Fallback to environment variable

Arguments can be marked to fall back to environment variable if no value is provided on the command line. This allows to implement convenient fallback mechanisms (such as automatically picking up the username) or 12 Factor Apps.

Example:

import argparse;

version (Windows)
    immutable UserVariable = "USERNAME";
else
    immutable UserVariable = "USER";

struct T
{
    @(PositionalArgument.EnvFallback("XDG_RUNTIME_DIR"))
    string dir;

    @(NamedArgument.EnvFallback(UserVariable))
    string user;
}

// This can be called argument-less on most Linux machines as both variables
// will be set. Calling with `./myprog some/path` will use `some/path` for `dir`,
// and `--user username` will take precedence over the `$USER` or `%USERNAME%
// environment variable.

Last modified: 26 August 2025