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

[Luke Palmer]

On Mon, Jun 23, 2008 at 5:58 AM, Luke Palmer <lrpalmer at gmail.com> wrote:
> On Mon, Jun 23, 2008 at 3:26 AM, Xiao-Yong Jin <xj2106 at columbia.edu> 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