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

[Casey Hawthorne]

>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
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