# Thun

A Dialect of Joy.

Version 0.5.0

> Simple pleasures are the best.

Joy is a programming language created by Manfred von Thun that is easy to use and understand and has many other nice properties. Thun is a dialect of Joy that attempts to stay very close to the spirit of Joy but does not precisely match the behaviour of the original version written in C. It started as a Python project called "Joypy", but after someone claimed that name on PyPI before me I renamed it to Thun in honor of Manfred Von Thun. Now there are interpreters implemented in several additional languages (C, Nim, Prolog, Rust).

Joy is:

- Purely Functional
- Stack-based
- Concatinative (See also concatenative.org)
- Categorical

The best source (no pun intended) for learning about Joy is the information made available at the website of La Trobe University | (mirror) which contains source code for the original C interpreter, Joy language source code for various functions, and a great deal of fascinating material mostly written by Von Thun on Joy and its deeper facets as well as how to program in it and several interesting aspects. It's quite a treasure trove.

- Wikipedia entry for Joy
- Homepage at La Trobe University (Kevin Albrecht's mirror)
- The original Thun/Joypy site

## Example Code

Here is an example of Joy code. This function `square_spiral` accepts two integers and increments or decrements one of them such that the new pair of numbers is the next coordinate pair in a square spiral (like the kind used to construct an Ulam Spiral). For more information see Square Spiral Example Joy Code.

    square_spiral [_p] [_then] [_else] ifte

    _p  [_p0] [_p1] &&
    _p0 [abs] ii <=
    _p1 [<>] [pop !-] ||

    _then [    !-] [[++]] [[--]] ifte dip
    _else [pop !-]  [--]   [++]  ifte

It might seem unreadable but with familiarity it becomes as legible as any other notation.

## Project Hosted on OSDN

- Source Repository (mirror)
- Bug tracker (old tracker)
- Forums
- Mailing list

## Documentation

This document describes Joy in a general way below, however most of the documentation is in the form of Jupyter Notebooks that go into more detail.

### Jupyter Notebooks

There's also a Function Reference that lists each function and combinator by name and gives a brief description. (It's usually out of date, I'm working on it.)

### Function Reference

### Building the Docs

Run `make` in the `docs` directory. (This is a lie, it's more complex than that. Really you need to run (GNU) make in the `docs/notebooks` and `docs/reference` dirs first, then run `make` in the `docs` directory.)

## Directory structure

    Thun
    |-- LICENSE - GPLv3
    |-- README.md - this file
    |
    |-- archive
    |   |-- Joy-Programming.zip
    |   `-- README
    |
    |-- docs
    |   |-- dep-graphs - Generated dependency graphs.
    |   |-- html - Generated HTML docs.
    |   |-- notebooks - Jupyter Notebooks and supporting modules
    |   `-- reference - Docs for each function.
    |
    |-- implementations
    |   |-- defs.txt - common Joy definitions for all interpreters
    |   |-- C - interpreter
    |   |-- GNUProlog - interpreter
        |               type inference
        |               work-in-progress compiler
    |   |-- Nim - interpreter
    |   |-- Ocaml - work-in-progress interpreter
    |   `-- Python - interpreter
    |
    `-- joy_code - Source code written in Joy.
        `-- bigints
            `-- bigints.joy

## Installation

Clone the repo and follow the instructions in the individual `implementations` directories. There isn't really any installation. You can put the binaries in your `PATH`.

(I had the Python package set up to upload to PyPI as "Thun", but the whole Python distribution story seems unsettled at the moment (2023) so I've gone back to the old ways: there is a single script `joy.py` that gets modified (`defs.txt` is inserted) to create a `joy` script that uses the "shebang" trick to pretend to be a binary. In other words, run `make` and put the resulting `joy` script in your PATH, if that's what you want to do. In a year or two the Python folks will have sorted things out and we can go back to `pip install Thun` or whatever.)

## Basics of Joy

The original Joy has several datatypes (such as strings and sets) but the Thun dialect currently only uses four:

- Integers, signed and unbounded by machine word length (they are bignums.)
- Boolean values `true` and `false`.
- Lists quoted in [ and ] brackets.
- Symbols (names).

Joy is built around three things: a stack of data items, an expression representing a program to evaluate, and a dictionary of named functions.

### Stack

Joy is stack-based. There is a single main stack that holds data items, which can be integers, bools, symbols (names), or sequences of data items enclosed in square brackets (`[` or `]`).

    23 dup [21 18 add] true false [1 [2 [3]]] cons

### Expression

A Joy expression is just a sequence or list of items. Sequences intended as programs are called "quoted programs". Evaluation proceeds by iterating through the terms in an expression putting all literals (integers, bools, or lists) onto the main stack and executing functions named by symbols as they are encountered. Functions receive the current stack, expression, and dictionary and return the next stack, expression, and dictionary.

### Dictionary

The dictionary associates symbols (names) with Joy expressions that define the available functions of the Joy system. Together the stack, expression, and dictionary are the entire state of the Joy interpreter.

### Interpreter

The Joy interpreter is extrememly simple. It accepts a stack, an expression, and a dictionary, and it iterates through the expression putting values onto the stack and delegating execution to functions which it looks up in the dictionary.

All control flow works by Continuation Passing Style. Combinators (see below) alter control flow by prepending quoted programs to the pending expression (aka "continuation".)

## Stack / Quote / List / Sequence

When talking about Joy we use the terms "stack", "quote", "sequence", "list", and others to mean the same thing: a simple linear datatype that permits certain operations such as iterating and pushing and popping values from (at least) one end.

> In describing Joy I have used the term quotation to describe all of the above, because I needed a word to describe the arguments to combinators which fulfill the same role in Joy as lambda abstractions (with variables) fulfill in the more familiar functional languages. I use the term list for those quotations whose members are what I call literals: numbers, characters, truth values, sets, strings and other quotations. All these I call literals because their occurrence in code results in them being pushed onto the stack. But I also call [London Paris] a list. So, [dup *] is a quotation but not a list.

From "A Conversation with Manfred von Thun" w/ Stevan Apter

From here it kinda falls apart...

### Literals and Simple Functions

TODO

### Combinators

The main loop is very simple as most of the action happens through what are called combinators. These are functions which accept quoted programs on the stack and run them in various ways. These combinators reify specific control-flow patterns (such as `ifte` which is like `if.. then.. else..` in other languages.) Combinators receive the current expession in addition to the stack and return the next expression. They work by changing the pending expression the interpreter is about to execute. (The combinators could work by making recursive calls to the interpreter and all intermediate state would be held in the call stack of the implementation language, in this joy implementation they work instead by changing the pending expression and intermediate state is put there.)

    joy? 23 [0 >] [dup --] while
    23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Copyright © 2014 - 2023 Simon Forman

This file is part of Thun

Thun is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

Thun is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Thun. If not see http://www.gnu.org/licenses/.