8.5 Structure Type Property Contracts

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

As an example, consider the following module. It creates a structure type property, prop, whose value should be a function mapping a structure instance to a numeric predicate. The module also exports app-prop, which extracts the predicate from a structure instance and applies it to a given value.

> (module propmod racket     (require racket/contract)     (define-values (prop prop? prop-ref)       (make-struct-type-property 'prop))     (define (app-prop x v)       (((prop-ref x) x) v))     (provide/contract      [prop? (-> any/c boolean?)]      [prop (struct-type-property/c             (-> prop? (-> integer? boolean?)))]      [app-prop (-> prop? integer? boolean?)])     (provide prop-ref))

The structmod module creates a structure type named s with a single field; the value of prop is a function that extracts the field value from an instance. Thus the field ought to be an integer predicate, but notice that structmod places no contract on s enforcing that constraint.

> (module structmod racket     (require 'propmod)     (struct s (f) #:property prop (lambda (s) (s-f s)))     (provide (struct-out s)))

> (require 'propmod 'structmod)

First we create an s instance with an integer predicate, so the constraint on prop is in fact satisfied. The first call to app-prop is correct; the second simply violates the contract of app-prop.

> (define s1 (s even?))

> (app-prop s1 5)

#f

> (app-prop s1 'apple)

app-prop: contract violation

expected: integer?

given: 'apple

in: the 2nd argument of

(-> prop? integer? boolean?)

contract from: propmod

blaming: top-level

at: eval:2.0

We are able to create s instances with values other than integer predicates, but applying app-prop on them blames structmod, because the function associated with prop—that is, (lambda (s) (s-f s))—does not always produce a value satisfying (-> integer? boolean?).

> (define s2 (s "not a fun"))

> (app-prop s2 5)

prop: contract violation

expected: a procedure

given: "not a fun"

in: the range of

...

(struct-type-property/c

(-> prop? (-> integer? boolean?)))

contract from: propmod

blaming: structmod

at: eval:2.0

> (define s3 (s list))

> (app-prop s3 5)

prop: contract violation

expected: boolean?

given: '(5)

in: the range of

the range of

...

(struct-type-property/c

(-> prop? (-> integer? boolean?)))

contract from: propmod

blaming: structmod

at: eval:2.0

The fix would be to propagate the obligation inherited from prop to s:

(provide (contract-out

[struct s ([f (-> integer? boolean?)])]))

Finally, if we directly apply the property accessor, prop-ref, and then misuse the resulting function, the propmod module is blamed:

> ((prop-ref s3) 'apple)

prop: broke its contract

promised: prop?

produced: 'apple

in: the 1st argument of

...

(struct-type-property/c

(-> prop? (-> integer? boolean?)))

contract from: propmod

blaming: propmod

at: eval:2.0

The propmod module has an obligation to ensure a function associated with prop is applied only to values satisfying prop?. By directly providing prop-ref, it enables that constraint to be violated (and thus it is blamed), even though the bad application actually occurs elsewhere.

Generally there is no need to provide a structure type property accessor at all; it is typically only used by other functions within the module. But if it must be provided, it should be protected thus:

(provide (contract-out

[prop-ref (-> prop? (-> prop? (-> integer? boolean?)))]))