On Sep 2, 2013, at 8:22 AM, Daniel Prager wrote:
> Consider this file:
>
> --------------------------------------------
> #lang racket
> (define/contract (twice x)
> (-> number? number?)
> (* 2 x))
>
> (twice 'foo)
> --------------------------------------------
>
> When run in DrRacket 5.3.6 this yields the following error:
>
> twice: contract violation
> expected: number?
> given: 'foo
> in: the 1st argument of
> (-> number? number?)
> contract from: (function twice)
> blaming: .../blame-test.rkt
> at: .../blame-test.rkt: 2.18
>
> The final line attributes blame to the twice function (line 2, col 18), but
> clearly the blame in this instance lies with the client code (line 6),
> consistent with the first six lines of the error message.
This is a misinterpretation of the last line. If you read the entire message as
a sentence, you will see that the "at: ..." means "this is where you find the
contract that was broken". The "expected:"/"given" pairing means that the
function was called, it expected a number, and the caller applied it to the
symbol 'foo instead.
If you run this program
#lang racket
(define/contract (twice x)
(-> number? number?)
'foo)
(twice 2)
the error message will say
twice: broke its contract
promised: number?
produced: 'foo
in: the range of
(-> number? number?)
contract from: (function twice)
blaming: (function twice)
at: unsaved-editor665:2.18
meaning that foo was called, it promised to return a number, but it produced
the symbol 'foo instead.
;; ---
I will admit that you're not the first one who has tripped over this error
message [format] and that you aren't the last one. But changing it isn't easy
and adding information isn't easy. The problem is that we wish to keep the
messages uniform across all functions but not all functions can benefit from
the actual contract system.
-- Matthias
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