argparse 0.5.0

Parser for command-line arguments

argparse is a self-contained flexible utility to parse command line arguments that can work at compile-time.

NOTICE: The API is not finalized yet so there might be backward incompatible changes until 1.0 version. Please refer to [releases](https://github.com/andrey-zherikov/argparse/releases) for breaking changes.

Getting started

Here is the simple example showing the usage of argparse utility. It uses the basic approach when all members are considered arguments with the same name as the name of member:

import argparse;

static struct Basic
{
  // Basic data types are supported:
    // --name argument
    string name;
  
    // --number argument
    int number;
  
    // --boolean argument
    bool boolean;

  // Argument can have default value if it's not specified in command line
    // --unused argument
    string unused = "some default value";


  // Enums are also supported
    enum Enum { unset, foo, boo }
    // --choice argument
    Enum choice;

  // Use array to store multiple values
    // --array argument
    int[] array;

  // Callback with no args (flag)
    // --callback argument
    void callback() {}

  // Callback with single value
    // --callback1 argument
    void callback1(string value) { assert(value == "cb-value"); }

  // Callback with zero or more values
    // --callback2 argument
    void callback2(string[] value) { assert(value == ["cb-v1","cb-v2"]); }
}

// This mixin defines standard main function that parses command line and calls the provided function:
mixin Main.parseCLIArgs!(Basic, (args)
{
  // 'args' has 'Baisc' type
  static assert(is(typeof(args) == Basic));

  // do whatever you need
  import std.stdio: writeln;
  args.writeln;
  return 0;
});

If you run the program above with -h argument then you'll see the following output:

usage: hello_world [--name NAME] [--number NUMBER] [--boolean [BOOLEAN]] [--unused UNUSED] [--choice {unset,foo,boo}] [--array ARRAY ...] [--callback] [--callback1 CALLBACK1] [--callback2 [CALLBACK2 ...]] [-h]

Optional arguments:
  --name NAME
  --number NUMBER
  --boolean [BOOLEAN]
  --unused UNUSED
  --choice {unset,foo,boo}

  --array ARRAY ...
  --callback
  --callback1 CALLBACK1

  --callback2 [CALLBACK2 ...]

  -h, --help              Show this help message and exit

Parser can even work at compile time, so you can do something like this:

enum values = ([
  "--boolean",
  "--number","100",
  "--name","Jake",
  "--array","1","2","3",
  "--choice","foo",
  "--callback",
  "--callback1","cb-value",
  "--callback2","cb-v1","cb-v2",
].parseCLIArgs!Basic).get;

static assert(values.name     == "Jake");
static assert(values.unused   == Basic.init.unused);
static assert(values.number   == 100);
static assert(values.boolean  == true);
static assert(values.choice   == Basic.Enum.foo);
static assert(values.array    == [1,2,3]);

For more sophisticated CLI usage, argparse provides few UDAs:

static struct Extended
{
    // Positional arguments are required by default
    @PositionalArgument(0)
    string name;

    // Named arguments can be attributed in bulk
    @NamedArgument
    {
        string unused = "some default value";
        int number;
        bool boolean;
    }

    // Named argument can have custom or multiple names
        @NamedArgument("apple","appl")
        int apple;
    
        @NamedArgument(["b","banana","ban"])
        int banana;
}

// This mixin defines standard main function that parses command line and calls the provided function:
mixin Main.parseCLIArgs!(Extended, (args)
{
	// 'args' has 'Extended' type
	static assert(is(typeof(args) == Extended));

	// do whatever you need
	import std.stdio: writeln;
	args.writeln;
	return 0;
});

If you run it with -h argument then you'll see the following:

usage: hello_world name [--unused UNUSED] [--number NUMBER] [--boolean [BOOLEAN]] [--apple APPLE] [-b BANANA] [-h]

Required arguments:
  name

Optional arguments:
  --unused UNUSED
  --number NUMBER
  --boolean [BOOLEAN]
  --apple APPLE
  -b BANANA, --banana BANANA, --ban BANANA

