具有两个参数的构造函数的适用样式解析器 [英] Applicative style parser for constructor with two arguments

问题描述

我想为尖括号中的逗号分隔的一对值编写解析器。我得到了以下方法：

  pair p1 p2 = do 
 x1 < -  p1 
逗号
 x2 < -  p2 
返回（x1，x2）
 
数据Foo = Foo（Bar，Bar）
 
 foo :: Parser Foo 
 foo = Foo< $> （angle $ pair bar bar）
  

然而我更喜欢Foo构造函数来取两个参数而不是元组：

  data Foo = Foo Bar Bar 
  

编写这样一个解析器的最好方法是什么？理想情况下，我想重复使用标准的Parsec解析器，例如 angles ，并尽可能使用applicative。

解决方案

写这样一个解析器的最好方法是什么？理想情况下，我想重复使用标准的Parsec解析器，并尽可能多地使用应用。

在应用样式中，解析器会be

  foo =角度$ Foo< $> bar< *逗号*> bar 
  

从头开始， bar 是然后是一个逗号，这会被丢弃，另一个 bar ，那么构造函数 Foo 应用于两个已解析的栏 s。最后，将所有元素都包装到角度组合器中，以便形成一个字符串

 <酒吧，酒吧> 
  

被解析（ bar $ b 结合解析器忽略 *> 和的结果， < * 应用组合器消除了对 pair 组合子的需要，并且可以轻松地将其推广到构造函数接受任意数量的参数。

由于 CA McCann 在评论中提到（< $）组合子（它是GHC的 Functor class，默认实现（< $）= fmap。const ;但它不是语言标准的一部分）也很有用，如果你想忽略领先的令牌。使用它，你可以写成：

  Foo< $ ignoreMe< *> bar< *逗号*> baz 
  

比使用圆括号更好

  Foo< $> （ignoreMe *>栏）< *逗号> baz 
  

或纯，

 纯Foo< * ignoreMe< *> bar< *逗号*> baz 
  

如果没有它，某些形式需要。

I want to write a parser for a comma separated pair of values in angle brackets. I got it to work with the following approach:

pair p1 p2 = do
    x1 <- p1
    comma
    x2 <- p2
    return (x1, x2)

data Foo = Foo (Bar, Bar)

foo :: Parser Foo
foo = Foo <$> (angles $ pair bar bar)

However I would prefer the Foo constructor to take two parameter rather than a tuple:

data Foo = Foo Bar Bar

What is the best way to write such a parser? Ideally I would like to reuse standard Parsec parsers such a angles and use applicative as much as possible.

解决方案

What is the best way to write such a parser? Ideally I would like to reuse standard Parsec parsers such a angles and use applicative as much as possible.

In applicative style, your parser would be

foo = angles $ Foo <$> bar <* comma <*> bar

Going inside out, a bar is parsed, then a comma, which is discarded, and another bar, then the constructor Foo is applied to the two parsed bars. Finally, all is wrapped into the angles combinator, so that a string of the form

< bar , bar >

is parsed (bar should probably consume trailing whitespace).

Combining parsers ignoring the result of one with the *> and <* applicative combinators eliminates the need for the pair combinator and easily generalises to constructors taking an arbitrary number of arguments.

As C.A. McCann mentioned in a comment, the (<$) combinator (which is part of GHC's implementation of the Functor class, with the default implementation (<$) = fmap . const; but it's not part of the language standard) is useful too, if you want to ignore a leading token. Using that, you can write

Foo <$ ignoreMe <*> bar <* comma <*> baz

which is nicer than using parentheses

Foo <$> (ignoreMe *> bar) <* comma <*> baz

or pure,

pure Foo <* ignoreMe <*> bar <* comma <*> baz

as would be required in some form without it.

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