Adrian Hey
height :: AVL e -> Int height = addHeight 0 where addHeight h E = h addHeight h (N l _ _) = addHeight h+2 l addHeight h (Z l _ _) = addHeight h+1 l addHeight h (P _ _ r) = addHeight h+2 r
It seems pretty obvious to me that addHeight is strict in its first argument if + is strict for Ints (as I guess it is).
No, this looks very obviously lazy to me. The first argument is always a pattern variable, h, and therefore no evaluation is required on it - the value is just bound lazily and used on the RHS.
height :: AVL e -> Int height = addHeight 0 where addHeight h E = h addHeight h (N l _ _) = let h' = h+2 in h' `seq` addHeight h' l addHeight h (Z l _ _) = let h' = h+1 in h' `seq` addHeight h' l addHeight h (P _ _ r) = let h' = h+2 in h' `seq` addHeight h' r
.. it still gives "DmdType LS".
Even though many uses of 'h' are strictified, the first clause addHeight h E = h is still lazy, because it simply binds the variable without forcing it.
height :: AVL e -> Int height = addHeight 0 where addHeight h E = h addHeight h (N l _ _) = h `seq` addHeight (h+2) l addHeight h (Z l _ _) = h `seq` addHeight (h+1) l addHeight h (P _ _ r) = h `seq` addHeight (h+2) r
Same again. Try addHeight h E = h `seq` h which, although it looks bizarre, actually forces the evaluation of h, whilst simply returning it does not. Regards, Malcolm