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@ -10,18 +10,15 @@ Forth was created by Charles H. Moore in the 70s.
Note: This article focuses predominantly on the Gforth implementation of Note: This article focuses predominantly on the Gforth implementation of
Forth, but most of what is written here should work elsewhere. Forth, but most of what is written here should work elsewhere.
> If Lisp is the ultimate high level lang, Forth is the ultimate low level lang.
```forth ```forth
\ Forth is an interactive programming language which is comprised of \ Forth is a low level interactive programming language which is comprised of
\ *words*. These are Forth subroutines which are executed once you press \ *words*. These are Forth subroutines which are executed once you press
\ <Cr>, from left to right. \ <Cr>, from left to right.
\ --------------------------------- Precursor ---------------------------------- \ --------------------------------- Precursor ----------------------------------
\ It's important to know how Forth processes instructions. All \ All programming in Forth is done by manipulating what's known as the parameter
\ programming in Forth is done by manipulating what's known as the parameter
\ stack (more commonly just referred to as "the stack"). Typing: \ stack (more commonly just referred to as "the stack"). Typing:
5 2 3 56 76 23 65 5 2 3 56 76 23 65
@ -78,9 +75,8 @@ see square \ dup * ; ok
\ -------------------------------- Conditionals -------------------------------- \ -------------------------------- Conditionals --------------------------------
\ In Forth, -1 is used to represent truth, and 0 is used to represent false. \ -1 == true, 0 == false. However, any non-zero value is usually treated as
\ The idea is that -1 is 11111111 in binary, whereas 0 is obviously 0 in binary. \ being true:
\ However, any non-zero value is usually treated as being true:
42 42 = \ -1 ok 42 42 = \ -1 ok
12 53 = \ 0 ok 12 53 = \ 0 ok
@ -123,21 +119,17 @@ threes \ 0 3 6 9 12 ok
\ ---------------------------- Variables and Memory ---------------------------- \ ---------------------------- Variables and Memory ----------------------------
\ Sometimes we'll be in a situation where we want more permanent variables: \ Use `variable` to declare `age` to be a variable.
\ First, we use `variable` to declare `age` to be a variable.
variable age variable age
\ Then we write 21 to age with the word `!`. \ Then we write 21 to age with the word `!`.
21 age ! 21 age !
\ Finally we can print our variable using the "read" word '@', which adds the \ Finally we can print our variable using the "read" word `@`, which adds the
\ value to the stack, or use `?` that reads and prints it in one go. \ value to the stack, or use `?` that reads and prints it in one go.
age @ . \ 12 ok age @ . \ 12 ok
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: \ Constants are quite simiar, except we don't bother with memory addresses:
100 constant WATER-BOILING-POINT \ ok 100 constant WATER-BOILING-POINT \ ok
WATER-BOILING-POINT . \ 100 ok WATER-BOILING-POINT . \ 100 ok
@ -174,8 +166,8 @@ create mynumbers 64 , 9001 , 1337 , \ the last `,` is important!
\ ------------------------------ The Return Stack ------------------------------ \ ------------------------------ The Return Stack ------------------------------
\ The return stack is used by Forth to the hold pointers to things when \ The return stack is used to the hold pointers to things when words are
\ words are executing other words, e.g. loops. \ executing other words, e.g. loops.
\ We've already seen one use of it: `i`, which duplicates the top of the return \ We've already seen one use of it: `i`, which duplicates the top of the return
\ stack. `i` is equivalent to `r@`. \ stack. `i` is equivalent to `r@`.
@ -190,12 +182,11 @@ create mynumbers 64 , 9001 , 1337 , \ the last `,` is important!
\ ------------------------- Floating Point Operations -------------------------- \ ------------------------- Floating Point Operations --------------------------
\ Most Forths tend to dislike the use of floating point operations. We write \ Most Forths tend to eschew the use of floating point operations. We write
\ floating point operations with scientific notation. \ floating point operations with scientific notation.
8.3e 0.8e f+ f. \ 9.1 ok 8.3e 0.8e f+ f. \ 9.1 ok
\ Usually we can just prepend arithmetic words with 'f' to use floating point \ Usually we simply prepend words with 'f' when dealing with floats:
\ arithmetic:
variable myfloatingvar \ ok variable myfloatingvar \ ok
4.4e myfloatingvar f! \ ok 4.4e myfloatingvar f! \ ok
myfloatingvar f@ f. \ 4.4 ok myfloatingvar f@ f. \ 4.4 ok