[racket] contracts ->i optional keyword syntax

From: Matthias Felleisen (matthias at ccs.neu.edu)
Date: Wed Aug 27 14:12:25 EDT 2014

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! 

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