Returning a list of every possible coordinate in a grid of that width and height

You could do that using a list comprehension.

[ (x,y) | x <- [0..1], y <- [0..2] ]

Will give the list in your example.

Your function would then need to be defined as:

type GridCoord = (Int, Int)

allCoords :: Int -> Int -> [GridCoord]
allCoords height width = [ (x,y) | x <- [0..width-1], y <- [0..height-1] ]

The range function does that.

import Data.Ix
allCoords h w = range ((0,0), (w,h))