Haskell monadic IO

compute fp = do
    text <- readFile fp
    let (a,b) = sth text
    let x = data b 
    --g <- x
    putStr $ print_matrix $ fst $ head $ x

It works when i get only first element but i want do this for all element on the list of pair. When i write g<- x i got Couldn't match expected type `IO t0' 开发者_StackOverflow中文版 with actual type [([[Integer]], [[Integer]])]

You're inside the IO Monad here, so any time you write a "bind" arrow <-, the thing on the right side has to be an IO operation. So the short answer is, you don't want to use <- on the value x.

Now, it looks like you want to call print_matrix for each element of a list rather than a single element. In that case I think Macke is on the right track, but I would use mapM_ instead:

mapM_ (putStr . print_matrix . fst) x

should do the trick.

The reason is that you are explicitly saying you want to putStr each element, one at a time, but you don't care about the result of putStr itself.

It sounds like mapM might fit your bill: Monad a => (b -> a c) -> [b] -> a [c]

It's used to apply a monadic function to a list, and get a list back, in the monad