[racket] contracts ->i optional keyword syntax

On Aug 27, 2014, at 11:12 AM, Matthias Felleisen <matthias at ccs.neu.edu> wrote:

> 
> On Aug 27, 2014, at 12:22 PM, Kevin Forchione <lysseus at gmail.com> wrote:
> 
>> 
>> #lang racket
>> 
>> (provide (contract-out [foo 
>>                        (-> string? list?)])
>>         (rename-out (foo bar)))
>> 
>> (define (foo n) (list n))
>> 
>> (module+ test
>>  (require (submod ".."))
>>  (foo 3)
>>  (bar "3”))
>> 
>> Here foo violates the contract.  Changing the argument to foo to “3” and  the argument to bar to 3 shows that bar does not violate the contract. So the contract is bound to foo and not to the rename. Quite interesting!
>> 
> 
> 
> contract-out attaches a contract to foo so when you can call foo on 3, you get an error. 
> 
> provide also exports foo as bar w/o contract, so you can call it on anything you want. 
> 
> This is clearly what the words of the program imply, and the semantics of the language gives it to you. No problems!

Yes, I agree. Very useful! Is there a way to attach a contract to a renamed identifier? I tried wrapping contract around a rename form and it didn’t appear to like it.

-Kevin

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