Re: [Haskell-cafe] What is a rigid type variable?

23 Jun 2008

      On Mon, Jun 23, 2008 at 5:58 AM, Luke Palmer  wrote:
...
On Mon, Jun 23, 2008 at 3:26 AM, Xiao-Yong Jin  wrote:
...
Hi all,
I'm writing a short function as follows, but I'm not able to
find a suitable type signature for `go'.  It uses
Numeric.LinearAlgebra from hmatrix.
-- | Map each element in a vector to vectors and thus form a matrix
-- | row by row
mapVecToMat :: (Element a, Element b) =>
              (a -> Vector b) -> Vector a -> Matrix b
mapVecToMat f v = fromRows $ go (d - 1) []
   where
     d = dim v
     go :: Element b => Int -> [Vector b] -> [Vector b]
     go 0 vs = f (v @> 0) : vs
     go !j !vs = go (j - 1) (f (v @> j) : vs)
If you want to give a type signature for 'go', you need a GHC
extension called ScopeTypeVariables (IIRC).
I was indeed correct on the name of this extension, but it would be no
help to you to know this since I made a typo :-)

The extension is called ScopedTypeVaraibles.

You probably already know that this can be enabled with:

{-# LANGUAGE ScopedTypeVariables #-}

Luke