A Parser a is an option parser returning a value of type a.

Builder for a flag parser.

A flag that switches from a "default value" to an "active value" when encountered. For a simple boolean value, use switch instead.

Note: Because this parser will never fail, it can not be used with combinators such as some or many, as these combinators continue until a failure occurs. See flag'.

Builder for a flag parser without a default value.

Same as flag, but with no default value. In particular, this flag will never parse successfully by itself.

It still makes sense to use it as part of a composite parser. For example

length <$> many (flag' () (short 't'))

is a parser that counts the number of "-t" arguments on the command line, alternatively

flag' True (long "on") <|> flag' False (long "off")

will require the user to enter '--on' or '--off' on the command line.

Builder for a boolean flag.

Note: Because this parser will never fail, it can not be used with combinators such as some or many, as these combinators continue until a failure occurs. See flag'.

switch = flag False True

Builder for an option taking a String argument.

Builder for an option using the given reader.

This is a regular option, and should always have either a long or short name specified in the modifiers (or both).

nameParser = option str ( long "name" <> short 'n' )

Builder for a String argument.

Builder for an argument parser.

Builder for a command parser. The command modifier can be used to specify individual commands.

By default, sub-parsers allow backtracking to their parent's options when they are completed. To allow full mixing of parent and sub-parser options, turn on subparserInline; otherwise, to disable backtracking completely, use noBacktrack.

Builder for a command parser with a "helper" option attached. Used in the same way as subparser, but includes a "--help|-h" inside the subcommand.

An option that always fails.

When this option is encountered, the option parser immediately aborts with the given parse error. If you simply want to output a message, use infoOption instead.

An option that always fails and displays a message.

A hidden "helper" option which always fails.

A common usage pattern is to apply this applicatively when creating a ParserInfo

opts :: ParserInfo Sample
opts = info (sample <**> helper) mempty

An option modifier.

Option modifiers are values that represent a modification of the properties of an option.

The type parameter a is the return type of the option, while f is a record containing its properties (e.g. OptionFields for regular options, FlagFields for flags, etc...).

An option modifier consists of 3 elements:

• A field modifier, of the form f a -> f a. These are essentially (compositions of) setters for some of the properties supported by f.
• An optional default value and function to display it.
• A property modifier, of the form OptProperties -> OptProperties. This is just like the field modifier, but for properties applicable to any option.

Modifiers are instances of Monoid, and can be composed as such.

One rarely needs to deal with modifiers directly, as most of the times it is sufficient to pass them to builders (such as strOption or flag) to create options (see Builder).

Specify a short name for an option.

Specify a long name for an option.

Specify the help text for an option.

Specify the help text for an option as a Doc value.

Specify a default value for an option.

Note: Because this modifier means the parser will never fail, do not use it with combinators such as some or many, as these combinators continue until a failure occurs. Careless use will thus result in a hang.

To display the default value, combine with showDefault or showDefaultWith.

Specify a function to show the default value for an option.

Show the default value for this option using its Show instance.

Specify a metavariable for the argument.

Metavariables have no effect on the actual parser, and only serve to specify the symbolic name for an argument to be displayed in the help text.

Specify the error to display when no argument is provided to this option.

Hide this option from the brief description.

Use internal to hide the option from the help text too.

Hide this option completely from the help text

Use hidden if the option should remain visible in the full description.

Apply a function to the option description in the usage text.

import Options.Applicative.Help
flag' () (short 't' <> style bold)

NOTE: This builder is more flexible than its name and example allude. One of the motivating examples for its addition was to use const to completely replace the usage text of an option.

Add a command to a subparser option.

Suggested usage for multiple commands is to add them to a single subparser. e.g.

sample :: Parser Sample
sample = subparser
       ( command "hello"
         (info hello (progDesc "Print greeting"))
      <> command "goodbye"
         (info goodbye (progDesc "Say goodbye"))
       )

Add a description to a group of commands.

Advanced feature for separating logical groups of commands on the parse line.

