139 lines
3.8 KiB
Racket
139 lines
3.8 KiB
Racket
#lang scribble/doc
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@(require scribble/manual
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scribble/eval
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scheme/sandbox
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"guide-utils.ss"
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"contracts-utils.ss"
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(for-label scheme/contract))
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@title[#:tag "contracts-gotchas"]{Gotchas}
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@ctc-section{Contracts and @scheme[eq?]}
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As a general rule, adding a contract to a program should
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either leave the behavior of the program unchanged, or
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should signal a contract violation. And this is almost true
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for PLT Scheme contracts, with one exception: @scheme[eq?].
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The @scheme[eq?] procedure is designed to be fast and does
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not provide much in the way of guarantees, except that if it
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returns true, it means that the two values behave
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identically in all respects. Internally, this is implemented
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as pointer equality at a low-level so it exposes information
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about how PLT Scheme is implemented (and how contracts are
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implemented).
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Contracts interact poorly with @scheme[eq?] because function
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contract checking is implemented internally as wrapper
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functions. For example, consider this module:
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@schememod[
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scheme
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(define (make-adder x)
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(if (= 1 x)
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add1
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(lambda (y) (+ x 1))))
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(provide/contract [make-adder (-> number? (-> number? number?))])
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]
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It exports the @scheme[make-adder] function that is the usual curried
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addition function, except that it returns Scheme's @scheme[add1] when
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its input is @scheme[1].
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You might expect that
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@schemeblock[
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(eq? (make-adder 1)
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(make-adder 1))
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]
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would return @scheme[#t], but it does not. If the contract were
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changed to @scheme[any/c] (or even @scheme[(-> number? any/c)]), then
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the @scheme[eq?] call would return @scheme[#t].
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Moral: do not use @scheme[eq?] on values that have contracts.
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@ctc-section{Defining recursive contracts}
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When defining a self-referential contract, it is natural to use
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@scheme[define]. For example, one might try to write a contract on
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streams like this:
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@(define e (make-base-eval))
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@(interaction-eval #:eval e (require scheme/contract))
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@interaction[
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#:eval e
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(define stream/c
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(promise/c
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(or/c
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null?
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(cons/c number? stream/c))))
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]
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@close-eval[e]
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Unfortunately, this does not work because the value of
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@scheme[stream/c] is needed before it is defined. Put another way, all
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of the combinators evaluate their arguments eagerly, even thought the
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values that they accept do not.
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Instead, use
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@schemeblock[
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(define stream/c
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(promise/c
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(or/c
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null?
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(cons/c 1
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(recursive-contract stream/c)))))
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]
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The use of @scheme[recursive-contract] delays the evaluation of the
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identifier @scheme[stream/c] until after the contract is first
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checked, long enough to ensure that @scheme[stream/c] is defined.
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See also @ctc-link["lazy-contracts"].
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@ctc-section{Using @scheme[set!] to Assign to Variables Provided via @scheme[provide/contract]}
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The contract library assumes that variables exported via
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@scheme[provide/contract] are not assigned to, but does not enforce
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it. Accordingly, if you try to @scheme[set!] those variables, you
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may be surprised. Consider the following example:
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@interaction[
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(module server scheme
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(define (inc-x!) (set! x (+ x 1)))
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(define x 0)
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(provide/contract [inc-x! (-> void?)]
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[x integer?]))
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(module client scheme
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(require 'server)
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(define (print-latest) (printf "x is ~s\n" x))
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(print-latest)
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(inc-x!)
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(print-latest))
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(require 'client)
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]
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Both calls to @scheme[print-latest] print @scheme[0], even though the
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value of @scheme[x] has been incremented (and the change is visible
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inside the module @scheme[x]).
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To work around this, export accessor functions, rather than
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exporting the variable directly, like this:
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@schememod[
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scheme
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(define (get-x) x)
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(define (inc-x!) (set! x (+ x 1)))
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(define x 0)
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(provide/contract [inc-x! (-> void?)]
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[get-x (-> integer?)])
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]
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Moral: This is a bug we hope to address in a future release.
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