> 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))
'+
'-)
```
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