ac26fe7554
Also, updates some of the mzlib files to point at `racket/*' libraries rather than to `scheme/*' ones.
43 lines
1.5 KiB
Racket
43 lines
1.5 KiB
Racket
#lang scribble/doc
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@(require scribble/manual
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scribble/struct
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scribble/decode
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scribble/eval
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"parse-common.rkt"
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(for-label racket/class))
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@title[#:tag "exprc"]{Experimental: Contracts on macro sub-expressions}
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@emph{This section involves facilities that are experimental and
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subject to change.}
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Just as procedures often expect certain kinds of values as arguments,
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macros often have expectations about the expressions they are
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given. And just as procedures express those expectations via
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contracts, so can macros, using the @racket[expr/c] syntax class.
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For example, here is a macro @racket[myparameterize] that behaves like
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@racket[parameterize] but enforces the @racket[parameter?] contract on
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the parameter expressions.
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@myinteraction[
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(define-syntax (myparameterize stx)
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(syntax-parse stx
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[(_ ((p v:expr) ...) body:expr)
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#:declare p (expr/c #'parameter?
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#:name "parameter argument")
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#'(parameterize ((p.c v) ...) body)]))
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(myparameterize ((current-input-port
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(open-input-string "(1 2 3)")))
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(read))
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(myparameterize (('whoops 'something))
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'whatever)
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]
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@bold{Important:} Make sure when using @racket[expr/c] to use the
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@racket[c] attribute. If the macro above had used @racket[p] in the
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template, the expansion would have used the raw, unchecked
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expressions. The @racket[expr/c] syntax class does not change how
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pattern variables are bound; it only computes an attribute that
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represents the checked expression.
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