[Haskell-cafe] Functional dependence nightmare

[Edgar Gomes Araujo]

Hi Stephen,
I've have done the following:

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE RankNTypes #-}
...
mbc :: forall a . (SubUnit a)=>[Point] -> SetActiveSubUnits a -> Box ->
StateMBC a [Unit a]
mbc p afl box = do
    cleanAFLs
    if (null afl)
        then do
            (unit, afl') <- case build1stUnit plane p1 p2 p of
                Just un  -> return (([un], fromList $ getAllSubUnits
un)::(SubUnit a)=>([Unit a], SetActiveSubUnits a))
                _        -> return ([]   , empty)
            analyze1stUnit unit afl'
.....


I hope that is right. Does it?

Edgar





On 26 March 2011 21:19, Stephen Tetley <stephen.tetley at gmail.com> wrote:

> Hi Edgar
>
> What did you try?
>
> My intuition is that this specific bit (there may be other problems)
> is because the type checker is introducing a new type variable. Thus
> you don't actually want the type operator (~) to say the new type
> variable is equal to the type variable in the function signature, you
> want to use scoped type variables so that the local type annotation is
> *the same type* type variable.
>
> Best wishes
>
> Stephen
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20110326/3c0ce826/attachment.htm>