It is worth noting that it is relatively easy to implement a
define/provide/contract syntax without any problems. In my two most
active projects I use it extensively. In one case even with
scribble/srcdoc to keep the contracts, scribblings and implementation in
one place.
The simplest version:
(define-syntax (define/provide/contract stx)
(syntax-case stx ()
((_ (name args ...)
contract
body ...)
#'(begin
(provide (contract-out (name contract)))
(define (name args ...)
body ...)))))
Of course, this is not the best approach for all situations (and I do
not use it everywhere), but it covers many common use-cases well.
Dominik
On 07. 04. 21 22:34, Robby Findler wrote:
> The short answer: you probably should use (provide (contract-out....))
> instead of define/contract.
>
> The slightly longer answer: when you write a contract, you are not just
> describing what the legal inputs and outputs are, you are also
> establishing a *boundary* between two regions of code. In the case of
> define/contract, you are establishing a boundary between the function
> (here: f) and the module that it contains (here the file "a.rkt"). In
> the case of (provide (contract-out ...)) the boundary is between a.rkt
> and b.rkt.
>
> The slightly uncomfortable answer: it is my (not completely solid yet)
> opinion that the boundary that is drawn for define/contract is perhaps
> not the right / best one. In a theoretical/mathematical sense it is a
> completely fine and completely defensible one. But it trips people up
> sometimes. (There are examples others have posted in the past that are
> perhaps even stranger than yours but boil down to the same specific
> design choice for define/contract.)
>
> Robby
>
>
>
> On Wed, Apr 7, 2021 at 2:10 PM epi via Racket Users
> <racket-users@googlegroups.com <mailto:racket-users@googlegroups.com>>
> wrote:
>
> Hello Racket users,
>
> I am trying to understand a contract violation message that I am
> getting.
> Here is the file a.rkt:
>
> #lang racket
> (provide f)
> (define/contract (f a)
> (-> boolean? any/c)
> '())
>
> and this is b.rkt:
>
> #lang racket
> (require "a.rkt")
> (f 3)
>
>
> I would expect that the caller is blamed for the contract violation,
> but the error message that I get is as follows:
>
>
> f: contract violation
> expected: boolean?
> given: 3
> in: the 1st argument of
> (-> boolean? any/c)
> contract from: (function f)
> blaming: /home/epi/snippets/a.rkt
> (assuming the contract is correct)
> at: /home/epi/snippets/a.rkt:3.18
> context...:
>
> /usr/share/racket/collects/racket/contract/private/blame.rkt:347:0:
> raise-blame-error
>
>
> /usr/share/racket/collects/racket/contract/private/arrow-higher-order.rkt:379:33
> body of "/home/dan/snippets/blameme.rkt"
>
> So, I am a bit surprised that the error message blames the file a.rkt.
> What am I missing here?
>
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