以冒号开头的运算符是构造函数 [英] An operator symbol starting with a colon is a constructor

问题描述

我学习Haskell.从Haskell 2010文档中:

I learn Haskell. From Haskell 2010 documentation:

• 以冒号开头的运算符是构造函数.
• 以其他任何字符开头的运算符是普通标识符.

• An operator symbol starting with a colon is a constructor.
• An operator symbol starting with any other character is an ordinary identifier.

我不明白第一句话.我知道存在数据构造函数和类类型构造函数.这种情况下是什么构造函数?也许我需要一个代码示例.

I don't understand first phrase. I know exist data constructors and class type constructors. What constructor this case? Maybe I need a code sample.

推荐答案

您可以定义类似

data Symbolic n
   = Constant n
   | Variable String
   | Symbolic n :+ Symbolic n
   | Symbolic n :* Symbolic n
  deriving (Show)

GHCi>令v =变量； c =常量
GHCi> c 2:* v"a":+ c 3
(常数2:*变量"a"):+常数3

GHCi> let v = Variable; c = Constant
GHCi> c 2 :* v"a" :+ c 3
    (Constant 2 :* Variable "a") :+ Constant 3

第一句话就是这个意思.

That's what the first phrase refers to.

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