如何摆脱$（...）和[| ... |]语法使用模板Haskell函数？ [英] How to get rid of $(...) and [| ... |] syntax when using a Template Haskell function?

问题描述

我试图学习一些模板哈斯克尔。作为练习，我编写了一个函数，可以生成 isLeft 和 isRight （灵感来自这个问题）。这是我谦虚的尝试：

  isA connam = do 
 ConE nam < -  connam 
 nn< -  newNamep
 lamE [varP nn] $ caseE（varE nn）[
 match（conP nam [wildP]）（normalB [| True |]）[]，
匹配wildP （normalB [| False |]）[] 
] 
  

问题是我有写 $（isA [| Left |]），而不是更直观的 isA Left 。是否有可能摆脱丑陋的语法？我似乎无法在文档中找到答案。

该函数仅适用于一个参数的构造函数，但这是针对另一个问题。

解决方案

语法对于原因;通知读者这里有编译时间的魔法。您只能在拼接处于顶层时消除 $（...）。

然而，我们可以消除 [|通过接收 Name 而不是 Exp ：



  isA nam = do 
 nn < -  newNamep$ b $ （normalB [| True |]）[]，
符合wildP（normalB [| False |]）[bPlamE [varPnn] $ caseE（varEnn）[
 match（conPnam [wildP] [] 
] 
  

要使用它，您可以编写 $（isA'Left），这对眼睛来说更容易一些。

作为奖励，如果你尝试给它以外的东西一个名称，你会得到一个类型错误，而不是无可辩驳的模式匹配错误。 模板Haskell语法

I'm trying to learn some Template Haskell. As an exercise, I wrote a function that can generate things like isLeft and isRight (inspired by this question). Here's my humble attempt:

isA connam = do
    ConE nam <- connam
    nn <- newName "p"
    lamE [varP nn] $ caseE (varE nn) [
                       match (conP nam [wildP]) ( normalB [| True |] ) [],
                       match wildP ( normalB [| False |] ) [] 
                     ]

The problem is that I have to write $(isA [| Left |]) instead of the more intuitive isA Left. Is it possible to get rid of the ugly syntax? I can't seem to find the answer in the documentation.

The function only works with one-argument constructors, but this is for another question.

解决方案

The syntax is there for a reason; to inform the reader that there is compile-time magic going on here. You can only eliminate the $(...) when your splice is at the top level.

However, we can eliminate the [| ... |] and also make the code more type-safe by taking in a Name instead of an Exp:

isA nam = do
    nn <- newName "p"
    lamE [varP nn] $ caseE (varE nn) [
                       match (conP nam [wildP]) ( normalB [| True |] ) [],
                       match wildP ( normalB [| False |] ) [] 
                     ]

To use this, you'd write $(isA 'Left), which is a little easier on the eyes.

As a bonus, if you try giving it something other than a Name, you get a type error instead of an irrefutable pattern match error.

See also: Template Haskell Syntax

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