[Haskell-cafe] Why is Haskell flagging this?
daniel.is.fischer at googlemail.com
Fri Dec 17 22:24:19 CET 2010
On Friday 17 December 2010 18:04:20, 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]
> f :: [Int] -> IO [Int]
> f lst = do return lst
> main = do let lst = f [1,2,3,4,5]
> fmap (+1) lst
The fmap is relative to IO, your code is equivalent to
do let lst = (return [1,2,3,4,5])
fmap (+1) lst
fmap (+1) (return [1,2,3,4,5])
do lst <- return [1,2,3,4,5]
return $ (+1) lst
but there's no instance Num [Int] in scope
You probably meant
do let lst = f [1,2,3,4,5]
fmap (map (+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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