax (or S-expression) and opens a macro stepper frame
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for stepping through the expansion. Returns an object of
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macro-stepper<%> with the following methods:
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> (send a-macro-stepper at-start?) -> boolean
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> (send a-macro-stepper at-end?) -> boolean
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> (send a-macro-stepper navigate-to-start) -> void
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> (send a-macro-stepper navigate-to-end) -> void
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> (send a-macro-stepper navigate-previous) -> void
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> (send a-macro-stepper navigate-next) -> void
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> (send a-macro-stepper navigate-forward/count n) -> void
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> (send a-macro-stepper navigate-forward/pred pred) -> void
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pred : syntax -> boolean
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Navigate forward to an expansion with redex (focus) matching pred.
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> (send a-macro-stepper at-top?) -> boolean
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> (send a-macro-stepper at-bottom?) -> boolean
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> (send a-macro-stepper navigate-up) -> void
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> (send a-macro-stepper navigate-down) -> void
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> (send a-macro-stepper navigate-down/pred pred) -> void
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pred : syntax -> boolean
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Navigate down to an expansion with initial syntax satisfying pred.
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_expand.ss_
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===========
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> (require (lib "expand.ss" "macro-debugger"))
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This module provides the following procedures:
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> (expand-only syntax list-of-identifiers)
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Expands the given syntax, but only shows the expansion of macros in
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the given identifier list.
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Warning: because of limitations in syntax, expansion, and hiding, the
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resulting syntax may not evaluate to the same thing as the original
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syntax.
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> (expand/hide syntax list-of-identifier)
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Expands the given syntax, but hides the expansion of macros in the
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given identifier list (conceptually, the complement of expand-only).
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Warning: because of limitations in syntax, expansion, and hiding, the
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resulting syntax may not evaluate to the same thing as the original
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syntax.
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_stepper-text.ss_
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=================
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> (require (lib "stepper-text.ss" "macro-debugger"))
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This module provides two procedures for stepping through macro
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expansion in text-only environments.
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> (expand/step-text syntax [identifier-predicate])
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Expands the syntax and prints the macro expansion steps. If the
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identifier predicate is given, it determines which macros are shown
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(if absent, no macros are hidden). A list of identifiers is also
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accepted.
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> (stepper-text syntax [identifier-predicate])
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Returns a procedure P that:
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- when called with no arguments (or on the symbol 'next),
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prints out individual steps until macro expansion finishes
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- when called on the symbol 'all,
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prints out all of the remaining steps
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_syntax-browser.ss_
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===================
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> (require (lib "syntax-browser.ss" "macro-debugger"))
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This module provides the following procedures:
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> (browse-syntax syntax) -> void
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Creates a frame with the given syntax object shown. More information
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on using the GUI is available below.
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> (browse-syntaxes list-of-syntax) -> void
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Like browse-syntax, but shows multiple syntax objects in the same
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frame. The coloring partitions are shared between the two,
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showing the relationships between subterms in different syntax
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objects.
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> (syntax-snip syntax) -> (is-a?/c snip%)
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Like 'browse-syntax', but creates a snip that can be displayed in an
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editor. In particular, evaluating
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(syntax-snip <stx>)
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in DrScheme's interactions window will insert a syntax browser into
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the interactions window.
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Using the Macro Stepper
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=======================
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Navigation
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----------
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The stepper presents expansion as a linear sequence of rewriting
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process, and it gives the user controls to step forward or backwards
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as well as to jump to the beginning or end of the expansion process.
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If the macro stepper is showing multiple expansions, then it also
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provides "Previous term" and "Next term" buttons to go up and down in
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the list of expansions. Horizontal lines delimit the current expansion
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from the others.
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Macro hiding
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------------
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Macro hiding lets one see how expansion would look if certain macros
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were actually primitive syntactic forms. The macro stepper skips over
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the expansion of the macros you designate as opaque, but it still
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shows the expansion of their subterms.
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The bottom panel of the macro stepper controls the macro hiding
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policy. The user changes the policy by selecting an identifier in the
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syntax browser pane and then clicking one of "Hide module", "Hide
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macro", or "Show macro". The new rule appears in the policy display,
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and the user may later remove it using the "Delete" button.
