[Haskell-cafe] Why is Haskell flagging this?

michael rice nowgate at yahoo.com
Sun Dec 19 00:12:00 CET 2010

Here's (attached) a screen shot of the post I received from you.  
There's your name after the "From:" at the top. Also, this post is a reply, so your message follows. Was there a mix up? 


--- On Sat, 12/18/10, Thomas Davie <tom.davie at gmail.com> wrote:

From: Thomas Davie <tom.davie at gmail.com>
Subject: Re: [Haskell-cafe] Why is Haskell flagging this?
To: chrisdone at googlemail.com
Cc: "michael rice" <nowgate at yahoo.com>, haskell-cafe at haskell.org
Date: Saturday, December 18, 2010, 2:38 PM

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]

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