Static Array with custom Index type in Haskell

Question

I am trying to build a static Array in Haskell with a custom Enum Index type and have implemented the required functions like this:

data Ind = One | Two | Three | Four deriving (Eq,Ord,Enum,Show)

instance Ix Ind where
  range (m,n)             = [m..n]
  inRange (m,n) i         = m <= i && i <= n
  index b i | inRange b i = fromEnum i
            | otherwise   = -1
  rangeSize (m,n) = (index (m,n) n ) + 1

With this I am now trying to build a matrix type:

type Row a    = Array Ind a
type Matrix a = Array Ind Row a

However, I am getting this error on the Matrix type:

 Illegal type "Array Ind Row a" in constructor application

Could anybody explain to me what this error means and what I am doing wrong?

you could also write ... deriving (Eq,Ord,Enum,Show, Ix ). — Will Ness
– Will Ness, Commented Apr 15, 2019 at 12:29

MathematicalOrchid · Accepted Answer · 2019-04-15 12:18:02Z

3

The error message is unhelpful, but you just need to add some brackets:

type Matrix a = Array Ind (Row a)

You're saying that the element type should be Row a. Without the brackets, it's trying to pass Row and a as separate type arguments — which doesn't work.

answered Apr 15, 2019 at 12:18

MathematicalOrchid

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