VFC

What is needed to write apps in Forth ?

C.H.Ting: "Forth Virtual Computer" contains 38 words.

memory      C@ C! @ !
arithmetic  + - * / MOD
logic       AND OR XOR INVERT
stack       DROP DUP SWAP OVER >R R@ R>
i/o         IN OUT
control     IF ELSE THEN  BEGIN WHILE REPEAT AGAIN  DO LOOP ' EXECUTE
            EXIT
defining    CONSTANT VARIABLE : ;

C.H.Moore: "Introductory vocabulary" contains 45 words, it has the same power as BASIC.

+ - * / MOD MIN MAX
< > =
AND OR XOR
NEGATE ABS NOT
*/
DUP DROP SWAP OVER
DECIMAL HEX OCTAL
. .R EMIT CR KEY
: ... ;
VARIABLE CREATE ALLOT , @ !
( ... )
EMPTY
IF ELSE THEN  FOR I NEXT

There are other words necessary to implement the interpreter, compiler, block storage and console interaction. But for most apps the above is enough.

G.Haydon: "Level 0 Forth", contains 37 words (C.H.Moore)

+ - * */ /MOD ABS NEGATE MIN MAX
AND OR XOR NOT
DROP DUP SWAP OVER
DECIMAL HEX OCTAL . n .R
CR EMIT KEY
: ; CREATE , ALLOT
IF ELSE THEN FOR NEXT I

Foreign function interface example

===> cat test.c
#include <stdio.h>
void test_c(int n) { printf("n = %d\n", n); }

===> cc -shared -fPIC -o libtest.so test.c -ldl

===> vfc run.f
library ./libtest.so

0 0 0 0 > 1 function: test_c

0 0 0 0 > 42 test_c drop
n = 42

0 0 0 0 > bye

According to Julian Fondren, there are 4 different levels of Forth.

1. Core Forth, and the Forth machine
2. Higher-level extensions: OO systems, quotations, data structures, FSL, etc.
3. DSLs that still aren't application level. Eg. Julian V. Noble's state machines and formula translator, even stuff like gl-helper.fs's SHADER: word.
4. Application lexicons. You're no longer writing in Forth; you're writing in RPG Scene Definition Language or Monster Description Language.

Tons and tons of level#1 docs, and tons of statements to the effect that level#4 is where you really want to be. Tons of level#2 code and discussions: coroutines, optimized tail recursion, lse64-style 'then'.

See the directory level4 for example code.