On Fri, Apr 17, 2020 at 9:45 AM Ben Greenman <[email protected]> wrote:
> Hi Marc, > > > >> For contracts, though, (provide (contract-out ...)) gives better error > >> messages than define/contract. > >> > > > > In what sense is this the case, and where can I read more about the > > differences, as well as how to improve errors of contracts? > > contract-out gives better error messages than define/contract in the > sense that it has better blame > > A define/contract needs to immediately decide who could be blamed for > future errors --- because the new definition can be used right away. > > A contract-out can wait until another module requires the definition > --- and tailor its blame errors to different clients. > My understanding is that contract-out only provides protection against inappropriate calls from clients *outside* the module, whereas define/contract enforces the contract against everyone, including things inside the module. Do I have that right? If so, I feel much more comfortable using the latter even if its error messages are not quite as good. On a related topic, I don't understand the concept of positive and negative blame. Can someone fill me in? > Here's an example to play with. Submod A provides two functions: f0 is > made with define/contract and f1 with contract-out. > > ``` > #lang racket > > (module A racket > (define/contract (f0 x) > (-> natural? natural?) > x) > > (define (f1 x) > x) > > (provide f0) > (provide (contract-out [f1 (-> natural? natural?)]))) > > (module B racket > (require (submod ".." A)) > (f0 'hello) > #;(f1 'hello)) > > (require 'B) > ``` > > If B makes a mistake with f0, the error blames submod A. > But if B makes a mistake with f1, the error blames B. > > > The contract library makes these blame errors internally. I don't > think there's any way to customize short of using `contract` directly. > > > Is it related to this part of the documentation of `contract-out` > > ( > https://docs.racket-lang.org/reference/attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29 > ) > > - which I admittedly don't understand: > > > > "The implementation of contract-out > > < > https://docs.racket-lang.org/reference/attaching-contracts-to-values.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._contract-out%29%29 > > > > uses syntax-property > > < > https://docs.racket-lang.org/reference/stxprops.html#%28def._%28%28quote._~23~25kernel%29._syntax-property%29%29 > > > > to attach properties to the code it generates that records the syntax of > > the contracts in the fully expanded program. Specifically, the symbol ' > > provide/contract-original-contract is bound to vectors of two elements, > the > > exported identifier and a syntax object for the expression that produces > > the contract controlling the export." > > I was only thinking of the "blaming: ...." part of error messages. > > Both define/contract and contract-out can print the whole contract; I > don't think this syntax-property gives contract-out any advantage > > (Sadly, the whole contract is sometimes too big to help me find a problem.) > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAFUu9R5BMG5ZFVrddrJ-6uceV%2B3936ZudE-8ESObycw9B%2BRjcg%40mail.gmail.com > . > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CAE8gKof2M%2BHuCf%2B2hfTKbVxCxGEb1Vtx%2BaXehYNZXhttpgMNPA%40mail.gmail.com.
