This repository contains two implementations of the Forth programming language.

The first is just called goforth and lives in the root directory of this repository. The second is called ctil and lives in the ctil directory.

goforth is implemented in Go and takes an unusual approach to Forth. It acts as a source-level interpreter. There is no intermediate representation of the program - the source string is interpreted directly. When a word definition is encountered, goforth links it to the offset in the input string where the word is defined, and when this word is called the interpreter just processes its string definition word by word.

This is an interesting experiment but clearly not the best way to implement Forth; in addition to the performance implications, it makes implementing things like loops quite difficult (having to carefully keep track of nested constructs like IF to enable proper LEAVE), and it's hard to support Forth's famous self-extension mechanisms - for example, implementing IF...THEN...ELSE in Forth itself using lower-level primitives.

To run goforth on a piece of Forth code, simply execute:

$ go run . < testdata/fizzbuzz.4th

... or ...
$ go build
$ ./goforth < testdata/fizzbuzz.4th

There are many small Forth programs in the testdata directory that can serve as examples to run and play with.

ctil (stands for "C Thread Interpretive Language") is a C implementation of Forth. It takes a much more traditional Forth implementation approach, where the Forth code is actually compiled into linked dictionary entries, and word invocations are replaced with the addresses of the dictionary entries for these words.

ctil still deviates from the Forth convention of Assembly language implementations, but it should be able to support pretty much everything. As an example, take a look at ctil/prelude.4th - it contains implementations of the variable word, IF...THEN...ELSE conditions and BEGIN...REPEAT loops in Forth itself.

To run ctil on a piece of Forth code, first build ctil. A Makefile is included - ctil has no external dependencies other than a standard C compiler.

$ cd ctil
$ make

To run realistic Forth programs, the "prelude" has to be included first - it's an implementation of several Forth primitives in Forth itself. If you're still in the ctil directory, run:

$ ./ctil -i prelude.4th < ../testdata/fizzbuzz.4th

The design of ctil is inspired by the jonesforth implementation, though some things work differently (I tried to stick closer to standard Forth).

goforth has an extensive automated test harness. Each test consists of a file in the testdata directory that contains some Forth code and the expected output from that code. The test harness runs the Forth code using each interpreter and compares the output with what's expected.

Both interpreters are tested using a Go test harness; goforth_test.go tests the Go implementation, and ctil_test.go tests the C implementation. Since the C implementation supports more features, some test files in testdata are prefixed by ctil- and are run only for ctil.