[Haskell-cafe] Why is Haskell flagging this?

[Miguel Mitrofanov]

On 17 Dec 2010, at 20:04, michael rice wrote:

> I don't understand this error message. Haskell appears not to understand that 1 is a Num.

As it clearly states in the error message, it doesn't understand that [Int] is a Num - and it's not.

"No instance for Num something" usually indicates that you're trying to use an integer literal - in this case, "1" - as this "something".

The problem is that your "lst" has the type "IO [Int]" (which is the same as "IO ([] Int)"). "fmap" has the type "(a -> b) -> f a -> f b", so, it tries to unify the type of "(+1)" with "[Int] -> something" - which, probably, isn't what you've meant. In fact, I'm pretty sure you wanted "lst" to have the type "[Int]" (= "[] Int"), without "IO". You can do that using "<-" instead of "let":

main =
   do lst <- fst [1,2,3,4,5]
      return (fmap (+1) lst)

> 
> 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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