Thanks Matthias. Why does the first case which doesn't cross a module boundary cause a contract violation.
#lang racket (provide/contract (amount positive?)) (define amount -7) On Fri, Nov 16, 2012 at 8:24 PM, Matthias Felleisen <matth...@ccs.neu.edu>wrote: > > Contracts really are like business contracts. They are between two > distinct parties; they are not by a party on itself. > > On occasion, you really do want to wish to break the contract inside a > module. This is well-known in the OO world, and it is often called the > re-entrance problem. Say you want a balanced tree object. Imagine it has a > method with a contract that requires 'balanced'ness but is also called > internally. Perhaps you don't need/wish to balance for internal calls. So > you do -- and you don't get caught. > > Contracts impose a run-time cost. To make the cost reasonable, we trust > the programmer and we let the programmer know that we trust him. > > > > > > > > > > On Nov 16, 2012, at 7:52 PM, Galler wrote: > > >> > >> Even if you use define/contract, you may violate the contract on > recursive > > calls. This is intentional. > >> > > > > Thanks. > > > > I hadn't run into that behavior before and wasn't aware. > > > > Could you possibly elaborate on the thinking wrt this design choice? > > > > (define/contract (my-natural-number n) > > (-> exact-positive-integer? any/c) > > (print n) (newline) > > (my-natural-number (sub1 n))) > > > > (my-natural-number 1) > > 1 > > 0 > > -1 > > -2 > > -3 > > -4 > > > > > > > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users > >
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