[Haskell-cafe] overloaded overloading?

[Brad Larsen]

Alberto,

On Tue, Jan 12, 2010 at 7:17 PM, Alberto G. Corona <agocorona at gmail.com> wrote:
> Hi,
>
> I sometimes strumble on the same quiestion that forces me to insert
> functions that process objects of a certain class inside their class
> definition.  This occurs when a computation uses the object internally,
> neiter as parameter or as a return value or in the case of existential
> types. An example of the first:
>
>
> class Example a where
>     irec ::  IO a
>     pr :: a →  IO String
>     sample2 ::  a  →   IO ()
>     sample2 _  =   do
>       x ←  irec :: IO a
>       pr x
>       return ()
>
> sample :: Example a ⇒ a  →   IO ()
> sample _  =   do
>   x ←  irec :: IO a
>   pr x
>   return ()
>
>
> With the flag -fglasgow-exts, the following error below appears in sample.
> without the flag, the error appears in both sample and sample2. I´m too lazy
> to find what concrete extension is involved and why, anyhow, in the case
> of sample, the compiler must generate a new type a1 with no context.
>
>     Could not deduce (Example a1) from the context ()
>       arising from a use of `irec' at Control\Workflow\Users.hs:73:7-10
>     Possible fix:
>       add (Example a1) to the context of an expression type signature
>     In a stmt of a 'do' expression: x <- irec :: IO a
>     In the expression:
>         do x <- irec :: IO a
>            pr x
>            return ()
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

In the code for `sample', you give a type signature to x, `IO a'.
However, the `a' there is different from the `a' in the signature for
`sample'.

Perhaps ScopedTypeVariables
<http://www.haskell.org/haskellwiki/ScopedTypeVariables> will help?

Sincerely,
Brad