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Bash Completion

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Introduction

optparse-applicative has built-in support for bash completion of command line options and arguments. Any parser, when run using execParser (and similar functions), is automatically extended with a few (hidden) options for bash completion:

  • --bash-completion-script: this takes the full path of the program as argument, and prints a bash script, which, when sourced into a bash session, will install the necessary machinery to make bash completion work. For a quick test, you can run something like (for a program called foo on the PATH):

    $ source <(foo --bash-completion-script `which foo`)

    Normally, the output of --bash-completion-script should be shipped with the program and copied to the appropriate directory (usually /etc/bash_completion.d/) during installation.

  • --bash-completion-index, --bash-completion-word: internal options used by the completion script to obtain a list of possible completions for a given command line.

Customizing completions

By default, options and commands are always completed. So, for example, if the program foo has an option with a long name output, typing

$ foo --ou<TAB>

will complete --output automatically.

Arguments (either of regular options, or top-level) are not completed by default. To enable completion for arguments, use one of the following modifiers on a regular option or argument:

  • completeWith: specifies a list of possible completions to choose from;
  • action: specifies a completion "action". An action dynamically determines a list of possible completions. A full list of actions can be found in the bash documentation;
  • completer: a completer is a function String -> IO [String], returning all possible completions for a given string. You can use this modifier to specify a custom completion for an argument.

Completion modifiers can be used multiple times: the resulting completions will call all of them and join the results.

How it works

When running a parser with execParser, the parser is extended with bashCompletionParser, which defines the above options.

When completion is triggered, the completion script calls the executable with the special --bash-completion-index and --bash-completion-word options.

The original parser is therefore run in completion mode, i.e. runParser is called on a different monad, which keeps track of the current state of the parser, and exits when all arguments have been processed.

The completion monad returns, on failure, either the last state of the parser (if no option could be matched), or the completer associated to an option (if it failed while fetching the argument for that option).

From that we generate a list of possible completions, and print them to standard output. They are then read by the completion script and put into the COMPREPLY variable.

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