Applicative option parser
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Applicative option parser

This package contains utilities and combinators to define command line option parsers.

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Table of Contents

Getting started

Install with

cabal install optparse-applicative

Here is a simple example of an applicative option parser:

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

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

sample :: Parser Sample
sample = Sample
     <$> strOption
         ( long "hello"
        <> metavar "TARGET"
        <> help "Target for the greeting" )
     <*> switch
         ( long "quiet"
        <> short 'q'
        <> help "Whether to be quiet" )
     <*> option auto
         ( long "repeat"
        <> help "Repeats for greeting"
        <> showDefault
        <> value 1
        <> metavar "INT" )

The parser is built using applicative style starting from a set of basic combinators. In this example, hello is defined as an option with a String argument, while quiet is a boolean flag (called switch) and repeat gets parsed as an Int with help of the Read typeclass.

A parser can be used like this:

greet :: Sample -> IO ()
greet (Sample h False n) = replicateM_ n $ putStrLn $ "Hello, " ++ h
greet _ = return ()

main :: IO ()
main = execParser opts >>= greet
    opts = info (helper <*> sample)
      ( fullDesc
     <> progDesc "Print a greeting for TARGET"
     <> header "hello - a test for optparse-applicative" )

The greet function is the entry point of the program, while opts is a complete description of the program, used when generating a help text. The helper combinator takes any parser, and adds a help option to it.

The hello option in this example is mandatory (since it doesn't have a default value), so running the program without any argument will display a short option summary:

Usage: hello --hello TARGET [--quiet]

Running the program with the --help option will display the full help text:

hello - a test for optparse-applicative

Usage: hello --hello TARGET [-q|--quiet] [--repeat INT]
  Print a greeting for TARGET

Available options:
  -h,--help                Show this help text
  --hello TARGET           Target for the greeting
  -q|--quiet               Whether to be quiet
  --repeat                 Repeats for greeting (default: 1)

containing a detailed list of options with descriptions.

The specified metavars are used as placeholders for the option arguments, and can be referred to in the program description. This makes it possible to explicitly describe the connection between the options and the behaviour of the program.

Parsers are instances of both Applicative and Alternative, and work with any generic combinator, like many and some. For example, to make a option return Nothing instead of failing when it's not supplied, you can use the optional combinator in Control.Applicative:

optional $ strOption
   ( long "output"
  <> metavar "DIRECTORY" )

Supported options

optparse-applicative supports four kinds of options: regular options, flags, arguments and commands.

Regular options

A regular option is an option which takes a single argument, parses it, and returns a value.

A regular option can have a default value, which is used as the result if the option is not found in the command line. An option without a default value is considered mandatory, and produces an error when not found.

Regular options can have long names, or short (one-character) names, which determine when the option matches and how the argument is extracted.

An option with a long name (say "output") is specified on the command line as

--output filename.txt



while a short name option (say "o") can be specified with

-o filename.txt



Options can have more than one name, usually one long and one short, although you are free to create options with an arbitrary combination of long and short names.

Regular options returning strings are the most common, and they can be created using the strOption builder. For example,

   ( long "output"
  <> short 'o'
  <> metavar "FILE"
  <> value "out.txt"
  <> help "Write output to FILE" )

creates a regular option with a string argument (which can be referred to as FILE in the help text and documentation), default value "out.txt", a long name "output" and a short name "o". See below for more information on the builder syntax and modifiers.

A regular option can return an object of any type, and takes a reader parameter which specifies how the argument should be parsed. A common reader is auto, which assumes a Read instance for the return type and uses it to parse its argument. For example:

lineCount :: Parser Int
lineCount = option auto
            ( long "lines"
           <> short 'n'
           <> metavar "K"
           <> help "Output the last K lines" )

specifies a regular option with an Int argument. We added an explicit type annotation here, since without it the parser would have been polymorphic in the output type. There's usually no need to add type annotations, however, because the type will be normally inferred from the context in which the parser is used.

