Hi,

thank you for the tip! It helps but it's not quite there yet. If you see the program in attachment, I can make it compile only by commenting the type declaration for the second function, otherwise it's the exact same error message as before.

In -Wall the compiler does suggest me the signature, but it's not enlightening for me at this point, I've yet to dig that deep in haskell's type system:
parseConfigMap :: forall t (m :: * -> *). Monad m => t -> m ()

Can you explain me why my type signature is not correct?

Thank you again!

Emmanuel

On Sun, Mar 17, 2013 at 1:23 PM, Andres Löh <andres@well-typed.com> wrote:

Hi there.

> -- | A JSON \"object\" (key\/value map).
> type Object = Map Text Value

See https://github.com/bos/aeson/blob/master/release-notes.markdown

Quoting:

"
0.3 to 0.4

[...]

We switched the intermediate representation of JSON objects from
Data.Map to Data.HashMap, which has improved type conversion
performance.

"

> So my clearly flawed plan is to get the value, pattern mach it against
> (Object hash) and then work on the hash. However for the program in
> attachment, which I would expect to compile, I get this compile error:
>
> question.hs:12:62:
> Couldn't match expected type `Map.Map T.Text Value'
> with actual type `Object'
> In the first argument of `parseConfigMap', namely `map'
> In the second argument of `($)', namely `parseConfigMap map'
> In the expression: return $ parseConfigMap map

This has nothing to do with "type" vs. "data". The type synonym
expansion of Object doesn't match your type, because it uses a HashMap
rather than a Map.

Cheers,
Andres

--
Andres Löh, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com

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