相关疑难解决方法(0)

令我感到困惑的是,haskell编译器有时会推断出比我预期的更不易变形的类型,例如在使用无点定义时.

似乎问题是"单态限制",默认情况下在旧版本的编译器上启用.

考虑以下haskell程序:

{-# LANGUAGE MonomorphismRestriction #-}

import Data.List(sortBy)

plus = (+)
plus' x = (+ x)

sort = sortBy compare

main = do
  print $ plus' 1.0 2.0
  print $ plus 1.0 2.0
  print $ sort [3, 1, 2]

如果我编译它,ghc我没有获得错误,可执行文件的输出是:

3.0
3.0
[1,2,3]

如果我将main身体改为:

main = do
  print $ plus' 1.0 2.0
  print $ plus (1 :: Int) 2
  print $ sort [3, 1, 2]

我没有编译时错误,输出变为:

3.0
3
[1,2,3]

正如所料.但是,如果我尝试将其更改为:

main = do
  print $ …

在ghci,我可以这样做:

ghci> (fmap . const) 5 [1,2,3,4,5]
[5,5,5,5,5]

但如果我尝试将子表达式提取(fmap . const)到变量中,我会收到错误:

ghci> let foo = (fmap . const)

<interactive>:3:12:
    No instance for (Functor f0) arising from a use of `fmap'
    The type variable `f0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance Functor ((,) a) -- Defined in `GHC.Base'
      instance Functor ((->) r) -- Defined in `GHC.Base'
      instance Functor IO -- Defined in `GHC.Base'
      ...plus two …

看起来翻转对我的功能做了意想不到的事情

例1:

let m = flip max
:t max
max :: Ord a => a -> a ->
:t m
m :: () -> () -> ()

例2:

let f x y = x + y
:t f
f :: Num a => a -> a -> a
let g = flip f
:t g
g :: Integer -> Integer -> Integer

f可以评估浮点数,但g在看到浮点数时会抛出错误.但是当我跑步的时候

(flip f) 1.5 1.7

评价很好!这些表达有什么区别？

我正在尝试编译简单的代码片段.

main = (putStrLn . show) (Right 3.423)

编译导致以下错误:

No instance for (Show a0) arising from a use of `show'
The type variable `a0' is ambiguous
Possible fix: add a type signature that fixes these type variable(s)
Note: there are several potential instances:
  instance Show Double -- Defined in `GHC.Float'
  instance Show Float -- Defined in `GHC.Float'
  instance (Integral a, Show a) => Show (GHC.Real.Ratio a)
    -- Defined in `GHC.Real'
  ...plus 42 others
In the second argument of `(.)', namely …