Contracts on parameters

[David Storrs]

(define/contract (foo x)
(-> boolean? any)
'ok)

(foo #t)
'ok
(foo 7)
; foo: contract violation
; expected: boolean?
; given: 7
; in: the 1st argument of
; (-> boolean? any)
; contract from: (function foo)
; blaming: top-level
; (assuming the contract is correct)
; at: readline-input:4.18
; [,bt for context]

Yup, all good.


(define/contract foo (make-parameter #f) boolean?)
(foo #t)
(foo)
#t
(foo 7)
#f


This isn't what I expected. What am I missing?

[Ben Greenman]

1. the contract comes first, right now foo = boolean?

2. try parameter/c

[Eric Griffis]

AFK, but it looks like the contract and function on your define/contract are swapped. Maybe the contract check on the one-arg call sets the parameter and then hilarity ensues?

Eric

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[David Storrs]

Thanks, that was exactly it.

On Mon, Mar 23, 2020 at 3:51 PM Michael MacLeod
<michael...@gmail.com> wrote:
>
> I think you are looking for parameter/c. See https://docs.racket-lang.org/reference/data-structure-contracts.html?q=parameterof#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._parameter%2Fc%29%29 for more information.
>
> Best,
> Michael

>
> On Mon, Mar 23, 2020, 12:36 PM David Storrs <david....@gmail.com> wrote:
>>