ARGPARSE_REF.md

argparse reference

Parser operates with arguments and commands, organised in a hierarchy. It is possible to define multiple commands, or none. Parsing always starts with root command, named after init:get_argument(progname). Empty command produces empty argument map:

1> parse("", #{}).
#{}

Options specification

It's possible to override program name using progname option:

2> io:format(args:help(#{}, #{progname => "readme"})).
usage: readme

To override default optional argument prefix (-), use prefixes option:

3> args:parse(["+sbwt"], #{arguments => [#{name => mode, short => $s}]}, #{prefixes => "+"}).
#{mode => "bwt"}

To define a global default for arguments that are not required, use default option:

4> args:parse([], #{arguments => [#{name => mode, required => false}]}, #{default => undef}).
#{mode => undef}

When global default is not set, resulting argument map does not include keys for arguments that are not specified in the command line and there is no locally defined default value.

Validation, help & usage information

Function validate/1 may be used to validate command with all sub-commands and options without actual parsing done.

4> args:validate(#{arguments => [#{short => $4}]}).
** exception error: {args,{invalid_option,["erl"],
    [],name,"argument must be a map, and specify 'name'"}}

Human-readable errors and usage information is accessible via help/2 and format_error/1,2.

Return value

If root level command does not contain any sub-commands, parser returns plain map of argument names to their values:

3> args:parse(["value"], #{arguments => [#{name => arg}]}).
#{arg => "value"}

This map contains all arguments matching command line passed, initialised with corresponding values. If argument is omitted, but default value is specified for it, it is added to the map. When no local default value specified, and argument is not present, corresponding key is not present in the map, unless there is a global default passed with parse/3 options.

Missing required (field required is set to true for optional arguments, or missing for positional) arguments raises an error.

When there are sub-commands, parser returns argument map, deepest matched command name, and a sub-spec passed for this command:

4> Cmd =  #{arguments => [#{name => arg}]}.
#{arguments => [#{name => arg}]}
5> args:parse(["cmd", "value"], #{commands => #{"cmd" => Cmd}}).
{#{arg => "value"},{"cmd",#{arguments => [#{name => arg}]}}}

Command specification

Command specification may contain following fields:

• commands - sub-commands, name to nested command spec
• arguments - list of argument specifications
• help - string to generate usage information, can be set to hidden, causing this command to be omitted in usage output
• handler - function expected to process this command, or optional atom
• user-defined fields

Missing handler field is automatically populated by CLI framework, when a module exporting cli/0 also exports function function with arity 1, named after command:

cli() -> #{commands => #{"run" => #{...}}}.

run(ArgMap) -> ....

If command contains sub-commands, and handler is not present, it is an error if arguments supplied to parse do not select one of the sub-commands. Set handler to optional to not require sub-command selected.

Command is matched as a positional argument, taking precedence over other positional arguments. For any positional argument found, parser will first attempt to match a sub-command in currently evaluated command. If there is no match, parser considers next argument as positional. Example:

1> Cmd = #{
    commands => #{sub => #{}},
    arguments => [#{name => positional}]
},
2> parse(["arg", "sub"], Cmd) == parse(["arg", "sub"], Cmd).
true

Command names cannot start with prefix character.

Argument specification

Every argument spec must have name field. Name defines key in the parsed arguments map, similar to dest field of Python library. It is allowed to have multiple arguments with the same name (option aliases):

    options => [
        #{name => foo, short => $f},
        #{name => foo, long => "-foo"},
        #{name => foo}
    ].

An argument can be:

• optional: matching command line arguments with prefix character
• positional: arguments not starting with a prefix

Negative number is considered positional argument, when - prefix is used, and there is no no optional argument spec that has short or long form defined as number (#{name => one, short => $1}).

Argument is optional when short or long field is defined. Short form may only contain a single character, and long form may contain any string. A single prefix character is automatically prepended to long forms, so for this example:

    #{name => myarg, short => $m, long => "-myarg"}

Argument myarg can be specified as -m or as --myarg. Please note that it is possible to have long forms with a single prefix character:

    #{name => kernel, long => "kernel"}

By default, optional arguments are not required and may be omitted from command line. Positional arguments are required. It is possible to override this behaviour by setting required field.

