Parsing customization

Sometime the functionality that is provided out of the box is not enough and needs to be tuned. argparse allows customizing of every step of command line parsing:

Pre-validation – argument values are validated as raw strings.
Parsing – raw argument values are converted to a different type (usually the type of the receiving data member).
Validation – converted value is validated.
Action – depending on a type of the receiving data member, for example, it can be an assignment of converted value to a data member, or appending value if type is an array.

In case if argument does not have any value to parse, then the only one step is involved in parsing:

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

Each of the steps above can be customized with UDA modifiers below.

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)
Result validate(string value)
In this case, function will be called once for every value specified in command line for an argument in case of multiple values.
bool validate(string[] value)
Result validate(string[] value)
bool validate(RawParam param)
Result validate(RawParam param)

Parameters:

value/param values to be parsed.

Return value:

true/Result.Success if validation passed or
false/Result.Error otherwise.

Parsing

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

ParseType parse(string value)
ParseType parse(string[] value)
ParseType parse(RawParam param)
ParseType is a type that a string value is supposed to be parsed to and it is not required be the same as a type of destination - argparse tries to detect this type from provided function.
ParseType must not be immutable or const.
void parse(ref ParseType receiver, RawParam param)
bool parse(ref ParseType receiver, RawParam param)
Result parse(ref ParseType receiver, RawParam param)

Parameters:

value/param raw (string) values to be parsed.
receiver is an output variable for parsed value.

Return value for functions that return bool or Result (in other cases parsing is always considered successful):

true/Result.Success if parsing was successful or
false/Result.Error otherwise.

Validation

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

bool validate(ParseType value)
Result validate(ParseType value)
bool validate(Param!ParseType param)
Result validate(Param!ParseType param)

Parameters:

value/param contains a value returned from Parse step.

Return value:

true/Result.Success if validation passed or
false/Result.Error otherwise.

Action

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

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

Parameters:

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

Return value:

true/Result.Success if operation was successful or
false/Result.Error otherwise.

Example

All the above modifiers can be combined in any way:

import argparse;

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

T t;
assert(CLI!T.parseArgs(t, ["-a","!4"]));
assert(t == T(4));

Last modified: 24 October 2025