A program is a special function that is an entry point into Birch code from the command line. It cannot be called from other Birch code. Declare a program with:

program example(x:Boolean, y:Integer <- 0, message:String,
    long_name:Real) {
    // do something
where x, y, message, and long_name are program options, with y given a default value of zero. A program has no return value.

To call a program from the command line, use birch, followed by the program name, following by a list of program options:

birch example --message "Hello!" --long-name 10.0 -x true -y 10

Program options may be given in any order but must be named. The name is usually prefixed with double-dash (--), but may be prefixed with single-dash (-) if the name is a single character.

Whenever an underscore (_) appears in the name of a program option, it should be replaced with a dash (-) on the command line. This occurs in the example above, where the long_name program option becomes --long-name on the command line.

Program options may be of any type for which an assignment from type String has been declared. This includes all basic types.

If a program is called with an unrecognized option, an error is produced.