Re: [Haskell-cafe] Why is Haskell flagging this?

17 Dec 2010

      To make that a little clearer, here is code that uses two calls to fmap to
drill through two monadic layers:
f :: [Int] -> IO [Int]
f lst = do return lst

main = do let lst = f [1,2,3,4,5]
          fmap (fmap (+1)) lst

So the order of operations is :
1. The first fmap converts an IO [Int] to [Int] and hands it off to the
second fmap
2. The second fmap applies the (+1) function to every element of the list.
3. The second fmap re-wraps the elements back into a [Int]
4. The first fmap re-wraps and returns the transformed [Int] into an IO
[Int].

-deech

On Fri, Dec 17, 2010 at 3:27 PM, aditya siram wrote:
...
I think it is giving you the error because you the "fmap" in your code is
operating on the IO monad and not the List monad. In order to get it to
work, you can remove the IO layer with ">>=" as below:
f :: [Int] -> IO [Int]
f lst = do return lst
main = do let lst = f [1,2,3,4,5]
          lst >>= return . fmap (+1)
Or you can not wrap the list in IO to begin with, my guess is that you
wrote 'f' to make the compiler happy at some point in development:
main = do let lst = [1,2,3,4,5]
          return $ fmap (+1) lst
-deech
On Fri, Dec 17, 2010 at 11:04 AM, michael rice  wrote:
...
I don't understand this error message. Haskell appears not to understand
that 1 is a Num.
Prelude> :t 1
1 :: (Num t) => t
Prelude> :t [1,2,3,4,5]
[1,2,3,4,5] :: (Num t) => [t]
Prelude>
Michael
===================
f :: [Int] -> IO [Int]
f lst = do return lst
main = do let lst = f [1,2,3,4,5]
          fmap (+1) lst
===============================
Prelude> :l test
[1 of 1] Compiling Main             ( test.hs, interpreted )
test.hs:5:17:
    No instance for (Num [Int])
      arising from the literal `1' at test.hs:5:17
    Possible fix: add an instance declaration for (Num [Int])
    In the second argument of `(+)', namely `1'
    In the first argument of `fmap', namely `(+ 1)'
    In the expression: fmap (+ 1) lst
Failed, modules loaded: none.
Prelude>
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