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The stepper also offers coarser-grained options that can hide
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collections of modules at once. These options have lower precedence
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than the rules above.
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Macro hiding, even with no macros marked opaque, also hides certain
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other kinds of steps: internal defines are not rewritten to letrecs,
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begin forms are not spliced into module or block bodies, etc.
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Using the Syntax Browser
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========================
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Selection (bold)
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----------------
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The user can click on any part of a subterm to *select* it. To select
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a parenthesized subterm, click on either of the parentheses. The
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selected syntax is bolded. Since one syntax object may occur
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inside of multiple other syntax objects, clicking on one occurrence
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will cause all occurrences to be bolded.
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The syntax browser displays information about the selected syntax
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object in the properties panel on the right, when that panel is
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shown. The selected syntax also determines the highlighting done by
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the secondary partitioning (see below).
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Primary partition (foreground color)
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------------------------------------
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The *primary partitioning* always assigns two syntax subterms the same
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color if they have the same marks. In the absence of unhygienic
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macros, this means that subterms with the same foreground color were
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either present in the original pre-expansion syntax or generated by
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the same macro transformation step.
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Syntax colored in black always corresponds to unmarked syntax. Such
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syntax may be original, or it may be produced by the expansion of a
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nonhygienic macro.
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Secondary partitioning (highlight)
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----------------------------------
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The user may select a *secondary partitioning* from a drop-down box
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(or in the macro stepper, through the Syntax menu). This partitioning
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applies only to identifiers. When the user selects an identifier, all
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terms in the same equivalence class as the selected term are
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highlighted in yellow.
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The available secondary partitionings are:
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* bound-identifier=?
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* module-identifier=?
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* module-or-top-identifier=?
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See the documentation for these predicates in the MzScheme manual.
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* symbolic-identifer=?
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Two identifiers are symbolic-identifier=? if discarding all lexical
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context information yields the same symbol.
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* same marks
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Two identifiers have the same marks if (barring nonhygienic macros)
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they were produced by the same macro transformation step.
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* same source module
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The bindings of the two identifiers come from definitions in the
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same module.
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* same nominal module
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The bindings of the two identifiers were imported into the current
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context by requiring the same module.
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Properties
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----------
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When the properties pane is shown, it displays properties of the
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selected syntax object. The properties pane has three tabbed pages:
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- Binding
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If the selection is an identifier, shows the binding information
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associated with the syntax object.
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*Note: See the warning in the section below.
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For more information, look up 'identifier-binding',
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'identifier-transformer-binding', and
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'identifier-template-binding' in the Help Desk.
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- Source
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Displays source location information about the syntax object.
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- Properties
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Displays properties (see 'syntax-property') of the selection
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when it has properties it knows the keys for.
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Warnings about interpreting syntax
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----------------------------------
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The binding information of a *syntax object* may not be the same as
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the binding structure of the *program* it represents. The binding
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structure of a *program* is only determined after macro expansion is
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complete.
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Example: (browse-syntax #'(lambda (foo) foo))
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The syntax browser will report that the inner 'foo' is unbound, even
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though in the *program* that this syntax represents, the inner 'foo'
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is bound to the outer 'foo'.
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Notes and Limitations
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---------------------
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The syntax browser does not have a way of extending the set of
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available secondary partitions.
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The syntax browser does not have a way of extending the set of known
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properties.
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The syntax browser does not preserve the distinction between
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parentheses and square brackets.
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Notes for DrScheme language implementors
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========================================
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The macro stepper works "out of the box" only with certain languages
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out of all the languages available from the DrScheme languages
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menu. For example, the macro stepper is disabled for the teaching
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languages.
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An implementor of a new DrScheme language can designate their language
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"macro-steppable" by overriding the 'enable-macro-stepper?' method of
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their implementation of 'drscheme:language:language<%>'. The default
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implementation in the mixin provided by
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'drscheme:language:get-default-mixin' returns false; override this
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method to return true if the macro stepper button should be shown for
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this language.
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Note: There is currently no way to customize the behavior of the macro
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stepper for different languages. When enabled, the macro stepper sees
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exactly those terms that pass through the 'current-eval' handler.
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