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 <[email protected]> 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
>
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