如何在Haskell的optparse-applicative中为子命令设置同义词

你可以借助optparse-applicative库的内置能力实现需求，既复用全部逻辑，又不会在帮助里出现重复条目，具体实现方式如下：

完整修改后的可运行代码 #

#!/usr/bin/env stack
-- stack --resolver lts-18.17 script --package optparse-applicative

import Options.Applicative
import Data.Semigroup ((<>))

data Options = CmdGreet GreetArgs | CmdGroot GreetArgs

newtype GreetArgs = GreetArgs String

main :: IO ()
main = do
    cmdOpts <- customExecParser (prefs showHelpOnEmpty) (info optParser fullDesc)
    runCmd cmdOpts

optParser :: Parser Options
optParser = subparser (
    command "greet" (info (CmdGreet <$> sample) (progDesc "Print greeting 1")) <>
    -- 注册别名gruut，复用完全相同的parser，加internal修饰符让它不在帮助列表中显示
    command "gruut" (info (CmdGreet <$> sample) (internal <> progDesc "Alias for greet")) <>
    command "groot" (info (CmdGroot <$> sample) (progDesc "Print greeting 2"))
    -- 自定义帮助的命令列表展示，实现greet|gruut的显示效果
    ) & help "Available commands:\n\
             \  greet|gruut              Print greeting 1\n\
             \  groot                    Print greeting 2"

runCmd :: Options -> IO ()
runCmd o = case o of
    CmdGreet opts -> greet opts
    CmdGroot opts -> groot opts

greet :: GreetArgs -> IO ()
greet (GreetArgs h) = putStrLn $ "Hello, " ++ h ++ "!"

groot :: GreetArgs -> IO ()
groot (GreetArgs h) = putStrLn $ "Howdy, " ++ h ++ "!"

sample :: Parser GreetArgs
sample = GreetArgs <$> strArgument ( metavar "TARGET" )

更简洁的高版本方案 #

如果你使用0.16及以上版本的optparse-applicative，可以用库内置的commandAlias函数进一步简化代码：

optParser :: Parser Options
optParser = subparser (
    command "greet" (info (CmdGreet <$> sample) (progDesc "Print greeting 1")) <>
    commandAlias "gruut" "greet" <> -- 直接声明gruut是greet的别名，无需重复写parser
    command "groot" (info (CmdGroot <$> sample) (progDesc "Print greeting 2"))
    ) & help "Available commands:\n\
             \  greet|gruut              Print greeting 1\n\
             \  groot                    Print greeting 2"

效果验证 #

• 运行./opt_ex.hs greet John和./opt_ex.hs gruut John都会返回Hello, John!，行为完全一致
• 直接运行./opt_ex.hs查看帮助，输出完全符合你期望的格式，没有重复的命令条目

内容的提问来源于stack exchange，提问作者nevrome