我正在 Haskell 中开发一个小项目，目前我正在尝试通过添加使用可选 cli 标志的可能性来使其更具可扩展性。
所需的界面如下所示：
program: [--command1 {n} ] [--help]
其中
command1helpcommand1n使用模式匹配，我得到了以下结果
main :: IO ()
main = do
args <- getArgs
case args of
[] -> command1'
["--command1", a] -> if isJust (readMaybe n::Maybe Int)
then command1 a-- handle proper integer
else -- call command1' but also show a warning message
["--command1"] -> command1'
["--help"] -> putStrLn showWarningMessage
_ -> putStrLn (showErrorMessage args) -- show an error message
由于程序不依赖于某个特定参数，因此即使 cli 参数无效，它也应该能够运行。
我不确定如何处理以下情况：
command1'--command2args=[--command2, --command1, n]args=[--command1, --command2]command2注意：我看到有专门设计来处理这些问题的库，但我有兴趣解决这些简单的案例，以便更好地学习 Haskell。当然，可能还有更多的边缘情况，但就我的使用而言，提到的两个是我唯一关心的。
不是专家，但这是我的答案：
1. 对您有用的是
doelsecommand1main :: IO ()
main = do
args <- getArgs
case args of
[] -> command1'
["--command1", a] -> if isJust (readMaybe n::Maybe Int)
then command1 a
-- here what I think you want to do
else do
-- here your warning message
putStrLn (showWarningMessage args)
-- here let's say 10 is the default value
command1 10
["--command1"] -> command1'
["--help"] -> putStrLn showWarningMessage
_ -> putStrLn (showErrorMessage args) -- show an error message
我发现在http://www.happylearnhaskelltutorial.com/1/times_table_train_of_terror.html#s18.4中你可以从头开始，它很好地解释了Haskell的概念。
2.我明白你想要做什么，但是命令行提示的目的不是快速失败吗？请记住，您必须关注默认值是什么，并且这样做，您的工具的默认行为应该是什么。也许您可以想到参数的不同实现。这是我从命令式语言学习几个月的 Haskell 中学到的东西。
无论如何，我有一些关于如何做到这一点的提示。如果添加
command3但是代码，即使不完美，也可以完美地处理标志/参数的每种组合，并捕获 4 个不同参数的所有错误。这是一些评论：
import Control.Monad (foldM)
import System.Console.GetOpt
import System.Environment ( getArgs, getProgName )
data Options = Options {
optVerbose :: Bool
, optShowVersion :: Bool
, optNbOfIterations :: Int
, optSomeString :: String
} deriving Show
defaultOptions = Options {
optVerbose = False
, optShowVersion = False
, optNbOfIterations = 1
, optSomeString = "a random string"
}
options :: [OptDescr (Options -> Either String Options)]
options =
[ Option ['v'] ["verbose"]
(NoArg (\ opts -> Right opts { optVerbose = True }))
"chatty output on stderr"
, Option ['V','?'] ["version"]
(NoArg (\ opts -> Right opts { optShowVersion = True })) -- here no need for a value for this option because of `NoArg`
"show version number"
, Option ['i'] ["iterations"]
(ReqArg (\ i opts -> -- here the value of the param is required `ReqArg`, but you can also have `OptArg` which makes the value optional
case reads i of
[(iterations, "")] | iterations >= 1 && iterations <= 20 -> Right opts { optNbOfIterations = iterations } -- here your conditions
_ -> Left "--iterations must be a number between 1 and 20" -- error message in case previous conditions are not met
) "ITERATIONS") -- here the name of the param in the cli help
"Number of times you want to repeat the reversed string" -- here the description of param's usage in the cli help
, Option ['s'] ["string"]
(ReqArg (\arg opts -> Right opts { optSomeString = return arg !! 0 } -- really dirty, sorry but could not make it work another way
"Outputs the string reversed"
]
parseArgs :: IO Options
parseArgs = do
argv <- getArgs
progName <- getProgName
let header = "Usage: " ++ progName ++ " [OPTION...]"
let helpMessage = usageInfo header options
case getOpt RequireOrder options argv of
(opts, [], []) ->
case foldM (flip id) defaultOptions opts of
Right opts -> return opts
Left errorMessage -> ioError (userError (errorMessage ++ "\n" ++ helpMessage))
(_, _, errs) -> ioError (userError (concat errs ++ helpMessage))
main :: IO ()
main = do
options <- parseArgs
if isVerbose options
then mapM_ putStrLn ["verbose mode activated !","You will have a lot of logs","With a lot of messages","And so on ..."]
else return () -- do nothing
mapM_ print (repeatReversedString options)
if getVersion options
then print "version : 0.1.0-prealpha"
else return ()
return ()
-- With those functions we extract the option setting from the data type Options (and maybe we play with it)
isVerbose :: Options -> Bool
isVerbose (Options optVerbose _ _ _) = optVerbose
getVersion :: Options -> Bool
getVersion (Options _ optShowVersion _ _) = optShowVersion
repeatReversedString :: Options -> [String]
repeatReversedString (Options _ _ optNbOfIterations optSomeString) =
replicate optNbOfIterations (reverse optSomeString)
getString :: Options -> String
getString (Options _ _ _ optSomeString) = optSomeString
我认为你可以通过使用 OptArg （而不是我使用的 NoArg 和 ReqArg）来实现你的目标，它有这样的定义：
OptArg (Maybe String -> a) String这是 hackage 的链接：
https://hackage.haskell.org/package/base-4.14.1.0/docs/System-Console-GetOpt.html
请记住，Haskell 与其他常见语言有很大不同，例如没有 for 循环！