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PF23 - A PostScript-style stack language

PF23 is a stack-based language inspired by PostScript. It is designed to be simple, easy to implement, and easy to use.

Language

PF23 is working with a Stack, which is a LIFO (Last In First Out) data structure. The stack is composed of values, which can be boolean or integers. The stack is initialized with an empty stack.

Program structure

A PF23 program is a sequence of operators, functions call, if/else statements. Each element is, followed by a space, a tabulation or a new line. The arguments are pushed on the stack before the instruction is executed. The instruction is executed, and the result is pushed on the stack.

Operators, functions and conditions

Arithmetic operators

Operator Description Stack before Stack after
+ Addition a b a+b
- Substraction a b a-b
* Multiplication a b a*b
/ Division a b a/b

Comparison operators

Operator Description Stack before Stack after
= Equality a b a=b
<> Inequality a b a!=b
< Less than a b a<b
> Greater than a b a>b

Stack operators

Operator Description Stack before Stack after
DUP Duplicate the top of the stack a a a
DROP Remove the top of the stack a
SWAP Swap the two top elements of the stack a b b a
ROT Rotate the three top elements of the stack a b c b c a

Conditions

Operator Description Stack before Stack after
IF If the top of the stack is true, execute the next instruction a
THEN End of an if statement
ELSE If the top of the stack is false, execute the next instruction a
ENDIF End of an if/else statement

Some examples of if/else statements:

In this example, if 1 = 2 is true, 10 is pushed on the stack. Else, 0 is pushed on the stack. And in every case, 100 is pushed on the stack.

1 2 = IF
    10
ELSE
    0
ENDIF
100

We will have the following result:

0
100

In this example, if 1 = 2 is true, 10 is pushed on the stack. Else, nothing is pushed on the stack. And in every case, 100 is pushed on the stack.

1 2 = IF
    10
THEN
    100

We will have the following result:

100

Functions

Operator Description
: NAME Beginning of the definition of a function called NAME
; End of the definition of a function

Some examples of functions:

: ADD1
    1 +
;

: FIB 
  DUP 1 > IF 
    DUP 1 - FIB SWAP 2 - FIB + 
  THEN 
;

⚠️ A function can't be defined inside another function.

The following code is not valid:

: FIB 
  : ADD1
    1 +
  ;
  DUP 1 > IF 
    DUP 1 - FIB SWAP 2 - FIB + 
  THEN
;

Interpreter

The interpreter is a program written in OCaml that reads a PF23 program and executes it. You can quit it by typing quit on a new line. You can also use the function list func to list all the functions defined in the program.

Compilation / Installation

This program is written in OCaml, using the dune build system. You can compile it by typing dune build in the root directory of the project. The executable will be compiled into a binary file called main.bc and a native file called main.exe. You will find them in the _build/default/bin directory.