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

[Antoine Latter]

2008/5/29 Olivier Boudry <olivier.boudry at gmail.com>:
> After some "read, guess, try, error" cycles I came up with this:
>
>     type Matrix = forall m. forall a. forall i. forall n. (Ix i, MArray a n
> m, Num i, Num n) => m (a (i,i) n)

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.

-Antoine





-Antoine