Haskell type error: Could not deduce (Show a) arising from a use of `show' from the context (Num a)

Question

I am configuring xmonad, and since I have to start a couple of dzen instances, I decided that it could be better to use a function that takes the parameters for x and y position, width, height and text align:

-- mydzen.hs

import Data.List

-- | Return a string that launches dzen with the given configuration.
myDzen :: Num a  => a -> a -> a -> Char -> String -> String

myDzen y x w ta e =
        intercalate " "
            [ "dzen2"
            , "-x"  , show x
            , "-w"  , show w
            , "-y"  , show y
            , "-h"  , myHeight
            , "-fn" , quote myFont
            , "-bg" , quote myDBGColor
            , "-fg" , quote myFFGColor
            , "-ta" , [ta]
            , "-e"  , quote e
            ]

quote :: String -> String
quote x = "'" x "'"

-- dummy values
myHeigth = "20"
myFont = "bitstream"
myDBGColor = "#ffffff"
myFFGColor = "#000000"

Could not deduce (Show a) arising from a use of `show'
from the context (Num a)
  bound by the type signature for
             myDzen :: Num a => a -> a -> a -> Char -> String -> String
  at mydzen.hs:(5,1)-(17,13)
Possible fix:
  add (Show a) to the context of
    the type signature for
      myDzen :: Num a => a -> a -> a -> Char -> String -> String
In the expression: show x
In the second argument of `intercalate', namely
  `["dzen2", "-x", show x, "-w", ....]'
In the expression:
  intercalate " " ["dzen2", "-x", show x, "-w", ....]

Obviously, deleting the signature, or changing Num a for Show a solves the issue, but I can't understand why. The 'arguments' x, w and y are supposed to be almost any kind of numbers (100, 550.2, 1366 * 0.7 ecc).

I'm new to haskell and until now I haven't been able to (clearly) understand the error or find what's wrong.

score 18 · Accepted Answer · answered Jun 25 '12 at 18:15

18

Previously, Show and Eq were superclasses of Num and the code would compile.

In the new GHC, from version 7.4, this has changed, now Num does not depend on Show and Eq, so you need to add them to signatures.

The reason is separation of concerns - there are numeric types with no sensible equality and show function (computable reals, rings of functions).

answered Jun 25 '12 at 18:15

sdcvvc

25,343
4
66
102

1

Thanks, even when you didn't give the "exact" answer, gave me the background (and history) to understand which was the origin of the error message :) – Pablo Olmos de Aguilera C. Jun 27 '12 at 00:40

score 7 · Answer 2 · answered Jun 25 '12 at 16:28

Not all numbers are showable (standard types like Int or Integer are, but you could have defined your own type; how would compiler know?) You’re using show x in your function — therefore, a must belong to Show typeclass. You can change your signature to myDzen :: (Num a, Show a) => a -> a -> a -> Char -> String -> String and get rid of the error.

score 3 · Answer 3 · answered Jun 25 '12 at 16:28

3

show is not part of the Num type class — to be able to use it, you have to add Show constraint alongside the Num one:

myDzen :: (Num a, Show a) => a -> a -> a -> Char -> String -> String

answered Jun 25 '12 at 16:28

Cat Plus Plus

125,936
27
200
224

Haskell type error: Could not deduce (Show a) arising from a use of `show' from the context (Num a)

3 Answers3