[racket-dev] [plt] Push #29023: master branch updated

I think the vast majority of contract errors that Racket programmers
see will be from contracts that the particular programmer didn't
write. For example: standard library contracts, or contracts from
packages they install, or contracts generated by Typed Racket, or
other such.

For example, here's a simple contract error:

-> (require scribble/core)
-> (part-parts 7)
; part-parts: contract violation
;   expected: part?
;   given: 7
;   in: the 1st argument of
;       (-> part? (listof part?))
;   contract from:
;       <pkgs>/scribble-lib/scribble/core.rkt
;   blaming: top-level
;    (assuming the contract is correct)
;   at: <pkgs>/scribble-lib/scribble/core.rkt:164.2

What does the "assuming the contract is correct" mean to the
programmer here? They can't change the contract, and it's not obvious
in what sense the contract being incorrect would change anything.

In general, I think that your new message makes a lot of sense if
programmers mostly experience contracts as strong invariants
protecting complex components specified publicly -- the sort of thing
you've talked about as a "marketplace of components". But I don't
think that's how Racket programmers typically experience contracts --
instead, they're more like the example above: simple specifications
protecting small functions implemented privately and used as
error-checking.

Sam

On Mon, Jul 14, 2014 at 9:30 AM, Robby Findler
<robby at eecs.northwestern.edu> wrote:
> I do not buy this argument: the user didn't write the compiler but they
> wrote the contract.
>
> Robby
>
>
> On Monday, July 14, 2014, Sam Tobin-Hochstadt <samth at cs.indiana.edu> wrote:
>>
>> This seems like a situation where the new error message is potentially
>> more confusing, even though it's technically more correct. There are
>> lots of other caveats we could add ("assuming there isn't a compiler
>> bug", etc) but I think adding them would make Racket harder to use.
>>
>> Sam
>>
>> On Mon, Jul 14, 2014 at 9:11 AM,  <robby at racket-lang.org> wrote:
>> > robby has updated `master' from 737330deb6 to 1dda800ca2.
>> >   http://git.racket-lang.org/plt/737330deb6..1dda800ca2
>> >
>> > =====[ One Commit ]=====================================================
>> > Directory summary:
>> >  100.0% racket/collects/racket/contract/private/
>> >
>> > ~~~~~~~~~~
>> >
>> > 1dda800 Robby Findler <robby at racket-lang.org> 2014-07-14 08:09
>> > :
>> > | add contract-correct caveat to contract violation error messages
>> > :
>> >   M racket/collects/racket/contract/private/blame.rkt | 1 +
>> >
>> > =====[ Overall Diff ]===================================================
>> >
>> > racket/collects/racket/contract/private/blame.rkt
>> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> > --- OLD/racket/collects/racket/contract/private/blame.rkt
>> > +++ NEW/racket/collects/racket/contract/private/blame.rkt
>> > @@ -320,6 +320,7 @@
>> >     from-line
>> >     on-line
>> >     blaming-line
>> > +   "   (assuming the contract is correct)"
>> >     at-line))
>> >
>> >  ;; combine-lines : (->* #:rest (listof (or/c string? #f))) string?)