| The library report defines | | -- Diagonal of a square matrix | diag :: (Ix a) => Array (a,a) b -> Array a b | diag x = ixmap (l,u) (\i->(i,i)) x | where ((l,l'),(u,u')) | l == l' && u == u' = bounds x And that is indeed a stupid definition. It's like saying x = if x>2 then 1 else 10 (the guard is really just an 'if'.) So of course l, u etc are all bottom. The guard needs to be on diag itself diag x | l==l' && u==u' = ixmap ...as before where ((l,l'),(u,u')) = bounds x thanks for pointing this out | I am also curious why, for example, | | row :: (Ix a, Ix b) => a -> Array (a,b) c -> Array b c | row i x = ixmap (l',u') (\j->(i,j)) x where ((l,l'),(u,u')) | = bounds x | | isn't written as | | row :: (Ix a, Ix b) => a -> Array (a,b) c -> Array b c | row i x = ixmap (l,u) (\j->(i,j)) x where ((_,l),(_,u)) = bounds x | ~~~ ~~~ ~~~ you can write it either way S