  -h, --help              Show this help message and exit

Features

• Positional arguments:
  • Automatic type conversion of the value.
  • Required by default, can be marked as optional.
• Named arguments:
  • Short and long names (-v, --verbose).
  • Multiple names are supported.
  • Case sensitive/insensitive parsing.
  • Bundling of short names (-vvv is same as -v -v -v).
  • Equals sign accepted (-v=debug, --verbose=debug).
  • Automatic type conversion of the value.
  • Optional by default, can be marked as required.
• User-defined conversion of argument value (string -> destination type).
• User-defined validation of argument value:
  • On raw (string) data (i.e. before parsing).
  • On parsed data (i.e. after parsing).
• Parsing of known arguments only (returning not recognized ones).
• Options terminator (e.g. parsing up to -- leaving any argument specified after it).
• Support different types of destination data member:
  • Scalar (e.g. int, float, bool).
  • String arguments.
  • Enum arguments.
  • Array arguments.
  • Hash (associative array) arguments.
  • Callbacks.
• Built-in reporting of error happened during argument parsing.
• Built-in help generation

Usage

Calling the parser

There is a top-level function parseCLIArgs that parses the command line. It has the following signatures:

• ParseCLIResult parseCLIArgs(T)(ref T receiver, string[] args, in Config config = Config.init)

  Parameters:

  • receiver - the object that's populated with parsed values.
  • args - raw command line arguments.
  • config - settings that are used for parsing.

  Return value:

  An object that can be cast to bool to check whether the parsing was successful or not.
  

• Nullable!T parseCLIArgs(T)(string[] args, in Config config = Config.init)

  Parameters:

  • args - raw command line arguments.
  • config - settings that are used for parsing.

  Return value:

  If there is an error happened during the parsing then null is returned. Otherwise, an object of type T filled with values from the command line.

• int parseCLIArgs(T, FUNC)(string[] args, FUNC func, in Config config = Config.init, T initialValue = T.init)

  Parameters:

  • args - raw command line arguments.
  • func - function that's called with object of type T filled with data parsed from command line.
  • config - settings that are used for parsing.
  • initialValue - initial value for the object passed to func.

  Return value:

  If there is an error happened during the parsing then int.max is returned. In other case if func returns a value that can be cast to int then this value is returned. Otherwise, 0 is returned.

Usage example:

struct T
{
    @NamedArgument string a;
    @NamedArgument string b;
}

enum result1 = parseCLIArgs!T([ "-a", "A", "-b", "B"]);
assert(result1.get == T("A","B"));

If you want to parse multiple command lines into single object then you can do this easily:

T result2;
result2.parseCLIArgs([ "-a", "A" ]);
result2.parseCLIArgs([ "-b", "B" ]);
assert(result2 == T("A","B"));

You can even write your own main function that accepts

int my_main(T command)
{
    // do something
    return 0;
}

int main(string[] args)
{
    return args.parseCLIArgs!T(&my_main);
}

Partial parsing

Sometimes a program may only parse a few of the command-line arguments, passing the remaining arguments on to another program. In these cases, parseCLIKnownArgs function can be used. It works much like parseCLIArgs except that it does not produce an error when extra arguments are present. It has the following signatures:

• ParseCLIResult parseCLIKnownArgs(T)(ref T receiver, string[] args, out string[] unrecognizedArgs, in Config config = Config.init)

  Parameters:

  • receiver - the object that's populated with parsed values.
  • args - raw command line arguments.
  • unrecognizedArgs - raw command line arguments that were not parsed.
  • config - settings that are used for parsing.

  Return value:

  An object that can be cast to bool to check whether the parsing was successful or not.
  

• ParseCLIResult parseCLIKnownArgs(T)(ref T receiver, ref string[] args, in Config config = Config.init)

  Parameters:

  • receiver - the object that's populated with parsed values.
  • args - raw command line arguments that are modified to have parsed arguments removed.
  • config - settings that are used for parsing.

  Return value:

  An object that can be cast to bool to check whether the parsing was successful or not.
  

• Nullable!T parseCLIKnownArgs(T)(ref string[] args, in Config config = Config.init)

  Parameters:

  • args - raw command line arguments that are modified to have parsed arguments removed.
  • config - settings that are used for parsing.

  Return value:

  If there is an error happened during the parsing then null is returned. Otherwise, an object of type T filled with values from the command line.

• int parseCLIKnownArgs(T, FUNC)(string[] args, FUNC func, in Config config = Config.init, T initialValue = T.init)

  Parameters:

  • args - raw command line arguments.
  • func - function that's called with object of type T filled with data parsed from command line and the unrecognized arguments having the type of string[].
  • config - settings that are used for parsing.
  • initialValue - initial value for the object passed to func.

