@ Miguel: Thanks for carrying out the fixpoint computation I was too lazy to do! I see: lazy evaluation programmers must not be lazy ;-) @ Josef: Oh yes, I mixed up x and y! In fact, I was confused about the semantical difference between eqrev and eqrev', although eqrev is just an iterative version of eqrev' -- if x and y were not mixed up. Peter Peter Padawitz wrote:
Can anybody give me a simple explanation why the second definition of a palindrome checker does not terminate, although the first one does?
pal :: Eq a => [a] -> Bool pal s = b where (b,r) = eqrev s r []
eqrev :: Eq a => [a] -> [a] -> [a] -> (Bool,[a]) eqrev (x:s1) ~(y:s2) acc = (x==y&&b,r) where (b,r) = eqrev s1 s2 (x:acc) eqrev _ _ acc = (True,acc)
pal :: Eq a => [a] -> Bool pal s = b where (b,r) = eqrev' s r
eqrev' :: Eq a => [a] -> [a] -> (Bool,[a]) eqrev' (x:s1) ~(y:s2) = (x==y&&b,r++[y]) where (b,r) = eqrev' s1 s2 eqrev' _ _ = (True,[])