[Haskell-cafe] Why is Haskell flagging this?
aditya.siram at gmail.com
Fri Dec 17 22:27:06 CET 2010
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
On Fri, Dec 17, 2010 at 11:04 AM, michael rice <nowgate at yahoo.com> 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]
> 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 )
> 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.
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