[Haskell-cafe] lazy evaluation

[Peter Padawitz]

@ 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,[])