One can also create a custom reader that doesn't use the Read typeclass, and use it to parse option arguments. A custom reader is a value in the ReadM monad. We provide eitherReader :: (String -> Either String a) -> ReadM a to help create these values, where a Left will hold the error message for a failure.

data FluxCapacitor = ...

parseFluxCapacitor :: ReadM FluxCapacitor
parseFluxCapacitor = eitherReader $ \s -> ...

option parseFluxCapacitor ( long "flux-capacitor" )

One can also use ReadM directly, using str to obtain the command line string, and readerAbort or readerError within the ReadM monad to exit with an error message.


A flag is just like a regular option, but it doesn't take any arguments: it is either present in the command line or not.

A flag has a default value and an active value. If the flag is found on the command line, the active value is returned, otherwise the default value is used. For example:

data Verbosity = Normal | Verbose

flag Normal Verbose
  ( long "verbose"
 <> short 'v'
 <> help "Enable verbose mode" )

is a flag parser returning a Verbosity value.

Simple boolean flags can be specified using the switch builder, like so:

  ( long "keep-tmp-files"
 <> help "Retain all intermediate temporary files" )

There is also a flag' builder, which has no default value. For example, to add a --version switch to a program, you could write:

flag' Nothing (long "version" <> hidden) <|> (Just <$> normal_options)


An argument parser specifies a positional command line argument.

The argument builder takes a reader parameter, and creates a parser which will return the parsed value every time it is passed a command line argument for which the reader succeeds. For example

argument str (metavar "FILE")

creates an argument accepting any string. To accept an arbitrary number of arguments, combine the argument builder with either the many or some combinator:

some (argument str (metavar "FILES..."))

Arguments are only displayed in the brief help text, so there's no need to attach a description to them. They should be manually documented in the program description.

Note that arguments starting with - are considered options by default, and will not be considered by an argument parser.

However, parsers always accept a special argument: --. When a -- is found on the command line, all the following words are considered by argument parsers, regardless of whether they start with - or not.


A command can be used to specify a sub-parser to be used when a certain string is encountered in the command line.

Commands are useful to implement command line programs with multiple functions, each with its own set of options, and possibly some global options that apply to all of them. Typical examples are version control systems like git, or build tools like cabal.

A command can be created using the subparser builder, and commands can be added with the command modifier. For example

  ( command "add" (info addOptions
      ( progDesc "Add a file to the repository" ))
 <> command "commit" (info commitOptions
      ( progDesc "Record changes to the repository" ))

Each command takes a full ParserInfo structure, which will be used to extract a description for this command when generating a help text.

Note that all the parsers appearing in a command need to have the same type. For this reason, it is often best to use a sum type which has the same structure as the command itself. For example, for the parser above, you would define a type like:

data Options = Options
  { optGlobalOpt :: String
  , optGlobalFlag :: Bool
  , optCommand :: Command }

data Command
  = Add AddOptions
  | Commit CommitOptions

Alternatively, you can directly return an IO action from a parser, and execute it using join from Control.Monad.

start :: String -> IO ()
stop :: IO ()

opts :: Parser (IO ())
opts = subparser
  ( command "start" (info (start <$> argument str idm) idm)
 <> command "stop"  (info (pure stop) idm) )

main :: IO ()
main = join $ execParser (info opts idm)

Option builders

Builders allow you to define parsers using a convenient combinator-based syntax. Each builder takes a modifier as parameter, and returns a parser.

A modifier is a composition of functions which act on the option, setting values for properties or adding features, and is used to build the option from scratch and finally lift it to a single-option parser, which can then be combined with other parsers using normal Applicative combinators.

Modifiers are instances of the Monoid typeclass, so they can be combined using the composition function mappend (or simply (<>)).

See the haddock documentation for Options.Applicative.Builder for a full list of builders and modifiers.

Advanced features

How it works

A Parser a is essentially a heterogeneous list of Options, implemented with existential types.

All options are therefore known statically (i.e. before parsing, not necessarily before runtime), and can, for example, be traversed to generate a help text.

See this blog post for a more detailed explanation based on a simplified implementation.