large hash/dc errors (was: Contracts for (partially) specifying dictionary key -> value-predicates)

[Ben Greenman]

> Ben: if you have an example of the bad hash/dc error message I'd be
> interested to see it.
>
> Robby

Here's the kind of message that turned me off:

```
ctc.rkt:34:0: broke its own contract
promised: (hash/dc (k (or/c (quote a) (quote b) (quote c))) (v (k)
(config-value/c k)) #:immutable #t #:kind (quote flat))
produced: '#hash((a . 1) (b . #t) (c . #<void>))
in: (hash/dc
(k (or/c 'a 'b 'c))
(v (k) (config-value/c k))
#:immutable
#t
#:kind
'flat)
contract from: +
blaming: +
(assuming the contract is correct)
```

It prints the whole hash and contract, leaving me to figure out the
two key parts: (1) what piece of the hash failed, and (2) what the
contract expected for that piece.

Now that I think of it ->* is bad in the same way --- especially for
`plot` functions.


Here's the code that made that error message. The #;(lambda ....)
comment is the contract that I ended up using instead of hash/dc (it
still doesn't explain bad values well).

```
#lang racket/base

(require racket/contract)

(provide config/c)

(define config-key/c
(or/c 'a 'b 'c))

(define (config-value/c k)
(case k
((a)
string?)
((b)
boolean?)
((c)
void?)))

(define config/c
#;(lambda (h)
(for ((k (in-list (list 'a 'b 'c))))
(unless (hash-has-key? h k)
(raise-arguments-error 'config/c "missing key" "key" k "hash" h))
(define v (hash-ref h k))
(unless ((config-value/c k) v)
(raise-arguments-error 'config/c "bad value for key" "key" k
"value" v "hash" h))
(void)))
(hash/dc
[k config-key/c]
[v (k) (config-value/c k)]
#:immutable #true
#:kind 'flat))

(contract
config/c
(hash 'a 1 'b #true 'c (void))
'+
'-)
```

[jackh...@gmail.com]

I have the same issue for ->i contracts. I'll write multiple #:pre/desc checks with nice messages, which are promptly rendered useless by the fact that ->i prints out the whole contract instead of just the precondition check that failed.

[Robby Findler]

I'm still thinking about Ben's example but I'm seeing the description right at the top in the examples below. What am I missing?

Robby

#lang racket

(module m racket
  (provide
   (contract-out
    [f (->i ([x integer?])
            #:pre/desc (x)
            (or (> x 5)
                "must be larger than 5")
            #:pre/desc (x)
            (or (even? x)
                "must be even")
            any)]))
  (define (f x) x))

(require 'm)

(display
 (exn-message
  (with-handlers ([values values])
    (f 1))))

(newline)
(newline)

(display
 (exn-message
  (with-handlers ([values values])
    (f 7))))

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[jackh...@gmail.com]

The message shows up at the top, but the message also includes the whole contract with all the other messages. Error messages that include too much information are frustrating, because I have to scan through the message to look for the important bit. The extra information has a cost. For instance, this is what my REPL looked like after running Robby's program:



Since the message is too big to fit in the REPL panel, I have to scroll around to find the "f: contract violation; must be even" and "f: contract violation; must be larger than five" parts of the message. And the message doesn't say what the actual value of x was!

I think when a precondition of an ->i contract is violated, the message should just include the precondition description and the values of the parameters the precondition depends on. The whole contract shouldn't be printed - I'd prefer it either be omitted entirely or shortened with ellipses. So I think that program should produce output like this:

f: contract violation;

 must be larger than 5

  x: 1
  in: a precondition of (->i ...)
  contract from: (anonymous-module m)
  blaming: anonymous-module

   (assuming the contract is correct)

  at: unsaved-editor:6.5

f: contract violation;
 must be even
  x: 7
  in: a precondition of (-> i ...)
  contract from: (anonymous-module m)
  blaming: anonymous-module

   (assuming the contract is correct)

  at: unsaved-editor:6.5