Kevin: one thing to keep in mind when working with contracts is that they come with a notion of a boundary -- this is critical for proper blame assignment; without being able to divide up your program into different pieces, blame assignment cannot point to a specific piece.
When you use define/contract, the boundary is drawn around that single definition, excluding the rest of the file. When you use 'contract-out' in a provide, the boundary is drawn around the entire module. Usually, but not always, the module is a more natural division, especially if you keep your modules relatively small. This is why you didn't see any contract checking at the REPL with the code from your earlier message; the REPL counts as "inside" the module. hth, Robby On Wed, Aug 20, 2014 at 2:47 PM, Kevin Forchione <[email protected]> wrote: > > On Aug 20, 2014, at 12:02 AM, Roman Klochkov <[email protected]> wrote: > >> Why does list? allow a vector in the following contract? > > Contract in `provide' is used only when you `require' your module from > other. If you want always test it, use define/contract > >> Also, how do I indicate an optional (in this case keyword) argument? > > http://docs.racket-lang.org/reference/function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%2A%29%29 > > (->* (list? exact-nonnegative-integer?) (#:splice boolean?) any) > > > Thanks! That got me started. This looks much better than what the > raise-xxxx-error approach: > > ;; subseq: list number number -> list > ;; returns the list of length len starting from pos elements of lst. > ;; > ;; Examples: > ;; > ;; > (subseq '(a b c d e f g) 3 2) > ;; '(d e) > (define/contract (subseq lst pos (len (- (length lst) pos))) > ; Contract (man ...) (opt ...) (result) > (->i ([lst list?] [pos (lst) (integer-in 0 (length lst))]) > ([len (lst pos) (integer-in 0 (- (length lst) pos))]) > (result list?)) > (take (drop lst pos) len)) > > -Kevin > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > ____________________ Racket Users list: http://lists.racket-lang.org/users
