第6章,练习7,Haskell from First Principles

对于以下代码:

module Main where

data EitherOr a b = Hello a | Goodbye b deriving Show

instance (Eq a, Eq b) => Eq (EitherOr a b) where
  (==) (Hello x) (Hello x') = x == x'
  (==) (Goodbye x) (Goodbye x') = x == x'
  (==) _ _ = False

main :: IO ()
main = do
  print (Hello 2 == Hello 2)
  -- print (Hello 3 == Hello 2)
  -- print (Goodbye 3 == Goodbye 3)
  -- print (Goodbye 4 == Goodbye 3)
  -- print (Hello 3 == Goodbye 3)

在runhaskell下执行,即在ghc下执行,得到以下错误:

    • Ambiguous type variable ‘b0’ arising from a use of ‘==’
      prevents the constraint ‘(Eq b0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘b0’ should be.
      These potential instances exist:
        instance Eq Ordering -- Defined in ‘GHC.Classes’
        instance Eq Integer
          -- Defined in ‘integer-gmp-1.0.2.0:GHC.Integer.Type’
        instance (Eq a, Eq b) => Eq (EitherOr a b)
          -- Defined at /tmp/runghcXXXX61964-0.hs:5:10. <-- This is because I am using org-mode source blocks
        ...plus 23 others
        ...plus 11 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the first argument of ‘print’, namely ‘(Hello 2 == Hello 2)’
      In a stmt of a 'do' block: print (Hello 2 == Hello 2)
      In the expression: do print (Hello 2 == Hello 2)
   |
12 |   print (Hello 2 == Hello 2)
   |          ^^^^^^^^^^^^^^^^^^

我想我可以给编译器一个类型提示

print ((Hello (2 :: Int)) == (Hello (2 :: Int)))

或者类似的,但这似乎还不够。我看到a和b是多态的,但我认为在main中使用==可能足以帮助编译器推断类型。

接下来,我在ghci中加载了数据类型和类型类实例,并对这些类型进行了一些探索,发现

λ> :t Hello (2 :: Int)
Hello (2 :: Int) :: EitherOr Int b

如预期。再次在ghci中,我做了更多的探索,并看到正在使用默认类型

>:t(你好2==你好2)

<interactive>:1:2: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Integer’
        (Eq a0) arising from a use of ‘==’ at <interactive>:1:2-19
        (Num a0) arising from the literal ‘2’ at <interactive>:1:8
    • In the expression: (Hello 2 == Hello 2)

<interactive>:1:2: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘()’
        Eq b0 arising from a use of ‘==’
    • In the expression: (Hello 2 == Hello 2)
(Hello 2 == Hello 2) :: Bool

当然,这是我想要的。

然后我实际在ghci中执行代码,得到正确的答案

λ> Hello 2 == Hello 2

<interactive>:27:1: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Integer’
        (Eq a0) arising from a use of ‘==’ at <interactive>:27:1-18
        (Num a0) arising from the literal ‘2’ at <interactive>:27:7
    • In the expression: Hello 2 == Hello 2
      In an equation for ‘it’: it = Hello 2 == Hello 2

<interactive>:27:1: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘()’
        Eq b0 arising from a use of ‘==’
    • In the expression: Hello 2 == Hello 2
      In an equation for ‘it’: it = Hello 2 == Hello 2

<interactive>:27:1: warning: [-Wtype-defaults]
    • Defaulting the following constraints to type ‘Integer’
        (Eq a0) arising from a use of ‘==’ at <interactive>:27:1-18
        (Num a0) arising from the literal ‘2’ at <interactive>:27:7
    • In the expression: Hello 2 == Hello 2
      In an equation for ‘it’: it = Hello 2 == Hello 2

<interactive>:27:1: warning: [-Wtype-defaults]
    • Defaulting the following constraint to type ‘()’
        Eq b0 arising from a use of ‘==’
    • In the expression: Hello 2 == Hello 2
      In an equation for ‘it’: it = Hello 2 == Hello 2
True

但是在runhaskell下执行的相同代码,即在ghc编译下,由于我首先给出的错误而失败。我需要在这里学到什么?