sallyforth/README.md

SallyForth: A simple Forth-like language implemented in Python.

SallyForth is a simple hobby implementation of a FORTH-like programming language. Possibly the most interesting thing about SallyForth is the name, which Michael Nygard suggested as a name for a FORTH implementation at exactly the same time that I happened to be writing this code.

Running SallyForth

SallyForth is writting in Python 3 and will happily run with either the standard C Python implementation or Pypy. To run SallyForth just kick off the sallyforth.py file:

$ python sallyforth/sallyforth.py

The Sally Language

Like FORTH, Sally is a stack oriented concatenative programming language. What this means is that any constant value in a sallyforth program, like a number or a string:

sallySh> "Hello, world!"

Has the effect of pushing the value onto an ever present data stack. There are also commands or functions -- called words -- that you can use to do things.

So the word p will pop the value off of the top of the stack and print it:

sallySh> "Hello, world!"
sallySh> p
Hello, world!

Sally parsing is about as simple as you can get: words and constant values are separated by whitespace. So if you wanted to print a number of values, you could do this:

sallySh> 1 2 3 p p p
3
2
1

The only execeptions to the separated by whitespace rule are double quoted strings, which work about the way you would expect:

sallySh> "I can have spaces in my string"
sallySh> p
I can have spaces in my string

Sally comes prepackaged with a host of useful words, everything from basic arithmetic:

sallySh> 1 2 + p
3
sallySh> 10 10 * 1 + p
101

To boolean logic:

sallySh> true false and p
False
sallySh> false true or p
True

To IO:

sallySh> "hello.txt" read-file
sallySh> p
This is the contents of hello.txt.
Use it wisely.

You can also define your own words. A word defintion starts with a colon, followed by the name of your new word, followed by the contents of your new word, followed by a semicolon. Keep in mind that everything -- including the colon and semicolon, needs to be set off with whitespace:

: hello-world "Hello, world!" p ;

Once your new word is defined you can use it like any other word:

sallySh> hello-world
Hello, world!