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Four years ago somebody posted a comment-thread describing how you could start writing a little reverse-polish calculator, in C, and slowly improve it until you had written a minimal FORTH-like system:

• https://news.ycombinator.com/item?id=13082825

At the time I read that comment I'd just hacked up a simple FORTH REPL of my own, in Perl, and I said "thanks for posting". I was recently reminded of this discussion, and decided to work through the process.

Using only minimal outside resources the recipe worked as expected!

The end-result is I have a working FORTH-lite, or FORTH-like, interpreter written in around 2000 lines of golang! Features include:

• Reverse-Polish mathematical operations.
• Comments between ( and ) are ignored, as expected.
  • Single-line comments \ to the end of the line are also supported.
• Support for floating-point numbers (anything that will fit inside a float64).
• Support for printing the top-most stack element (., or print).
• Support for outputting ASCII characters (emit).
• Support for outputting strings (." Hello, World ").
• Support for basic stack operations (drop, dup, over, swap)
• Support for loops, via do/loop.
• Support for conditional-execution, via if, else, and then.
• Load any files specified on the command-line
  • If no arguments are included run the REPL
• A standard library is loaded, from the present directory, if it is present.

To give a flavour here we define a word called star which just outputs a single start-character:

: star 42 emit ;

Now we can call that (NOTE: We didn't add a newline here, so the REPL prompt follows it, that's expected):

> star
*>

To make it more useful we define the word "stars" which shows N stars:

> : stars dup 0 > if 0 do star loop else drop then ;
> 0 stars
> 1 stars
*> 2 stars
**> 10 stars
**********>

This example uses both if to test that the parameter on the stack was greater than zero, as well as do/loop to handle the repetition.

Finally we use that to draw a box:

> : squares 0 do over stars cr loop ;
> 4 squares
****
****
****
****

> 10 squares
**********
**********
**********
**********
**********
**********
**********
**********
**********
**********

For fun we allow decompiling the words too:

> #words 0 do dup dump loop
..
Word 'square'
 0: dup
 1: *
Word 'cube'
 0: dup
 1: square
 2: *
Word '1+'
 0: store 1.000000
 2: +
Word 'test_hot'
  0: store 0.000000
  2: >
  3: if
  4: [cond-jmp 7.000000]
  6: hot
  7: then
..

Anyway if that is at all interesting feel free to take a peak. There's a bit of hackery there to avoid the use of return-stacks, etc. Compared to gforth this is actually more featureful in some areas:

• I allow you to use conditionals in the REPL - outside a word-definition.
• I allow you to use loops in the REPL - outside a word-definition.

Find the code here:

• https://github.com/skx/foth

Tags: forth, github, go, golang, hackernews | No comments