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
https://groups.google.com/forum/#!searchin/racket-users/contract$20twice%7Csort:relevance/racket-users/SUzcozdPh6Q/bjgEjTyQEAAJ
.
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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