Rulz Programming Language

Rulz and Rulz Language is Copyright © G.A.Jennings, 2015-2022.

This is language specification 2-1/2; last updated 24 June 2022.

This document is slighty out of date.


Subroutines are a set of rules—a Rule. Subroutines are defined only (there is no need to declare them). Subroutine names are one or more lowercase letters within square brackets.

Say you did not like the ^ operator, and you like Perl. You can do this.


Which shows that say is now a print function and that subroutine arguments pass as the parameter array, and that ^ echos all its arguments similar to Perl. (See #Subroutine Parameters next.)

Rulz' $0 (which is not like Perl's) is used as a return value for almost all operators and builtins; and $_ (which is like Perl's) for many default arguments—but not all.

A subroutine is called just like any builtin, followed by any arguments which are stored in the special parameter variable @_ as a list. Parameters are passed by copy.

product 2 4 6 8           ; call subroutine with four numbers
^                         ; print result (in $0)

[product]                 ; subroutine name
= 1                       ; set $0 to 1
@for @_                   ; each argument value in $_
*= $_                     ; result stays in $0

A subroutine ends at the end of the file or with any following subroutine definition. The loop termination operator, @end, is not needed if it would be the last operator of a subroutine.[Which is how all block end operaters work.]

The parameter variables are unique and no others starts with @—it's syntax was stolen from Perl just for the parameter arguments.

Currently there is no return value of subroutines, though any variable will do. The "return value" is generally by $0, as in the example, but since all variables are global (except as defined below) any variable can be used.

product 2 4 6 8
^ $p
= p 1
@for @_
*= p $_                   ; result in $p

Like Perl, arguments are passed as an array, with multiple arguments concatenated.

product (2,4,6,8)
product (2,4) 6 8
= b (6,8)
product (2,4) $b

The Rulz convention for subroutine names is "the shorter the better"; which, for short Rulz programs, "p" would be used rather than "product".

Redefining a subroutine is not an error; it wil simply overwrite the existing one.

Subroutine Parameters

@_                      list of parameters
@#                      count of parameters
@*                      all parameters expanded to a string
@0                      first parameter
@1                      second parameter, etc., with @9 the limit
@A                      first parameter
@B                      second parameter, etc., with @Z the limit

The special variable $@ is an alias to @*.[Which is only useful for in an interpolated string and might go away.]

Subroutines with or without any parameters, whether expecting them or not. All of the following would print nothing (but a newline) if called with no arguments.

^@0 @1 @2

The print operator sees NULL and empty lists as empty strings.

For Rulz any variables not defined are read as NULL. (And NULL is always converted to the "zero" or "empty" value for other type; which are false, 0, 0.0, "" and () for boolean, integer, float, string and list respectively.)

Subroutine Attributes

Subroutine attributes are for saving special variables, "private" variables (same as Perl's "my"), "local" variables (same as "static" in PHP) and named arguments.

Attributes have the format id:var[=value][,var[=value]...] and follow the subroutine name separated by spaces.


The $_ special variable is automatically saved and restored. Other special variables are saved by the s: attribute. This attribute does not separate it's arguments with commas (so to allow the saving of $,). (See RulzSplat.)

sub foo bar
[sub s:.]
= . "\r\n"
^ @0

That subroutine changes the line termination special variable, $., temporarily.


Private variables, Perl's "my", is implemented as p:.

[sub p:p]
= p 1
@for @_
*= p $_
= $p

There can be multiple variables per attribute and optionally can be set to a default value. (The default is NULL.)

[p p:a,b=1]


A Local variable, like a PHP static varibale, which retains it's value between subroutine calls, is defined by l:.[That is the letter "el", not the number "1". It is, frankly, really bad of font designers to allow two different charcaters to be nearly indistinguishable—in some fonts they differ by a single pixel.]

@for (1..4)
[sub l:s]
+= s
^ $s
$ ./rulz local.rul

Subroutines can recurse.

[rsub l:s=1]
+= s
>? $s 4 ..
^ $s


Named subroutine arguments are designated by a:.

sub foo
[sub a:s]

Assigning a value make the argument default (if not provided to the subroutine) no longer NULL but the attribute value.

exit                  ; "normal" exit
exit 2                ; "error" exit
[exit a:e=0]
.^ $e

The arguments are positional, not sorted.

Multiple Entry Points

It is possible to enter a subroutine at any point. However, an entry point into the body of a loop currently results in undefined behavior.

Subroutine entry occurs by the jump operator along with a subroutine name index, just like a string or array index—but only syntactically.

. sub[1]

A subroutine call, a subroutine name as an operator (first in a rule), is basically shorthand for a jump to a subroutine's 0th entry point.

. sub[0]

Jumping past the end of a subroutine is a no-op.

Rulz File Format

There are three file "forms" for a Rulz file. The first is with one or more rules.

# form one
^ just some rules

The second contains rules followed by one or more subroutines.

# form two
^ in subroutine

In this form all rules before any subroutine(s) are call "Top Rules".

The final form contains just subroutines and are useful as loadable libraries. Given this file:

# lib.rul - an API
^ sub

This Rule:

.< lib.rul

Prints "sub".



Special Subroutines

Like Perl there are some special subroutines that are run internally.

For the _sub subroutine to run the $SUB Rulz variable needs to be set to true (default is false), which is:

= SUB 1

Unlike Perl there can be only one of those subroutines per program.

A _stop is not really a subroutine, of course; it just has the same format.

that line and all others disappear into...

(It just tells the load function to immediately stop.)

The begin routine disappears after being run.

Data Subroutines

These special subroutines, which are processed after a progam is loaded but before any Top Rules are run, are for creating data—not really subroutines, they just share the syntax.

^ $help
[_text help]
Program xyz usage:

xyy [options] <file>


While something like a "help text" output for a program is trivial no matter the way it is done—a number of print statements or a here-doc for example— if variables are not needed, the _text way is nearly the simplest way to display some text.

^ $var            ; prints (1,2,"a")
[_list var]
^ FOO BAR	        ; prints 10 20
FOO 10
BAR 20
^ FOO BAR         ; prints 1 2

Any leading and trailing whitespace, blank lines and comments are ignored.

None of those are retained after the data are created.