[Haskell-cafe] Question on type synonym definition and language extensions

[Olivier Boudry]

On Thu, May 29, 2008 at 7:36 PM, Antoine Latter <aslatter at gmail.com> wrote:

> I've tried similar things before.  You may run into subtle problems later.
>
> Such as:
>
> > transpose :: Matrix -> Matrix
>
> won't expand into the type signature you want it to, I think.
>
> You probably want that to be equivalent to:
>
> transpose ::  forall m. forall a. forall i. forall n. (Ix i, MArray a
> n m, Num i, Num n) => m (a (i,i) n) -> m (a (i,i) n)
>
> But you'll get:
>
> transpose ::  forall m. forall a. forall i. forall n. (Ix i, MArray a
> n m, Num i, Num n) => Matrix -> m (a (i,i) n)
>
> which means that the first argument must be a polymorphic value, which
> isn't very useful.
>
>
Right, this is exactly what I'm getting when creating a transpose function:

    *Main> :t transpose
    transpose :: (MArray a Double m) => Matrix Int Double -> m (a (Int, Int)
Double)

Thanks for sharing your experience. I'll try to stay out of the subtle bugs
and keep it verbose but simple.

Regards,

Olivier.
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