Gob 🐱

Gob is a very small stack based programming language.

Every program is a sequence of values that operate on a single global stack. For example, the following program evaluates to 9:

# examples/sum.gob
4 5 +

$ gob examples/sum.gob --debug
 | 4 5 +
4 | 5 +
4 5 | +
9 | 
9

Everything on the left of the | is the current state of the stack and everything on the right is the current state of the program. First 4 is pushed on the stack, followed by 5, at which point + pops both numbers off the stack, sums them, and pushes the answer back on the stack. At that point the program is complete so the execution finishes by outputting 9.

More examples can be found in ./examples.

Features

The data types Gob currently supports are strings, integers, booleans, and quotations.

"Hello World!"
-1729
true
[10 445 +]

The full list of supported functions can be found in the code.

Piping programs

Due to the nature of stack based programs, you can compose two together simply by concatenating them. Gob takes advantage of this by allowing you to pipe a program into the interpreter and combine it with the program read from a file.

For example, the factorial program reads its argument from stdin

$ echo 7 | gob examples/factorial.gob --pipe
5040

This first argument can be any program, so equivalently we can do:

$ echo [3 4 +] apply | gob examples/factorial.gob --pipe
5040

I think this idea could be very useful for shell scripting because of the ability to easily compose programs directly on the command line. I have not explored it much however, and right now practical benefits are limited by Gob's very small feature set.

Defs

Gob programs consist of any number of definitions followed by the body of the program. Defs are useful for readability and easily implementing recursion. When a def is referenced in the body of the program, the reference to the def is replaced by its contents. For example, factorial can be implemented like this:

# examples/factorialDefs.gob
fact: 1 - isZero [drop] [dup dig * swap fact] branch apply
isZero: dup 0 =

dup fact

$ echo 10 | gob examples/factorialDefs.gob --pipe
3628800

CLI

To see the rest of the CLI options, run

gob --help

Building from source

Gob's interpreter is implemented in Roc, a delightful functional language.

To use the language, you will need the Roc compiler.

To build the interpreter, run

roc build main.roc --output ./gob

Inspiration

Gob was originally inspired by the excellent talk Concatenative programming and stack-based languages by Douglas Creager at Strange Loop 2023.