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6.1.1
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5 Contracts for Macro Subexpressions

Ryan Culpepper <ryanc@racket-lang.org>

This library is unstable; compatibility will not be maintained. See Unstable: May Change Without Warning for more information.

 (require unstable/wrapc) package: base

This library provides a procedure wrap-expr/c for applying contracts to macro subexpressions.

procedure

(wrap-expr/c contract-expr    
  expr    
  [#:positive pos-blame    
  #:negative neg-blame    
  #:name expr-name    
  #:macro macro-name    
  #:context context]) → syntax?
  contract-expr : syntax?
  expr : syntax?
  pos-blame : 
(or/c syntax? string? module-path-index?
      'from-macro 'use-site 'unknown)
   = 'use-site
  neg-blame : 
(or/c syntax? string? module-path-index?
      'from-macro 'use-site 'unknown)
   = 'from-macro
  expr-name : (or/c identifier? symbol? string? #f) = #f
  macro-name : (or/c identifier? symbol? string? #f) = #f
  context : (or/c syntax? #f) = (current-syntax-context)
Returns a syntax object representing an expression that applies the contract represented by contract-expr to the value produced by expr.

The other arguments have the same meaning as for expr/c.

Examples:

> (define-syntax (myparameterize1 stx)
    (syntax-case stx ()
      [(_ ([p v]) body)
       (with-syntax ([cp (wrap-expr/c
                          #'parameter? #'p
                          #:name "the parameter argument"
                          #:context stx)])
         #'(parameterize ([cp v]) body))]))
> (myparameterize1 ([current-input-port
                     (open-input-string "(1 2 3)")])
    (read))

'(1 2 3)

> (myparameterize1 (['whoops 'something])
    'whatever)

the parameter argument of myparameterize1: broke its

contract

  promised: parameter?

  produced: 'whoops

  in: parameter?

  contract from: top-level

  blaming: top-level

   (assuming the contract is correct)

  at: eval:4.0

> (module mod racket
    (require (for-syntax unstable/wrapc))
    (define-syntax (app stx)
      (syntax-case stx ()
        [(app f arg)
         (with-syntax ([cf (wrap-expr/c
                            #'(-> number? number?)
                            #'f
                            #:name "the function argument"
                            #:context stx)])
           #'(cf arg))]))
    (provide app))
> (require 'mod)
> (app add1 5)

6

> (app add1 'apple)

the function argument of app: contract violation

  expected: number?

  given: 'apple

  in: the 1st argument of

      (-> number? number?)

  contract from: top-level

  blaming: (quote mod)

   (assuming the contract is correct)

  at: eval:8.0

> (app (lambda (x) 'pear) 5)

the function argument of app: broke its contract

  promised: number?

  produced: 'pear

  in: the range of

      (-> number? number?)

  contract from: top-level

  blaming: top-level

   (assuming the contract is correct)

  at: eval:9.0

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