It is supported to specify long-form value using this syntax:

    mycli --arg=value

It is treated the same way as mycli --arg value.

For every argument matched, parser may consume several following positional arguments (not starting with a prefix). Analysis is based on nargs, action and type fields.

• when nargs is 'maybe' and next argument is positional, it gets consumed and produced as value, and if next argument starts with option prefix, default value is produced (when no default present, default value deduced from argument type)
• when nargs is {'maybe', Term}, and next argument starts with option prefix, Term is produced
• when nargs is set to a positive integer, parser tries to consume exactly this number of positional arguments, and fails with an error if there is not enough. Produced value is a list.
• when nargs is set to nonempty_list, parser consumes at least one following positional argument (fails if first argument is not positional), until it finds next optional argument. Produced value is a list.
• when nargs is list, parser consumes positional arguments until it finds next optional. Produced value is a list.
• when nargs is all, all remaining arguments (whether positional or not) are folded into single produced list.

When nargs is not specified, action determines how many arguments are consumed. For store/append, single argument is consumed. Special handling is done for boolean type: no argument is consumed, even for store/append. If it is necessary to consume an argument for boolean type, use type => atom with choices => ["true", "false"].

Action can have following values:

• store - replace value in the argument map (last value wins)
• append - append value to existing list (when value is also a list, resulting value can be a list of lists)
• {store, Term} - do not consume argument, replace current value with Term
• {append, Term} - do not consume argument, append Term to list of existing values
• count - do not consume any arguments, bump the counter - useful for bumping logging verbosity level, e.g. cmd -vvv
• extend - valid only for nargs set to list/nonempty_list/all/(pos_integer()), explodes this list and appends every single value to existing list, so result is just a list, not a list of lists

Arguments have type associated. Default type is string, taken unprocessed from command line input.

• int, {int, Limits} - where Limits is a list containing {min, Int}, {max, Int} tuples
• float, {float, Limits} - same as int, but for floating point
• string, {string, Regex}, {string, Regex, RegexOption} - string validated with Regex
• binary, {binary, Regex}, {binary, Regex, RegexOption} - same as string, but of binary type
• atom - existing atom (error when atom does not exist)
• {atom, unsafe} - atom, creates new atoms when needed
• {custom, Fun} - custom type conversion from string() into term()

For int, float, string, binary and atom, it is also possible to specify list of choices that will pass validation:

    #{name => choices, short => $c, type => {string, ["one", "two", "three"]}}

Custom function may throw erlang:error(invalid_argument), to utilise built-in validation information. If any other exception is raised, it is passed with no conversion.

An error is thrown when argument cannot be converted to required type, or does not pass validation. If custom conversion function throws error(invalid_argument), exception is augmented with necessary parser state information (any other exception passes through unchanged).

It is not allowed to store user-defined fields in arguments.

Errors

Argparse throws exceptions of class error, and Reason tuple:

    {args, ArgParseError}

To get human-readable representation:

    try args:parse(Args, Command)
    catch error:{args, Reason} ->
        io:format(args:format_error(Reason, Command, #{}))
    end.

Help templates

To completely hide the argument or command from the output, supply help => hidden.

It is possible to override help text generated for arguments. By default, options are formatted with "help text, type, default", e.g.:

crawl [-s <shard>...] [-z <last>] [-c <choice>]

Optional arguments:
  -s     initial shards, int
  -z     last shard, between, 100 < int < 200, 150
  -c     tough choice, choice: 1, 2, 3

It is possible to override the description, and print it this way:

crawl [-s SHARD] [-c CHOICE]

Optional arguments:
  -s     initial number, int, with a default value of 0
  -z     150, number of last shard, unless overridden
  -c     custom parameter, to choose from 1, 2 or 3

Example:

  #{
    arguments => [
      #{
        name => shard,
        default => 0,
        help => {"[-s SHARD]", ["initial number, ", type, " with a default value of ", default]}}
    ]}

First element of the tuple replaces [-s <shard>] with [-s SHARD] in command line example, and second defines detailed help template.