[Haskell-cafe] lazy evaluation
Peter Padawitz
peter.padawitz at udo.edu
Wed Feb 6 10:19:41 EST 2008
@ 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,[])
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