价值建构者的别名 [英] Aliases for Value Constructors

问题描述

是否可以为值构造函数定义别名？上下文是这样的：我正在编写一个程序，该程序实现图灵机的原始递归功能。为此，我正在使用一元整数编码。我的图灵机类型定义如下：

Is it possible to define aliases for value constructors? The context is this: I am writing a program, that implements primitive recursive functions as Turing machines. For that I am working with unary integer encoding. My Turing machine type is defined like this:

-- definition of a Turing machine
data TuringMachine a = TuringMachine
                     State      -- q0
                     (Set State) -- F
                     (Set State) -- Q
                     (Set (Symbol a)) -- Gamma
                     (Set (Symbol a)) -- Sigma
                     (Map (State, Symbol a) (State, Symbol a, Instruction)) -- delta
                     deriving (Show)

其中，我的符号类型定义如下：

where my symbol type is defined like this:

data Symbol a = Symbol a | Blank | Delim | Final | One deriving (Eq, Ord)

我有一个一个为方便起见，所以我不需要到处写符号1 。但这有点不整洁。我想在 Symbol 类型之外定义别名，例如

I have One as a convenience in there, so I don't need to write Symbol 1 everywhere. But that is a little untidy. I would like to define an alias outside of the type Symbol, like

alias One = Symbol 1

和

alias Zero = Blank

是否可能这样？

推荐答案

{-# Language PatternSynonyms #-}

pattern Zero :: Symbol a
pattern Zero = Blank

pattern One :: (Eq a, Num a) => Symbol a
pattern One = Symbol 1

有效（双向模式同义词）

works (bidirectional pattern synonym)

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