[Haskell-cafe] Why is Haskell flagging this?

[Thomas Davie]

On 17 Dec 2010, at 21:44, Christopher Done wrote:

> On 17 December 2010 18:04, michael rice <nowgate at yahoo.com> wrote:
> ===================
> 
> f :: [Int] -> IO [Int]
> f lst = do return lst
> 
> main = do let lst = f [1,2,3,4,5]
>           fmap (+1) lst
>  
> The problem is that you are applying fmap to a type IO a.
> 
> fmap (+1) (return [1,2,3])
> 
> But to achieve the behaviour you expect, you need another fmap:
> 
> fmap (fmap (+1)) (return [1,2,3])

Which can be more neatly written with Conal's semantic editor cominators as

(fmap . fmap) (+1) (return [1,2,3])

Of course, I question why the list is put in the IO monad at all here... surely this would be much better

return $ fmap (+1) [1,2,3]

Finally, that has the wrong type for main... perhaps you meant to print it out?

main :: IO ()
main = print $ fmap (+1) [1,2,3]

Bob
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