  Return value:

  If there is an error happened during the parsing then int.max is returned. In other case if func returns a value that can be cast to int then this value is returned. Otherwise, 0 is returned.

Usage example:

struct T
{
    @NamedArgument string a;
}

auto args = [ "-a", "A", "-c", "C" ];

assert(parseCLIKnownArgs!T(args).get == T("A"));
assert(args == ["-c", "C"]);

Wrappers for main function

argparse offers two mixins for convenience that one can use to wrap custom main function that accepts parameter object instead of raw command line in the form of string[]:

• Main.parseCLIKnownArgs(TYPE, alias newMain, Config config = Config.init)
• Main.parseCLIArgs(TYPE, alias newMain, Config config = Config.init)

They define standard main function that calls corresponding parsing function for arguments in TYPE type providing newMain callback that is called upon successful parsing.

Positional arguments

Positional arguments are expected to be at a specific position within the command line. This argument can be declared using PositionalArgument UDA:

struct Params
{
    @PositionalArgument(0)
    string firstName;

    @PositionalArgument(0, "lastName")
    string arg;
}

Parameters of PositionalArgument UDA:

#	Name	Type	Optional/ Required	Description
1	position	uint	required	Zero-based unsigned position of the argument.
2	name	string	optional	Name of this argument that is shown in help text. If not provided then the name of data member is used.

Named arguments

As an opposite to positional there can be named arguments (they are also called as flags or options). They can be declared using NamedArgument UDA:

struct Params
{
    @NamedArgument
    string greeting;

    @NamedArgument(["name", "first-name", "n"])
    string name;

    @NamedArgument("family", "last-name")
    string family;
}

Parameters of NamedArgument UDA:

#	Name	Type	Optional/ Required	Description
1	name	string or string[]	optional	Name(s) of this argument that can show up in command line.

Named arguments might have multiple names, so they should be specified either as an array of strings or as a list of parameters in NamedArgument UDA. Argument names can be either single-letter (called as short options) or multi-letter (called as long options). Both cases are fully supported with one caveat: if a single-letter argument is used with a double-dash (e.g. --n) in command line then it behaves the same as a multi-letter option. When an argument is used with a single dash then it is treated as a single-letter argument.

The following usages of the argument in the command line are equivalent: --name John, --name=John, --n John, --n=John, -nJohn, -n John. Note that any other character can be used instead of = - see Config for details.

Trailing arguments

A lone double-dash terminates argument parsing by default. It is used to separate program arguments from other parameters (e.g., arguments to be passed to another program). To store trailing arguments simply add a data member of type string[] with TrailingArguments UDA:

struct T
{
    @NamedArgument  string a;
    @NamedArgument  string b;

    @TrailingArguments string[] args;
}

static assert(["-a","A","--","-b","B"].parseCLIArgs!T.get == T("A","",["-b","B"]));

Note that any other character sequence can be used instead of -- - see Config for details.

Optional and required arguments

Arguments can be marked as required or optional by adding Required() or .Optional() to UDA. If required argument is not present parser will error out. Positional agruments are required by default.

struct T
{
    @(PositionalArgument(0, "a").Optional())
    string a = "not set";

    @(NamedArgument.Required())
    int b;
}

static assert(["-b", "4"].parseCLIArgs!T.get == T("not set", 4));

Limit the allowed values

In some cases an argument can receive one of the limited set of values so AllowedValues can be used here:

struct T
{
    @(NamedArgument.AllowedValues!(["apple","pear","banana"]))
    string fruit;
}

static assert(["--fruit", "apple"].parseCLIArgs!T.get == T("apple"));
static assert(["--fruit", "kiwi"].parseCLIArgs!T.isNull);              // "kiwi" is not allowed

For the value that is not in the allowed list, this error will be printed:

Error: Invalid value 'kiwi' for argument '--fruit'.
Valid argument values are: apple,pear,banana

Note that if the type of destination variable is enum then the allowed values are automatically limited to those listed in the enum.

Help generation

Command

To customize generated help text one can use Command UDA that receives optional parameter of a program name. If this parameter is not provided then Runtime.args[0] is used. Additional parameters are also available for customization:

• Usage - allows custom usage text. By default, the parser calculates the usage message from the arguments it contains but this can be overridden with Usage call. If the custom text contains %(PROG) then it will be replaced by the program name.
• Description - used to provide a brief description of what the program does and how it works. In help messages, the description is displayed between the usage string and the list of the arguments.
• Epilog - custom text that is printed after the list of the arguments.

