[Haskell-cafe] foldl to foldl' , ok ; but with foldM ?
noteed at gmail.com
Sun May 14 07:38:33 EDT 2006
Hi, I know the question is raised often but i cannot find the answer.
In a previous mail, I was said to use foldl' instead of foldl and it
worked really well :
acc1 values = foldl1' (+) values
How is foldl' defined ?
I have two other related questions :
What if I want to write something like this :
foldl' f emptyArray values
where f accumulates a value into an array.
What's the best representation for an array here ?
Also, given the fact that I though to represent my array with a c-like
Ptr Word8 array,
i tried this code, where the accumulator is referenced by an IORef :
acc2 ints = do
accRef <- newIORef 0
foldM add accRef ints
acc <- readIORef accRef
putStrLn $ show acc
where add accRef i = do
modifyIORef accRef (+ i)
but it's far worse than the code with foldl'.
I guess someone will tell me to try to strictify something... but I
don't know what and how.
I think trying to strictify the accRef is worthless : it's the
referenced acc that needs to be evaluated. Is-it correct ?
VO Minh Thu
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