JFORTH

A simple one-file interpreter for a custom FORTH language.

>>> 3 4 6 + * .
30 :)
>>> : DOUBLE DUP DUP + ;
Function definition for 'DOUBLE' created. :)
>>> 30 DOUBLE .
60 :)
>>> 3 4 > IF ."3 > 4 " ELSE ."3 <= 4 " THEN
3 <= 4 :)

Implementations

For a C# implementation, please look at the repo here.

Introduction

JFORTH is a language that is an imperative stack-based language that is ran via a shell. It uses a stack for storing numbers for the calculations and expressions the programmer writes, this naturally means the language follows a RPN style.

There are only two types of tokens in the language, symbols, and numbers. Symbols (called "words" in typical FORTH) are your typical functions, they do stuff. Numbers are just numbers. When numbers are encountered they are simply pushed onto the stack. When symbols are encountered, the interpreter first checks if the symbol is a built in, such as CR (carriage return), . (print), DUP (duplicate the top of the stack), etc. if it is not a built in, it then checks the user defined functions.

Examples

>>> 3 4 6 + * .
30 :)

Explanation: First 3, 4, and 6 are pushed onto the stack. The + pops the two most recent numbers on the stack (the 4 and 6), adds them together and pushes 10 back onto the stack. The * pops the two most recent numbers on the stack, 10 and 3, multiplies them and pushes 30 back onto the stack. Then finally . simply outputs the top of the stack.

>>> : FAC DUP 1 <> IF DUP 1 - FAC * THEN ;
Function definition for 'FAC' created. :)
>>> 5 FAC .
120 :)

Recursive factorial function.

Documentation

Builtins

Symbol	Explanation
CR	Outputs a new line character.
.	Ouputs the number on top of the stack.
+ - * /	Pops two numbers off the stack and pushes the result.
DUP	Takes the number on top of the stack, and pushes it again.
DROP	Discards the number on top of the stack.
== > < >= <= <>	Pops the two numbers on top of the stack, pushes 1 if true, 0 if false.
0< 0>	Compares the top of the stack with 0, pushes 1 if true, 0 if false.
SWAP	Swaps the two numbers on top of the stack.
IF	If the number on top of the stack is a non-zero number, proceed as normal. Else skip ahead to an ELSE or THEN token.
ELSE	Skip ahead to a THEN token.
THEN	Do nothing.

A string has the following syntax ."Hello, world!". When a string is encountered it will instantly print to the console.

Writing functions

Function definitions have the following syntax

: <FUNCTION NAME> <FUNCTION BODY> ;

The function will then entered into a function map, and then if the function name is encountered in a later statement the function body will be executed.