Argument

There are some customizations supported on argument level for both PositionalArgument and NamedArgument UDAs:

• Description - provides brief description of the argument. This text is printed next to the argument in the argument list section of a help message.
• HideFromHelp - can be used to indicate that the argument shouldn't be printed in help message.
• Placeholder - provides custom text that it used to indicate the value of the argument in help message.

Example

Here is an example of how this customization can be used:

@(Command("MYPROG")
 .Description("custom description")
 .Epilog("custom epilog")
)
struct T
{
  @NamedArgument  string s;
  @(NamedArgument.Placeholder("VALUE"))  string p;

  @(NamedArgument.HideFromHelp())  string hidden;

  enum Fruit { apple, pear };
  @(NamedArgument("f","fruit").Required().Description("This is a help text for fruit. Very very very very very very very very very very very very very very very very very very very long text")) Fruit f;

  @(NamedArgument.AllowedValues!([1,4,16,8])) int i;

  @(PositionalArgument(0).Description("This is a help text for param0. Very very very very very very very very very very very very very very very very very very very long text")) string param0;
  @(PositionalArgument(1).AllowedValues!(["q","a"])) string param1;

  @TrailingArguments string[] args;
}

parseCLIArgs!T(["-h"]);

This example will print the following help message:

usage: MYPROG [-s S] [-p VALUE] -f {apple,pear} [-i {1,4,16,8}] [-h] param0 {q,a}

custom description

Required arguments:
  -f {apple,pear}, --fruit {apple,pear}
                          This is a help text for fruit. Very very very very
                          very very very very very very very very very very
                          very very very very very long text
  param0                  This is a help text for param0. Very very very very
                          very very very very very very very very very very
                          very very very very very long text
  {q,a}

Optional arguments:
  -s S
  -p VALUE
  -i {1,4,16,8}
  -h, --help              Show this help message and exit

custom epilog

Supported types

Boolean

Boolean types usually represent command line flags. argparse supports multiple ways of providing flag value:

struct T
{
    @NamedArgument bool b;
}

static assert(["-b"]        .parseCLIArgs!T.get == T(true));
static assert(["-b","true"] .parseCLIArgs!T.get == T(true));
static assert(["-b","false"].parseCLIArgs!T.get == T(false));
static assert(["-b=true"]   .parseCLIArgs!T.get == T(true));
static assert(["-b=false"]  .parseCLIArgs!T.get == T(false));

Numeric

Numeric arguments are converted using std.conv.to:

struct T
{
    @NamedArgument  int i;
    @NamedArgument  uint u;
    @NamedArgument  double d;
}

static assert(["-i","-5","-u","8","-d","12.345"].parseCLIArgs!T.get == T(-5,8,12.345));

String

argparse supports string arguments as pass trough:

struct T
{
    @NamedArgument  string a;
}

static assert(["-a","foo"].parseCLIArgs!T.get == T("foo"));

Enum

If an argument is bound to an enum, an enum symbol as a string is expected as a value, or right within the argument separated with an "=" sign:

struct T
{
    enum Fruit { apple, pear };

    @NamedArgument Fruit a;
}

static assert(["-a","apple"].parseCLIArgs!T.get == T(T.Fruit.apple));
static assert(["-a=pear"].parseCLIArgs!T.get == T(T.Fruit.pear));

Counter

Counter argument is the parameter that tracks the number of times the argument occurred on the command line:

struct T
{
    @(NamedArgument.Counter()) int a;
}

static assert(["-a","-a","-a"].parseCLIArgs!T.get == T(3));

Array

If an argument is bound to 1D array, a new element is appended to this array each time the argument is provided in command line. In case if an argument is bound to 2D array then new elements are grouped in a way as they appear in command line and then each group is appended to this array:

struct T
{
    @NamedArgument int[]   a;
    @NamedArgument int[][] b;
}

static assert(["-a","1","2","3","-a","4","5"].parseCLIArgs!T.get.a == [1,2,3,4,5]);
static assert(["-b","1","2","3","-b","4","5"].parseCLIArgs!T.get.b == [[1,2,3],[4,5]]);

