Declarative command line option parser https://github.com/tanakh/optparse-declarative

Latest on Hackage:0.3.0

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MIT licensed by Hideyuki Tanaka


optparse-declarative is a declarative and easy to use command-line option parser.


$ cabal install optparse-declarative


Writing a simple command

First, you need to enable DataKinds extension and import Options.Declarative module.

{-# LANGUAGE DataKinds #-}
import           Options.Declarative

Then, define the command line option as a type of the function. For example, this is a simple greeting program:

greet :: Flag "g" '["greet"] "STRING" "greeting message" (Def "Hello" String)
      -> Arg "NAME" String
      -> Cmd "Greeting command" ()
greet msg name =
    liftIO $ putStrLn $ get msg ++ ", " ++ get name ++ "!"

There are two type of options, Flag and Arg. Flag is named argument and Arg is unnamed argument. Last argument of both options is value type. If you need to specify default value, use the modifiers such as Def.

In above, variable msg has a very complex type (Flag "g" '["greet"] "STRING" "greeting message" (Def "Hello" String)). In order to get the value of usual type (in this case, that is String), you can use get function.

The whole type of command is Cmd. Cmd is an instance of MonadIO and it has some extra information.

After defining a command, you just invoke it by run_.

main :: IO ()
main = run_ greet

You can execute this program like this:

$ ghc simple.hs

$ ./simple
simple: not enough arguments
Try 'simple --help' for more information.

$ ./simple --help
Usage: simple [OPTION...] NAME
  -g STRING  --greet=STRING  greeting message
  -?         --help          display this help and exit

$ ./simple World
Hello, World!

$ ./simple --greet=Goodbye World
Goodbye, World!

Writing multiple sum-commands

You can write (nested) sub-commands.

Just groupe subcommands by Group, you got sub-command parser.

This is the example:

{-# LANGUAGE DataKinds #-}

import           Options.Declarative

main :: IO ()
main = run_ $
    Group "Test program for library"
    [ subCmd "greet"   greet
    , subCmd "connect" connect

greet :: Flag "g" '["greet"] "STRING" "greeting message" (Def "Hello" String)
      -> Flag "" '["decolate"] "" "decolate message" Bool
      -> Arg "NAME" String
      -> Cmd "Greeting command" ()
greet msg deco name = do
    let f x | get deco = "*** " ++ x ++ " ***"
            | otherwise = x
    liftIO $ putStrLn $ f $ get msg ++ ", " ++ get name ++ "!"

connect :: Flag "h" '["host"] "HOST" "host name"   (Def "localhost" String)
        -> Flag "p" '["port"] "PORT" "port number" (Def "8080"      Int   )
        -> Cmd "Connect command" ()
connect host port = do
    let addr = get host ++ ":" ++ show (get port)
    liftIO $ putStrLn $ "connect to " ++ addr

And this is the output:

$ ./subcmd --help
Usage: subcmd [OPTION...] <COMMAND> [ARGS...]
  -?  --help  display this help and exit

  greet       Greeting command
  port        Server command

$ ./subcmd connect --port=1234
connect to localhost:1234

For more examples, please see example directory.



  • Simplify API
  • Support -vv -vvv verbosity
  • Improve help message


  • Verbosity support


  • First release
Depends on:
Used by 4 packages:
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