▼Unstable: May Change Without Warning
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struct-type-property/ c
Version: 5.1.2
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6 Contracts for struct type properties

Ryan Culpepper <ryanc@racket-lang.org>

 (require unstable/prop-contract)

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

(struct-type-property/c value-contract) → contract?
  value-contract : contract?
Produces a contract for struct type properties. When the contract is applied to a struct type property, it produces a wrapped struct type property that applies value-contract to the value associated with the property when used to create a new struct type (via struct, make-struct-type, etc).

The struct type property’s accessor function is not affected; it must be protected separately.

Examples:

> (module propmod racket
    (require racket/contract
             unstable/prop-contract)
    (define-values (prop prop? prop-ref)
      (make-struct-type-property 'prop))
    (define (prop-app x v)
      (((prop-ref x) x) v))
    (provide/contract
     [prop? (-> any/c boolean?)]
     [prop (struct-type-property/c
            (-> prop? (-> number? boolean?)))]
     [prop-app (-> prop? number? boolean?)])
    (provide prop-ref))
> (module structmod racket
    (require 'propmod)
    (struct s (f) #:property prop (lambda (s) (s-f s)))
    (provide (struct-out s)))
> (require 'propmod 'structmod)
> (define s1 (s even?))
> (prop-app s1 5)

#f

> (prop-app s1 'apple)

prop-app: contract violation, expected <number?>, given:

'apple

  contract from propmod, blaming top-level

  contract: (-> prop? number? boolean?)

        at: eval:2.0

> (define s2 (s "not a fun"))
> (prop-app s2 5)

prop: contract violation, expected a procedure that accepts

1 mandatory argument without any keywords, given: "not a

fun"

  contract from propmod, blaming structmod

  contract:

    (struct-type-property/c

    (-> prop? (-> number? boolean?)))

  at: eval:2.0

> (define s3 (s list))
> (prop-app s3 5)

prop: contract violation, expected <boolean?>, given: '(5)

  contract from propmod, blaming structmod

  contract:

    (struct-type-property/c

    (-> prop? (-> number? boolean?)))

  at: eval:2.0

> ((prop-ref s3) 'apple)

prop: self-contract violation, expected <prop?>, given:

'apple

  contract from propmod, blaming propmod

  contract:

    (struct-type-property/c

    (-> prop? (-> number? boolean?)))

  at: eval:2.0

The first contract error above is a simple function contract violation on prop-app. The second and third contract errors above blame the structmod module, because it accepted the struct type property contract. To avoid blame, structmod should have placed a contract on s. The final contract error, involving s3, blames propmod because the struct type property contract obliges propmod to make sure the property’s value is not misused, but propmod allows direct access to the property value via prop-ref. To avoid blame, propmod should remove the export of prop-ref or protect it with a contract.

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