Alternatively you can set Config.arraySep to allow multiple elements in one parameter:

struct T
{
    @NamedArgument int[] a;
}

Config cfg;
cfg.arraySep = ',';

assert(["-a","1,2,3","-a","4","5"].parseCLIArgs!T(cfg).get == T([1,2,3,4,5]));

Specifying number of values

In case the argument is bound to static array then the maximum number of values is set to the size of the array. For dynamic array, the number of values is not limited. The minimum number of values is 1 in all cases. This behavior can be customized by calling the following functions:

• NumberOfValues(ulong min, ulong max) - sets both minimum and maximum number of values.
• NumberOfValues(ulong num) - sets both minimum and maximum number of values to the same value.
• MinNumberOfValues(ulong min) - sets minimum number of values.
• MaxNumberOfValues(ulong max) - sets maximum number of values.

struct T
{
  @(NamedArgument.NumberOfValues(1,3))
  int[] a;
  @(NamedArgument.NumberOfValues(2))
  int[] b;
}

assert(["-a","1","2","3","-b","4","5"].parseCLIArgs!T.get == T([1,2,3],[4,5]));
assert(["-a","1","-b","4","5"].parseCLIArgs!T.get == T([1],[4,5]));

Associative array

If an argument is bound to an associative array, a string of the form "name=value" is expected as the next entry in command line, or right within the option separated with an "=" sign:

struct T
{
    @NamedArgument int[string] a;
}

static assert(["-a=foo=3","-a","boo=7"].parseCLIArgs!T.get.a == ["foo":3,"boo":7]);

Alternatively you can set Config.arraySep to allow multiple elements in one parameter:

struct T
{
    @NamedArgument int[string] a;
}

Config cfg;
cfg.arraySep = ',';

assert(["-a=foo=3,boo=7"].parseCLIArgs!T(cfg).get.a == ["foo":3,"boo":7]);
assert(["-a","foo=3,boo=7"].parseCLIArgs!T(cfg).get.a == ["foo":3,"boo":7]);

In general, the keys and values can be of any parsable types.

Callback

An argument can be bound to a function with one of the following signatures (return value, if any, is ignored):

• ... function()

In this case, the argument is treated as a flag and the function is called every time when the argument is seen in command line.

• ... function(string)

In this case, the argument has exactly one value and the function is called every time when the argument is seen in command line and the value specified in command line is provided into string parameter.

• ... function(string[])

In this case, the argument has zero or more values and the function is called every time when the argument is seen in command line and the set of values specified in command line is provided into string[] parameter.

• ... function(RawParam)

In this case, the argument has one or more values and the function is called every time when the argument is seen in command line and the set of values specified in command line is provided into parameter.

static struct T
{
    int a;

    @(NamedArgument("a")) void foo() { a++; }
}

static assert(["-a","-a","-a","-a"].parseCLIArgs!T.get.a == 4);

Parsing customization

Some time the functionality provided out of the box is not enough and it needs to be tuned.

Parsing of a command line string values into some typed receiver member consists of multiple steps:

• Pre-validation - argument values are validated as raw strings.
• Parsing - raw argument values are converted to a different type (usually the type of the receiver).
• Validation - converted value is validated.
• Action - depending on a type of the receiver, it might be either assignment of converted value to a receiver, appending value if receiver is an array or other operation.

In case if argument does not expect any value then the only one step is involved:

• Action if no value - similar to Action step above but without converted value.

If any of the steps fails then the command line parsing fails as well.

Each of the step above can be customized with UDA modifiers below. These modifiers take a function that might accept either argument value(s) or Param struct that has these fields (there is also an alias, RawParam, where the type of the value field is string[]):

• config- Config object that is passed to parsing function.
• name - Argument name that is specified in command line.
• value - Array of argument values that are provided in command line.

Pre-validation

PreValidation modifier can be used to customize the validation of raw string values. It accepts a function with one of the following signatures:

• bool validate(string value)
• bool validate(string[] value)
• bool validate(RawParam param)

The function should return true if validation passed and false otherwise.

Parsing

Parse modifier allows providing custom conversion from raw string to typed value. It accepts a function with one of the following signatures:

• ParseType parse(string value)
• ParseType parse(string[] value)
• ParseType parse(RawParam param)
• bool parse(ref ParseType receiver, RawParam param)
• void parse(ref ParseType receiver, RawParam param)

Parameters:

• ParseType is a type that the string value will be parsed to.
• value/param values to be parsed.
• receiver is an output variable for parsed value.

