argparse 0.1.0

Parser for command-line arguments

To use this package, run the following command in your project's root directory:

Manual usage
Put the following dependency into your project's dependences section:


Parser for command-line arguments

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

Getting started

Here is "Hello World" example showing the usage of this utility:

<!-- READMECONTENTBEGIN file=examples/hello_world.d -->

import argparse;

struct Params
    // Positional arguments are required by default
    string name;

    // Named argments are optional by default
    string unused = "some default value";

    // Numeric types are converted automatically
    int number;

    // Boolean flags are supported
    bool boolean;

    // Enums are also supported
    enum Enum { unset, foo, boo };
    Enum enumValue;

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

// Can even work at compile time
enum params = (["--flag","--num","100","Jake","--array","1","2","3","--enum","foo"].parseCLIArgs!Params).get;

static assert(      == "Jake");
static assert(params.unused    == Params.init.unused);
static assert(params.number    == 100);
static assert(params.boolean   == true);
static assert(params.enumValue ==;
static assert(params.array     == [1,2,3]);



  • 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).
  • Passing 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.


  • Help text: Automatic generation of help file during compile time.

    • Automatically generated with slight ability for customisation.

    • Works for the default command.

    • Works for exact matches for named commands.

    • Works for partial matches for named commands.

    • Arguments can be displayed in organised groups.

  • Provided help output

  • Generated help output for your program and sub-commands

  • Can be configured to suit your needs, such as disabling colored output.

  • Provided usage, help and version information.

  • Completely detached from core Program, giving you complete freedom in writing your own help output.

  • You can categorize commands for better help output

  • Validators

  • Provided validators for common cases: EnumValidator, FileSystemValidator and DelegateValidator



Calling the parser

There is a top-level function parseCLIArgs(T)(string[] args, Config config) that parses command line specified in args parameter and returns Nullable!T which is null if there is an error happened during parsing. Otherwise it returns object of type T filled with data from command line.

struct T
    @NamedArgument("a") string a;
    @NamedArgument("b") string b;

enum result = parseCLIArgs!T([ "-a", "A", "-b", "B"]);

assert(result.get == T("A","B"));

If you want to reuse the parser and parse multiple command lines then you can use CommandLineParser struct:

struct T
    @NamedArgument("a") string a;
    @NamedArgument("b") string b;

enum parser = CommandLineParser!T(Config.init);

T result;
parser.parseArgs(result, [ "-a", "A" ]);
parser.parseArgs(result, [ "-b", "B" ]);

assert(result == T("A","B"));

CommandLineParser.parseArgs returns false if there is an error happened during the parsing.

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(T)(ref string[] args, Config config) method can be used. It works much like parseCLIArgs() except that it does not produce an error when extra arguments are present. Instead, it removes parsed arguments from args parameter leaving remaining arguments.

struct T
    @NamedArgument("a") string a;
    @NamedArgument("b") string b;

enum args = [ "-a", "A", "-c", "C" ];
enum result = parseCLIKnownArgs!T();

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

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
    string firstName;

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

Parameters of PositionalArgument UDA:

1positionuintrequiredZero-based unsigned position of the argument.
2namestringoptionalName 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
    string greeting;

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

Parameters of NamedArgument UDA:

1namestring or string[]requiredName(s) of this argument that can show up in command line.

Named arguments might have multiple names, so they should be specified as an array of strings 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("a")  string a;
    @NamedArgument("b")  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";

    int b;

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

Supported types


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

struct T
    @NamedArgument("b") 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 arguments are converted using

struct T
    @NamedArgument("i")  int i;
    @NamedArgument("u")  uint u;
    @NamedArgument("d")  double d;

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


argparse supports string arguments as pass trough:

struct T
    @NamedArgument("a")  string a;

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


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("a")) Fruit a;

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


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

struct T
    @(NamedArgument("a").Counter()) int a;

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


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("a")) int[]   a;
    @(NamedArgument("b")) 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("a")) int[] a;

Config cfg;
cfg.arraySep = ',';

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

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("a")) 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("a")) 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.

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.


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.


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)


  • 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 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)


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

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


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)


  • receiver is a receiver (destination field that has @*Argument UDA) 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("a").AllowNoValue  !10) int a;
        @(NamedArgument("b").RequireNoValue!20) int b;

    static assert(["-a"].parseCLIArgs!T.get.a == 10);       // use value from UDA
    static assert(["-b"].parseCLIArgs!T.get.b == 20);       // use vlue from UDA
    static 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
         .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);


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("a")) 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.

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.

  • Andrey Zherikov
1.3.0 2023-May-10
1.2.0 2022-Sep-16
1.1.1 2022-Aug-29
1.1.0 2022-Aug-24
1.0.0 2022-Jun-09
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