Grzegorz wrote:
hammingDistance [] _ = 0
hammingDistance _ [] = 0
hammingDistance (x:xs) (y:ys) | x==y = hammingDistance xs ys
| otherwise = 1 + hammingDistance xs ys
hammingDistance xs ys = h xs ys 0
where
h [] _ n = n
h _ [] n = n
h (x:xs) (y:ys) n | x==y = h xs ys n
| otherwise = h xs ys $! (n+1)
It is also possible to use a bang pattern on the parameter n. I'm too
lazy to look up how to do it.
But that is not the end of the problem.
modify (\ (sum,count) -> ((,) $! hammingDistance xs ys + sum) $!
count + 1)
modify still delays state update, i.e., its code says
s <- get
put (f s)
therefore s nor f s is evaluated now. Loop over it, and you will
accumulate a thunk equivalent to
f (f (f (f (... (f s)...
and that costs stack.
Try your own version of modify, e.g.,
modifies f =
do s <- get
put $! f s
and that does it.
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