If using the same metavar for each group of commands, it may yield a more attractive usage text combined with hidden for some groups.

Add a list of possible completion values.

Add a bash completion action. Common actions include file and directory. See http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html#Programmable-Completion-Builtins for a complete list.

Add a completer to an argument.

A completer is a function String -> IO String which, given a partial argument, returns all possible completions for that argument.

Trivial option modifier.

An associative operation

NOTE: This method is redundant and has the default implementation mappend = (<>) since base-4.11.0.0. Should it be implemented manually, since mappend is a synonym for (<>), it is expected that the two functions are defined the same way. In a future GHC release mappend will be removed from Monoid.

A newtype over 'ReaderT String Except', used by option readers.

Option reader based on the Read type class.

String Option reader.

Polymorphic over the IsString type class since 0.14.

Convert a function producing a Maybe into a reader.

Convert a function producing an Either into a reader.

As an example, one can create a ReadM from an attoparsec Parser easily with

import qualified Data.Attoparsec.Text as A
import qualified Data.Text as T
attoparsecReader :: A.Parser a -> ReadM a
attoparsecReader p = eitherReader (A.parseOnly p . T.pack)

Null Option reader. All arguments will fail validation.

Abort option reader by exiting with a ParseError.

Abort option reader by exiting with an error message.

Create a ParserInfo given a Parser and a modifier.

A full description for a runnable Parser for a program.

Modifier for ParserInfo.

Show a full description in the help text of this parser (default).

Only show a brief description in the help text of this parser.

Specify a header for this parser.

Specify a header for this parser as a Doc value.

Specify a footer for this parser.

Specify a footer for this parser as a Doc value.

Specify a short program description.

Specify a short program description as a Doc value.

Specify an exit code if a parse error occurs.

Disable parsing of regular options after arguments. After a positional argument is parsed, all remaining options and arguments will be treated as a positional arguments. Not recommended in general as users often expect to be able to freely intersperse regular options and flags within command line options.

Intersperse matched options and arguments normally, but allow unmatched options to be treated as positional arguments. This is sometimes useful if one is wrapping a third party cli tool and needs to pass options through, while also providing a handful of their own options. Not recommended in general as typos by the user may not yield a parse error and cause confusion.

Run a program description.

Parse command line arguments. Display help text and exit if any parse error occurs.

Run a program description with custom preferences.

The most general way to run a program description in pure code.

Extract the actual result from a ParserResult value.

This function returns Nothing in case of errors. Possible error messages or completion actions are simply discarded.

If you want to display error messages and invoke completion actions appropriately, use handleParseResult instead.

Handle ParserResult.

Generate a ParserFailure from a ParseError in a given Context.

This function can be used, for example, to show the help text for a parser:

handleParseResult . Failure $ parserFailure pprefs pinfo ShowHelpText mempty

Create a ParserPrefs given a modifier

Global preferences for a top-level Parser.

Include a suffix to attach to the metavar when multiple values can be entered.

Turn on disambiguation.

See https://github.com/pcapriotti/optparse-applicative#disambiguation

Show full help text on any error.

Show the help text if the user enters only the program name or subcommand.

This will suppress a "Missing:" error and show the full usage instead if a user just types the name of the program.

Turn off backtracking after subcommand is parsed.

Allow full mixing of subcommand and parent arguments by inlining selected subparsers into the parent parser.

NOTE: When this option is used, preferences for the subparser which effect the parser behaviour (such as noIntersperse) are ignored.

Set the maximum width of the generated help text.

Show equals sign, rather than space, in usage and help text for options with long names.

Show global help information in subparser usage.

Default preferences.

A shell complete function.

Smart constructor for a Completer

Create a Completer from an IO action

Create a Completer from a constant list of strings.

Run a compgen completion action.

Common actions include file and directory. See http://www.gnu.org/software/bash/manual/html_node/Programmable-Completion-Builtins.html#Programmable-Completion-Builtins for a complete list.

Result of execParserPure.