Le dimanche 7 mai 2017 07:27:08 UTC+2, Daniel Prager a écrit :
> 1. Default argument goes missing from post-condition, leading to an
> unexpected error ...

You should use unsupplied-arg? . But I couldn't find a way to get the default 
value from the contract. I would guess that the problem is that the contract 
can be specified separately from the function, and the function itself knows 
the default argument's value. In the general case, extracting the default 
argument's value (without running the default expression multiple times) might 
be tricky.

> 2. contract-exercise seems to miss explicit #pre-conditions ...

This does look like a real (pun intended) problem.

> 3. post-conditions appear to be called twice ...

With the code below, the post-condition only gets called once:

#lang racket

(define/contract (real-sqrt x)
  (->i ([x real?])
       #:pre (x)
       (>= x 0)
       [result any/c]
       #:post (result)
       (begin (displayln result) (>= result 0)))
  (sqrt x))

(real-sqrt 1)

With this code, however, the contract does get called twice:

#lang racket

(define/contract (real-sqrt x)
  (->i ([x real?])
       #:pre (x)
       (>= x 0)
       [result (λ (result) (displayln result) (>= result 0))]
       #:post (result) #t)
  (sqrt x))

(real-sqrt 1)

See this very interesting thread on the mailing list for a detailed discussion 
about this 

Short story: if one of the arguments is a function, and a dependent #:pre, 
#:post, argument or result contract calls that function, you want it to be 
protected. The contract therefore needs to be applied to the argument before it 
is passed to the other dependent clause, and is applied again when actually 
calling the function.

> Finally, some timings.

Thanks for taking the time to test this out :) .

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