learnxinyminutes-docs/forth.html.markdown

---
language: forth
contributors:
    - ["Horse M.D.", "http://github.com/HorseMD/"]
filename: learnforth.fs
---

Forth was created by Charles H. Moore in the 70s.

Note: This article focuses predominantly on the Gforth implementation of Forth, but most
of what is written here should work elsewhere. 

> If Lisp is the ultimate high level language, Forth is the ultimate low level language.

```forth

\ Forth is an interactive programming language which is comprised of *words*. These are
\ Forth subroutines which are executed once you press <Cr>, from left to right.

\ ------------------------------ Precursor ------------------------------

\ It's important to know how forth processes instructions. All programming in Forth is
\ done by manipulating what's known as the parameter stack (more commonly just referred
\ to as "the stack"). The stack is a typical last-in-first-out (LIFO) stack. Typing:
5 2 3 56 76 23 65

\ Means 5 gets put on the stack first, then 2, then 3, etc all the way to 65, which
\ is now at the top of the stack. We can see the length and contents of the stack by
\ passing forth the word `.s`:
.s <7> 5 2 3 56 76 23 65    \ ok

\ Forth's interpreter interprets what you type in one of two ways: as *words* (i.e. the
\ name of subroutines) or as *numbers*. Words are essentially "symbols that do things".

\ Finally, as the stack is LIFO, we obviously must use postfix notation to manipulate
\ the stack. This should become clear shortly.

\ ------------------------------ Basic Arithmetic ------------------------------

\ Lets do a simple equation: adding 5 and 4. In infix notation this would be 5 + 4,
\ but as forth works in postfix (see above about stack manipulation) we input it like so:
5 4 +    \ ok

\ However, this alone yields "ok", yet no answer. Typing the word `.` will yield
\ the result.
.    \ 9 ok

\ This should illustrate how Forth's stack works. Lets do a few more arithmetic tests:
6 7 * .     \ 42 ok
1360 23 - . \ 1337 ok
12 12 / .   \ 1 ok

\ And so on.

\ ------------------------------ Stack Maniulation ------------------------------

\ Naturally, as we do so much work with the stack, we'll want some useful methods.

drop \ drop (remove) the item at the top of the stack (note the difference between this and `.`)
dup  \ duplicate the item on top the stack
rot  \ rotate the top three items (third -> first, first -> second, second -> third)
swap \ swaps the top item with the second item

\ Examples:

dup *            \ square the top item
2 5 dup * swap / \ half the top item squared
6 4 5 rot * -    \ sometimes we just want to reorganize
4 0 drop 2 /     \ add 4 and 0, remove 0 and divide the top by 2 

\ ------------------------------ More Advanced Stack Manipulation ------------------------------

tuck   \ acts like dup, except it duplicates the top item into the 3rd* position in the stack
over   \ duplicate the second item to the top of the stack
n roll \ where n is a number, *move* the stack item at that position to the top of the stack
n pick \ where n is a number, *duplicate* the item at that position to the top of the stack

\ When referring to stack indexes, they are zero-based.

\ ------------------------------ Creating Words ------------------------------

\ Quite often one will want to write their own words.
: square ( n -- n ) dup * ;    \ ok

\ Lets break this down. The `:` word says to Forth to enter "compile" mode. After that,
\ we tell Forth what our word is called - "square". Between the parentheses we have a
\ comment depicting what this word does to the stack - it takes a number and adds a
\ number. Finally, we have what the word does, until we reach the `;` word which
\ says that you've finished your definition, Forth will add this to the dictionary and
\ switch back into interpret mode.

\ We can check the definition of a word with the `see` word:
see square     \ dup * ; ok

\ ------------------------------ Conditionals ------------------------------

\ Booleans:
\ In forth, -1 is used to represent truth, and 0 is used to represent false.
\ The idea is that -1 is 11111111 in binary, whereas 0 is obviously 0 in binary.
\ However, any non-zero value is usually treated as being true:

42 42 =    / -1 ok
12 53 =    / 0 ok

\ `if` is a *compile-only word*. This means that it can only be used when we're compiling a word.
\ when creating conditionals, the format is `if` <stuff to do> `then` <rest of program>.

: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" then ; \ ok
100 ?>64                                                  \ Greater than 64! ok

\ Else:

: ?>64 ( n -- n ) DUP 64 > if ." Greater than 64!" else ." Less than 64!" then ; \ ok
100 ?>64                                                                         \ Greater than 64! ok
20 ?>64                                                                          \ Less than 64! ok

\ ------------------------------ Loops ------------------------------

\ `do` is like `if` in that it is also a compile-only word, though it uses `loop` as its
\ terminator:
: myloop ( -- ) 5 0 do cr ." Hello!" loop ; \ ok
test
\ Hello!
\ Hello!
\ Hello!
\ Hello!
\ Hello! ok

\ `do` expects two numbers on the stack: the end number and the index number, respectively.

\ Get the value of the index as we loop with `i`:
: one-to-15 ( -- ) 15 0 do i . loop ;     \ ok
one-to-15                                 \ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ok
: squares ( -- ) 10 0 do i DUP * . loop ; \ ok
squares                                   \ 0 1 4 9 16 25 36 49 64 81 ok

\ Change the "step" with `+loop`:
: threes ( -- ) 15 0 do i . 3 +loop ; \ ok
threes                                \ 0 3 6 9 12 ok

\ Finally, while loops with `begin` <stuff to do> <flag> `unil`:
: death ( -- ) begin ." Are we there yet?" 0 until ;

\ ------------------------------ Variables and Memory ------------------------------

\ Sometimes we'll be in a situation where we want more permanent variables:
\ First, we use `variable` to declare `age` to be a variable.
variable age

\ Then we write 21 to age with the word `!`.
21 age !

\ Finally we can print our variable using the "read" word '@', which adds the value
\ to the stack, or use a handy word called `?` that reads and prints it in one go.
age @ . \ 12 ok
age ?   \ 12 ok

\ What's happening here is that `age` stores the memory address, and we use `!`
\ and `@` to manipulate it.

\ Constants are quite simiar, except we don't bother with memory addresses:
100 constant WATER-BOILING-POINT \ ok
WATER-BOILING-POINT .            \ 100 ok

\ Arrays!

\ Set up an array of length 3:
variable mynumbers 2 cells allot

\ Initialize all the values to 0
mynumbers 3 cells erase
\ (alternatively we could do `0 fill` instead of `erase`, but as we're setting
\ them to 0 we just use `erase`).

\ or we can just skip all the above and initialize with specific values:

create mynumbers 64 , 9001 , 1337 , \ the last `,` is important!

\ ...which is equivalent to:

\ [64, 9001, 1337]
64 mynumbers 0 cells + !
9001 mynumbers 1 cells + !
1337 mynumbers 2 cells + !

\ Reading values at certain array indexes:
0 cells mynumbers + ? \ 64 ok
1 cells mynumbers + ? \ 9001 ok
2 cells mynumbers + ? \ 1337 ok

\ Of course, you'll probably want to define your own words to manipulate arrays:
: ?mynumbers ( n -- n ) cells mynumbers + ; \ ok
64 mynumbers 2 cells + !                    \ ok
2 ?mynumbers ?                              \ 64 ok

\ ------------------------------ The Return Stack ------------------------------

\ TODO

\ ------------------------------ Final Notes ------------------------------

\ Floats
\ Commenting (types)
\ bye

```

##Ready For More?

* [Starting Forth](http://www.forth.com/starting-forth/)
* [Thinking Forth](http://thinking-forth.sourceforge.net/)