[Haskell-cafe] From Rabhi's & Lapalme's book 'Algorithms - A Functional Programming Approach' I'm getting a non-exhaustive pattern match

From Rabhi's & Lapalme's book 'Algorithms - A Functional Programming Approach' 1999, I'm getting a non-exhaustive pattern match

weight x y g = w where (Just w) = g!(x,y) I can't figure out why