>b <- mapArray id a
The reason it is slow is because the array type is copied every time
a member is assigned.
There are two solutions:
1) Use a mutable-array in the StateMonad then freeze it.
2) In this particular case where processing is sequential (IE you
are only altering values based on *nearby* values, you can use streams.
One of the nicest features of Haskell is how lists (being lazy) operate
just like streams...
So read the data into a list and define a filter of the type
filter :: [a] -> [a]
Your program then becomes:
main :: IO ()
main = do
s <- getMyDataAsList()
putMyList (filter s)
where the commands getMyDataAsList and putMyList have the types:
getMyDataAsList :: IO [a]
putMyList :: [a] -> IO ()
This should be fast, and also use very little memory.
Regards,
Keean Schupke.
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