Haskell类型错误:无法从上下文中使用"show"引起的(显示a)(Num a)

Question

我正在配置xmonad,因为我必须启动几个dzen实例,所以我认为最好使用一个函数来获取x和y位置,宽度,高度和文本对齐的参数:

-- 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", ....]

显然,删除签名,或更改Num a为Show a解决问题,但我无法理解为什么.在"参数" x,w而y被认为是几乎任何类型的数字(100,550.2,1366 * 0.7ECC).

我是haskell的新手,直到现在我还没能(明确地)理解错误或发现错误.

Answer 1

此前,Show和Eq人的超类Num和代码将编译.

在新的GHC,从7.4版本,这种情况已经改变,现在Num不依赖于Show和Eq,所以你需要将它们添加到签名.

原因是关注点分离 - 有数字类型没有明智的相等和显示功能(可计算的实数,函数环).

Answer 2

并非所有数字都是可显示的(标准类型喜欢Int或者Integer是,但你可以定义自己的类型;编译器如何知道？)你show x在函数中使用 - 因此,a必须属于Show类型类.您可以将签名更改为myDzen :: (Num a, Show a) => a -> a -> a -> Char -> String -> String并删除错误.