了解 Haskell 流行的 optparse-applicative 库的介绍性代码片段

Question

来自流行 optparse 库的介绍性代码片段：

data Sample = Sample
  { hello :: String
  , quiet :: Bool }

sample :: Parser Sample
sample = Sample
  <$> strOption -- Q1
      ( long "hello"
     <> metavar "TARGET" -- Q2
     <> help "Target for the greeting" )
  <*> switch
      ( long "quiet"
     <> help "Whether to be quiet" )

查看我的 questions/confusion 代码片段中的注释。

Q1：<$>如何作为类型构造函数Sample的第一个参数？我以为这个操作必须用在函数和仿函数之间。

问题 2：此代码段中使用的操作 <> 是什么？

Answer 1

Q1: How is it that <$> can be used as the first argument of the type constructor Sample? I thought this operation had to be used between a function and a functor.

恰恰相反：Sample 是 (<$>) 的第一个参数，它被映射到 strOption 产生的 Parser String 上。

Q2: What is the operation <> used throughout this code snippet?

(<>) 是 mappend 的同义词，来自 Monoid class。在这种情况下，它用于将各个设置组合到一组设置中，以用于您正在定义的每个命令行选项。

Answer 2

使用ApplicativeDo和NamedFieldPuns扩展，也可以这样写：

{-# language ApplicativeDo #-}
{-# language NamedFieldPuns #-}
import Options.Applicative
import Data.Monoid

data Sample = Sample
  { hello :: String
  , quiet :: Bool }

sample :: Parser Sample
sample = do
    hello <- strOption ( long "hello"
                      <> metavar "TARGET"
                      <> help "Target for the greeting" )
    quiet <- switch ( long "quiet"
                   <> help "Whether to be quiet" )
    pure (Sample {hello,quiet})

了解 Haskell 流行的 optparse-applicative 库的介绍性代码片段

Understanding the introductory code snippet for Haskell's popular optparse-applicative library

haskell

optparse