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Forth is a programming language that uses two stacks and a dictionary of words.

A Forth environment combines the compiler with an interactive shell, where the user defines and runs subroutines called words. Words can be tested, redefined, and debugged as the source is entered without recompiling or restarting the whole program.

Forth programmers enjoy the immediacy of an interpreter while at the same time the performance and efficiency of a compiler.

—Forth Programming Aesthetics

I use a custom version of the lbforth.c REPL, modified to work on Plan9(ARM) which can be downloaded here. To experiment with a subset of Forth's primitives and learn Reverse Polish Notation, have a look at Firth. For a graphical exploration of forth's concepts, try Postscript, if you're looking to build GUI applications, try Uxn.

Basics

Forth reads from left to right, spaces are separators, when you wish to quit, type BYE. A stack is a way of managing data. With a stack, data is added to and taken from the "top", as with a stack of dishes. The acronym for this is LIFO: Last In First Out.

To inspect the stack, type .S

10 20 30 .S
30 20 10 OK
DROP .S
20 10 OK
BYE

Forth has no operator precedence, and does not need parentheses.

Reverse PolishInfix
3 4 + 2 *(3 + 4) * 2
2 3 4 * +(3 * 4) + 2
2 3 * 4 +(2 * 3) + 4
5 6 + 7 *(5 + 6) * 7

Words

The dictionary comes with the Forth system. The programmer writes a program by adding to the dictionary words defined in terms of words in the dictionary. As a rule, Forth finds a word by starting with the most recently defined word and working backwards. If two or more words in the dictionary have the same name, Forth will find the most recently defined and be satisfied.

A colon definition starts with the Forth word :

: HELLO ." Hi! " ;  
HELLO
Hi!  OK

Because in Forth data is passed implicitly, it is considered insane to define a word without documenting what data it takes from the stack and what data it returns to the stack. The canonical way of doing this is to use the Forth word ( which tells the system to ignore what follows up to and including the next ). Expectations ("before") and results ("after") are separated by --. The resulting ( before -- after ) is a "stack-effect comment".

: SQUARED DUP * ;
5 SQUARED .
25

You program in Forth by teaching a machine new actions that you and the machine know by name. Each new action is a novel arrangement of known actions, perhaps mixing in some data. By being added to the dictionary the new action can be used to teach still newer actions.

: SQUARED  ( n -- n**2 )  DUP * ;
: CUBED  ( n -- n**3 )  DUP SQUARED * ;
: 4TH  ( n -- n**4 )  SQUARED SQUARED ;

Logic

There’s actually no boolean type in Forth. The number 0 is treated as false, and any other number is true, although the canonical true value is -1 (all boolean operators return 0 or -1). Conditionals in Forth can only be used inside definitions.

The simplest conditional statement in Forth is if then, which is equivalent to a standard if statement in most languages. Here’s an example of a definition using if then. In this example, we’re also using the mod word, which returns the modulo of the top two numbers on the stack. In this case, the top number is 5, and the other is whatever was placed on the stack before calling buzz?. Therefore, 5 mod 0 = is a boolean expression that checks to see if the top of the stack is divisible by 5.

s
: BUZZ? 5 MOD 0 = IF ." BUZZ" THEN ;

Loops

: STAR [CHAR] * EMIT ;
: STARS 0 DO STAR LOOP CR ;
10 STARS
**********

Stack Manipulators

SWAPa bb aReverses the top two stack items
DUPaa aDuplicates the top stack item
OVERa ba b aCopies second item to top
ROTa b cb c aRotates third item to top
DROPaDiscards the top stack item

Cons cells

: car! ( v a - ) ! ; 
: cdr! ( v a - ) cell+ ! ; 
: car@ ( a - v ) @ ;
: cdr@ ( a - v ) cell+ @ ;
: cons ( k v - a ) swap 2 cells allocate throw tuck ! tuck ! ;
: cons0 ( - a ) 0 0 cons ;