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The Maentwrog Programming Language

This is the reference distribution for the Maentwrog programming language, designed and implemented in 1993.

This distribution contains the reference interpreter, written in ANSI C, a rudimentary specification (below) and some example Maentwrog programs. It also contains a few other miscellaneous contemporaneous C programs by the author.

The current released version of the Maentwrog distribution is version 1.1 revision 2014.0923.


This is a rudimentary specification for the Maentwrog programming language.

This information, and the example programs in this distribution, were taken from the wiki page for Maentwrog, written by User:Marinus and (like all esowiki articles) placed in the public domain. Thanks Marinus!


A Maentwrog program consists of a series of words, separated by whitespace. Words can contain any character except whitespace. The way these words are executed depends on the character they begin with.

  • A word that consists of digits is taken as an integer and pushed. A minus sign may be used to make negative numbers, so 25 and -14 are words that push 25 and -14 onto the stack. Extra characters at the end of the word are allowed, so 25abc and 25.14 both also push 25.
  • A word that isn't a number is treated as either a function or a variable. If the word is defined as a function, it is executed; if it's a variable, its current value is pushed to the stack. Using an undefined word results in an error, though this doesn't stop further execution of the program.
  • To define a word as a function, use the syntax : new-word words to execute ; (as in Forth). Redefining a word is not allowed, and neither are nested function definitions.
  • To define a word as a variable, use the syntax *varname. This must be done before using a variable.

Additionally, words (except number words) can take one of a list of prefixes, changing how the word is executed.

Prefix   Action               Example     Result
-----    ------               -------     ------
=        Assign to variable   =foo        A value popped from the stack is
                                          assigned to the variable foo.
@        If                   @bye        Pop value, stop the program (see
                                          predefined words) if it isn't 0.
[        While                [xyz        Pop value, if it's not 0 execute
                                          word xyz, then pop another value
                                          and do it again; continue until a
                                          0 is popped.
$        Repeat               $.          Pop value, then output that many
                                          values from the stack.

Predefined words

Word           Stack effect   Description
----           ------------   -----------
bye                           Stop program immediately.
rem ... ;                     Comment. (Ignore all words between rem and ;.)
: word ... ;                  Define a new word.
debug                         Turn on debugging (outputs all words executed).
vars                          Output a list of currently defined variables
                                and their values.
words                         Output a list of currently defined words.
alloc            n | ptr      Allocate memory for n C longs, returns a pointer.
free           ptr | -        Free memory at pointer.
size             - | n        Push stack size.
dup              a | a a      Duplicate top of stack.
swap           a b | b a      Swap the two topmost stack values.
pop              a | -        Remove top value from stack.
get            ptr | value    Push value at pointer to stack (C `*ptr`).
put            p v | -        Store value at pointer (C `*ptr = val`).
rnd              - | n        Push random value.
>              a b | (a>b)    Push 1 if a is greater than b, else 0.
<              a b | (a<b)    Push 1 if a is less than b, else 0.
==             undefined      Undefined.
.                n | -        Pop a value, output as integer, adding a newline.
..               n | -        Pop a value, output as an ASCII character.
mod            a b | (a%b)    Modulo.
+              a b | (a+b)    Addition.
-              a b | (a-b)    Subtraction.
*              a b | (a*b)    Multiplication.
/              a b | (a/b)    Division, result is rounded towards 0.