Parse function is supposed to parse values from value/param parameter into ParseType type and optionally return boolean type indicating whether parsing was done successfully (true) or not (false).

Validation

Validation modifier can be used to validate the parsed value. It accepts a function with one of the following signatures:

• bool validate(ParseType value)
• bool validate(ParseType[] value)
• bool validate(Param!ParseType param)

Parameters:

• value/param has a value returned from Parse step.

The function should return true if validation passed and false otherwise.

Action

Action modifier allows providing a custom logic of how receiver should be changed when argument has a value in command line. It accepts a function with one of the following signatures:

• bool action(ref T receiver, ParseType value)
• void action(ref T receiver, ParseType value)
• bool action(ref T receiver, Param!ParseType param)
• void action(ref T receiver, Param!ParseType param)

Parameters:

• receiver is a receiver (destination field) which is supposed to be changed based on a value/param.
• value/param has a value returned from Parse step.

Arguments with no values

Sometimes arguments are allowed to have no values in command line. Here are two cases that arise in this situation:

• Argument should get specific default value if there is no value provided in command line. AllowNoValue modifier should be used in this case.
• Argument must not have any values in command line. In this case RequireNoValue modifier should be used.

Both AllowNoValue and RequireNoValue modifiers accept a value that should be used when no value is provided in command line. The difference between them can be seen in this example:

    struct T
    {
        @(NamedArgument.AllowNoValue  !10) int a;
        @(NamedArgument.RequireNoValue!20) int b;
    }

    assert(["-a"].parseCLIArgs!T.get.a == 10);       // use value from UDA
    assert(["-b"].parseCLIArgs!T.get.b == 20);       // use value from UDA
    assert(["-a", "30"].parseCLIArgs!T.get.a == 30); // providing value is allowed
    assert(["-b", "30"].parseCLIArgs!T.isNull);      // providing value is not allowed

Usage example

All the above modifiers can be combined in any way:

    struct T
    {
        @(NamedArgument
         .PreValidation!((string s) { return s.length > 1 && s[0] == '!'; })
         .Parse        !((string s) { return s[1]; })
         .Validation   !((char v) { return v >= '0' && v <= '9'; })
         .Action       !((ref int a, char v) { a = v - '0'; })
        )
        int a;
    }

    static assert(["-a","!4"].parseCLIArgs!T.get.a == 4);

Config

Assign character

Config.assignChar - the assignment character used in arguments with value: -a=5, -b=foo.

Default is equal sign =.

Array separator

Config.arraySep - when set to char.init, value to array and associative array receivers are treated as an individual value. That is, only one argument is appended inserted per appearance of the argument. If arraySep is set to something else, then each value is first split by the separator, and the individual pieces are treated as values to the same argument.

Default is char.init.

struct T
{
    @NamedArgument string[] a;
}

assert(["-a","1,2,3","-a","4","5"].parseCLIArgs!T.get == T(["1,2,3","4","5"]));

Config cfg;
cfg.arraySep = ',';

assert(["-a","1,2,3","-a","4","5"].parseCLIArgs!T(cfg).get == T(["1","2","3","4","5"]));

Named argument character

Config.namedArgChar - the character that named arguments begin with.

Default is dash -.

End of arguments

Config.endOfArgs - the string that conventionally marks the end of all arguments.

Default is double-dash --.

Case sensitivity

Config.caseSensitive - by default argument names are case-sensitive. You can change that behavior by setting thia member to false.

Default is true.

Bundling of single-letter arguments

Config.bundling - when it is set to true, single-letter arguments can be bundled together, i.e. -abc is the same as -a -b -c.

Default is false.

Adding help generation

   Add a -h/--help option to the parser.
   Defaults to true.

Config.addHelp - when it is set to true then -h and --help arguments are added to the parser. In case if the command line has one of these arguments then the corresponding help text is printed and the parsing will be stopped. If parseCLIKnownArgs or parseCLIArgs is called with function parameter then this callback will not be called.

Default is true.

Error handling

Config.errorHandler - this is a handler function for all errors occurred during parsing the command line. It might be either a function or a delegate that takes string parameter which would be an error message.

The default behavior is to print error message to stderr.