Question

启用语言扩展程序TypeOperators后，可以定义自己的类型运算符。此外，可以使用infix*设置其相对优先级。但是，例如(->)的优先级是什么？

> :i (->)
data (->) a b   -- Defined in `GHC.Prim'
instance Monad ((->) r) -- Defined in `GHC.Base'
instance Functor ((->) r) -- Defined in `GHC.Base'
instance Applicative ((->) a) -- Defined in `Control.Applicative'
instance Arrow (->) -- Defined in `Control.Arrow'
instance Monoid b => Monoid (a -> b) -- Defined in `Data.Monoid'
instance ArrowLoop (->) -- Defined in `Control.Arrow'
instance ArrowChoice (->) -- Defined in `Control.Arrow'
instance ArrowApply (->) -- Defined in `Control.Arrow'

Answer 1

以下是compiler/basicTypes/BasicTypes.lhs中GHC来源的相关位：

maxPrecedence, minPrecedence :: Int
maxPrecedence = 9
minPrecedence = 0
defaultFixity :: Fixity
defaultFixity = Fixity maxPrecedence InfixL
negateFixity, funTyFixity :: Fixity
-- Wired-in fixities
negateFixity = Fixity 6 InfixL  -- Fixity of unary negate
funTyFixity  = Fixity 0 InfixR  -- Fixity of '->'

因此->的固定性为infixr 0。

您还可以从错误消息中推断出这一点。创建以下Haskell源文件：

{-# LANGUAGE TypeOperators #-}
data a // b
infixl 0 //

然后：

GHCi> :kind Int // Int -> Int

<interactive>:1:5:
    Precedence parsing error
        cannot mix ‘//’ [infixl 0] and ‘(->)’ [infixr 0] in the same infix expression

Haskell类型的运算符优先级

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