I'm trying to use foldl or foldr to run a computation over a list, in such a way that only as much of the list as may be needed is actually examined. I believe the 'and' function behaves like this (e.g. see testand below), but when I use a function that performs pairwise operations on the list elementsa, it seems that I always end up constructing the entire list, even if I don't need to examine the value of each element. I guess I could try and figure out my own 2-place folding function, but this seems as if there should be a common solution. My test code is below. The tests are evaluated as: test allSame1 or test allSame2 to use a foldl- or foldr-versions of the function respectively If I comment out the indefinitely long list case (test7) in the definition of 'test', the foldl version will pass all these test cases and foldr throws an error on test6. But if I include test7, using Hugs the foldr test case returns a C stack overflow, and the foldl case silently disappears. Is there a standard method for dealing with cases like this? (I ask out of curiosity rather than need: for my real application, I am using relatively short list with quite expensive combining operations, so I think the 'foldl' variant which doesn't actually examine list elements once the result is known, will work quite well for me.) #g -- -- Spike-allsame.hs -- -- Test use of folding functions to evaluate whether all members -- of a list are the same. -- -- What I'm really interested in is whether the list gets evaluated -- beyond what is needed to obtain the desired result. -- import Maybe ( Maybe(..), isJust ) test allSame = and [ test1 allSame , test2 allSame , test3 allSame , test4 allSame , test5 allSame , test6 allSame , test7 allSame ] test1 allSame = allSame [1,1,1,1] == True test2 allSame = allSame [1,2,3,4] == False test3 allSame = allSame [1,2,undefined,4] == False test4 allSame = allSame [1] == True test5 allSame = allSame [] == True test6 allSame = allSame [1,2,undefined] == False test7 allSame = allSame (1:2:repeat 3) == False testand = and (True:True:False:repeat True) longList :: [Int] longList = (1:2:repeat 3) allSame1 :: (Eq a) => [a] -> Bool allSame1 [] = True allSame1 [_] = True allSame1 (a:as) = isJust $ foldr nextSame1 (Just a) as nextSame1 :: (Eq a) => a -> Maybe a -> Maybe a nextSame1 _ Nothing = Nothing nextSame1 a1 (Just a) | a1 == a = Just a | otherwise = Nothing {- foldr f z [] = z foldr f z (x:xs) = f x (foldr f z xs) foldr nextSame (Just 1) [2,undefined,4] ..> nextSame 2 (foldr nextSame (Just 1) [undefined,4]) ..> nextSame 2 (nextSame undefined (foldr nextSame (Just 1) [4])) ..> nextSame 2 (nextSame undefined (nextSame 4 (foldr nextSame (Just 1) []))) ..> nextSame 2 (nextSame undefined (nextSame 4 ((Just 1)))) -} allSame2 :: (Eq a) => [a] -> Bool allSame2 [] = True allSame2 [_] = True allSame2 (a:as) = isJust $ foldl nextSame2 (Just a) as nextSame2 :: (Eq a) => Maybe a -> a -> Maybe a nextSame2 Nothing _ = Nothing nextSame2 (Just a) a1 | a1 == a = Just a | otherwise = Nothing {- foldl f z [] = z foldl f z (x:xs) = foldl f (f z x) xs foldl nextSame (Just 1) [2,undefined,4] ..> foldl nextSame (nextSame (Just 1) 2) [undefined,4] ..> foldl nextSame (nextSame (nextSame (Just 1) 2) undefined) [4] ..> foldl nextSame (nextSame (nextSame (nextSame (Just 1) 2) undefined) 4) [] ..> (nextSame (nextSame (nextSame (Just 1) 2) undefined) 4) -} ------------ Graham Klyne For email: http://www.ninebynine.org/#Contact