Haskell: Parsing command line arguments

Question

This more of a style question, rather than a how to.

So I\'ve got a program that needs two command line arguments: a string and an integer.

I implemented it this

Answer 1

I agree the optparse-applicative package is very nice. Awesome! Let me give an up-to-date example.

The program takes as arguments a string and an integer n, returns the string replicated n times, and it has a flag which reverses the string.

-- file: repstring.hs
import Options.Applicative
import Data.Monoid ((<>))

data Sample = Sample
  { string :: String
  , n :: Int
  , flip :: Bool }

replicateString :: Sample -> IO ()
replicateString (Sample string n flip) = 
    do 
      if not flip then putStrLn repstring else putStrLn $ reverse repstring
          where repstring = foldr (++) "" $ replicate n string

sample :: Parser Sample
sample = Sample
     <$> argument str 
          ( metavar "STRING"
         <> help "String to replicate" )
     <*> argument auto
          ( metavar "INTEGER"
         <> help "Number of replicates" )
     <*> switch
          ( long "flip"
         <> short 'f'
         <> help "Whether to reverse the string" )

main :: IO ()
main = execParser opts >>= replicateString
  where
    opts = info (helper <*> sample)
      ( fullDesc
     <> progDesc "Replicate a string"
     <> header "repstring - an example of the optparse-applicative package" )

Once the file is compiled (with ghc as usual):

$ ./repstring --help
repstring - an example of the optparse-applicative package

Usage: repstring STRING INTEGER [-f|--flip]
  Replicate a string

Available options:
  -h,--help                Show this help text
  STRING                   String to replicate
  INTEGER                  Number of replicates
  -f,--flip                Whether to reverse the string

$ ./repstring "hi" 3 
hihihi
$ ./repstring "hi" 3 -f
ihihih

Now, assume you want an optional argument, a name to append at the end of the string:

-- file: repstring2.hs
import Options.Applicative
import Data.Monoid ((<>))
import Data.Maybe (fromJust, isJust)

data Sample = Sample
  { string :: String
  , n :: Int
  , flip :: Bool
  , name :: Maybe String }

replicateString :: Sample -> IO ()
replicateString (Sample string n flip maybeName) = 
    do 
      if not flip then putStrLn $ repstring ++ name  else putStrLn $ reverse repstring ++ name
          where repstring = foldr (++) "" $ replicate n string
                name = if isJust maybeName then fromJust maybeName else ""

sample :: Parser Sample
sample = Sample
     <$> argument str 
          ( metavar "STRING"
         <> help "String to replicate" )
     <*> argument auto
          ( metavar "INTEGER"
         <> help "Number of replicates" )
     <*> switch
          ( long "flip"
         <> short 'f'
         <> help "Whether to reverse the string" )
     <*> ( optional $ strOption 
          ( metavar "NAME"
         <> long "append"
         <> short 'a'
         <> help "Append name" ))

Compile and have fun:

$ ./repstring2 "hi" 3 -f -a rampion
ihihihrampion