Hi > findAllPath :: (a -> Bool) -> (BTree a) -> [[a]] > findAllPath pred = g where > g (Leaf l) | pred l = [[l]] > g (Branch lf r rt) | pred r = map (r:) $ (findAllPath pred > lf) ++ (findAllPath pred rt) > g _ = [] > > without even using maybe. However, 2 questions remained: > 1 - why is the first version strict in its arguments?
Because in all call paths findAllPath will call g with its second argument. g will always evaluate (by pattern matching on) its value argument. > 2 - if it really is strict in its arguments, is there any automated > way to know when a function is strict in its arguments? Yes, strictness analysis is a very well studied subject - http://haskell.org/haskellwiki/Research_papers/Compilation#Strictness . Essentially, an argument is strict if passing _|_ for that value results in _|_. So to take your example, evaluating: findAllPath a _|_ g _|_ _|_ Since g tests what value _|_ has, we get bottom. Thanks Neil _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
