[Haskell-beginners] Using Show

[mukesh tiwari]

I modified your code little bit and I got this error.

λ> :t (pure (+) <*> ([1, 2, 3] :: [Int]))
(pure (+) <*> ([1, 2, 3] :: [Int])) :: [Int -> Int]


λ> show (pure (+) <*> ([1, 2, 3] :: [Int]))

<interactive>:13:1:
    No instance for (Show (Int -> Int)) arising from a use of ‘show’
    In the expression: show (pure (+) <*> ([1, 2, 3] :: [Int]))
    In an equation for ‘it’:
        it = show (pure (+) <*> ([1, 2, 3] :: [Int]))

You have list of functions ( [Int -> Int] ) and functions are not instance
of Show class so error is pointing towards this fact.

λ> :i Show
class Show a where
  showsPrec :: Int -> a -> ShowS
  show :: a -> String
  showList :: [a] -> ShowS
      -- Defined in ‘GHC.Show’
instance Show a => Show [a] -- Defined in ‘GHC.Show’
instance Show Ordering -- Defined in ‘GHC.Show’
instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
instance Show Integer -- Defined in ‘GHC.Show’
instance Show Int -- Defined in ‘GHC.Show’
instance Show Char -- Defined in ‘GHC.Show’
instance Show Bool -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k, Show l, Show m, Show n, Show o) =>
         Show (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k, Show l, Show m, Show n) =>
         Show (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k, Show l, Show m) =>
         Show (a, b, c, d, e, f, g, h, i, j, k, l, m)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k, Show l) =>
         Show (a, b, c, d, e, f, g, h, i, j, k, l)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j, Show k) =>
         Show (a, b, c, d, e, f, g, h, i, j, k)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i, Show j) =>
         Show (a, b, c, d, e, f, g, h, i, j)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h, Show i) =>
         Show (a, b, c, d, e, f, g, h, i)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f, Show g,
          Show h) =>
         Show (a, b, c, d, e, f, g, h)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f,
          Show g) =>
         Show (a, b, c, d, e, f, g)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e, Show f) =>
         Show (a, b, c, d, e, f)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d, Show e) =>
         Show (a, b, c, d, e)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c, Show d) => Show (a, b, c, d)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b, Show c) => Show (a, b, c)
  -- Defined in ‘GHC.Show’
instance (Show a, Show b) => Show (a, b) -- Defined in ‘GHC.Show’
instance Show () -- Defined in ‘GHC.Show’
instance (Show a, Show b) => Show (Either a b)
  -- Defined in ‘Data.Either’
instance Show a => Show (ZipList a)
  -- Defined in ‘Control.Applicative’
instance Show Float -- Defined in ‘GHC.Float’
instance Show Double -- Defined in ‘GHC.Float’

See  the [1] [2].

[1] https://wiki.haskell.org/Show_instance_for_functions
[2]
http://stackoverflow.com/questions/15823732/why-is-there-no-show-instance-for-functions

On Wed, Apr 15, 2015 at 6:17 PM, Shishir Srivastava <
shishir.srivastava at gmail.com> wrote:

> hi,
>
> I am trying to naively print an applicative functor using 'show' like this
> -
>
> show (pure (+) <*> [1, 2, 3])
>
> I know there is something fundamentally wrong with that expression but not
> sure what.
>
> Can anyone please point it out.
>
> Thanks,
> Shishir
>
>
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