[Haskell-cafe] Why is Haskell flagging this?

[Mads Lindstrøm]

Hi Michael

The type of lst is "IO [Int]" and therefore "fmap (+1)" applies (+1) to
the hole lists of integers, and not to each member of the list. That is:

fmap (+1) lst   <=>
fmap (+1) (return [1,2,3,4,5])  <=>
return ([1,2,3,4,5] + 1)

and you cannot say [1,2,3,4,5] + 1.

Does that make sense?

Maybe you want to say:

main = do let lst = [1,2,3,4,5]
          print $ map (+1) lst

/Mads

On Fri, 2010-12-17 at 09:04 -0800, 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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