An interpreter for a small dialect of Forth written in Go.
Fibonacci sequence:
: fib over over + ; ( duplicates top two values and adds them )
Count down from 10:
10 @ lbl ( start at 10 and define label )
dup . 10 emit ( print current value and newline )
1 - dup if ( subtract 1 and check if not zero )
lbl goto then ( loop )
Duplicate the top 4 elements of the stack:
: 4dup cross cross cross dup
back dup rot rot
back dup cross rot rot
back back dup cross swap cross rot rot
back back ;
Based on the Forth programming language, this is a stack-based language with space-separated commands called words. Most words manipulate the stack in some way. Integers are pushed to the top of the stack. Running 1 2 + will result in a stack with 3 on top.
This dialect only supports integer values and does not have variables. Instead it has a second stack that you can push and pop from (using cross and back). Additionally, it supports labels and goto rather than loops. For control flow, zero evaluates to false and any non-zero value evaluates to true. Referencing a label pushes an integer on the top of the stack where execution will continue. Words and labels must be unique. Comments beging with ( and end with ) with a space before and after each.
The table below describes all of the built-in words. The stacks are shown bottom to top (i.e., the right most element is the top).
| Word | Stack effect | Description |
|---|---|---|
| dup | ( 1 -- 1 1 ) | Duplicates the top element. |
| drop | ( 1 -- ) | Pops the top element. |
| swap | ( 1 2 -- 2 1 ) | Swaps the top two elements. |
| over | ( 1 2 -- 1 2 1 ) | Duplicates the second element and puts it on top. |
| rot | ( 1 2 3 -- 2 3 1 ) | Rotates the top three elements. |
| cross | ( 1 , -- , 1 ) | Pops the top element and pushes it to the secondary stack. |
| back | ( , 1 -- 1 , ) | Pops the top element of the second stack and pushes it to stack. |
| + | ( 1 2 -- 3 ) | Pops the top two elements and pushes the sum. |
| - | ( 1 2 -- -1 ) | Pops the top two elements and pushes the difference. |
| * | ( 1 2 -- 2 ) | Pops the top two elements and pushes the product. |
| / | (1 2 -- 0 ) | Pops the top two elements and pushes the quotient. |
| mod | ( 1 2 -- 1 ) | Pops the top two elements and pushes the remainder. |
| > | ( 1 2 -- 0 ) | Compares if the second element is greater than the top element. |
| < | ( 1 2 -- 1 ) | Compares if the second element is less than the top element. |
| if | ( 1 -- ) | If the top is non-zero, continue execution at the next word. |
| else | no effect | Optional after an If, continue executing if the If was false. |
| then | no effect | Ends an if/if-else block. |
| : | no effect | Start of a word definition. The next token is the name followed by the body. |
| ; | no effect | End of a word definition. |
| @ | no effect | Label definition. The next token is the name. |
| . | ( 1 -- ) | Prints the top. |
| emit | ( 1 -- ) | Prints the top as an ASCII character. |
| key | ( -- 1 ) | Gets keyboard input and pushes the ASCII value. |
Just exploring how Forth works. It isn't an optimal implementation in any way.