[Haskell-cafe] Strange type error with associated type synonyms

[Bulat Ziganshin]

Hello Simon,

Wednesday, May 27, 2009, 11:42:22 PM, you wrote:

while we are here - i always had problems understanding what is
inferred and what is expected type. may be problem is just that i'm
not native speaker

are other, especially beginners, had the same problem?

> Claus made a suggestion about type error messages:

> | Apart from bracketing fully applied type synonyms, the error message
> | could be improved by providing the missing bit of information about
> | 'Memo':
> |
> |     D:\home\Haskell\tmp\desktop\types.hs:11:11:
> |         Couldn't match expected type `Memo d1'
> |                against inferred type `Memo d'
> |         (type Memo d :: * -> *)
> |         In the second argument of `(.)', namely `appl'
> |         In the expression: abst . appl
> |         In the definition of `f': f = abst . appl

> I've implemented this idea; note the "NB" line below:

> NoMatchErr.hs:20:11:
>     Couldn't match expected type `Memo d'
>            against inferred type `Memo d1'
>       NB: `Memo' is a (non-injective) type function
>     In the second argument of `(.)', namely `appl'
>     In the expression: abst . appl
>     In the definition of `f': f = abst . appl

> (Rather than give its kind, I thought it was better to focus on the
> reason for the mis-match, namely the non-injectivity.)

> Simon
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-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com