# 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. This project implements interpreters for a dialect 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.

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 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

The original Thun/Joypy site

## Example Code

Here is an example of 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 a little familiarity it becomes just as legible as any other notation.

This function accepts two integers on the stack 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

## Project Hosted on OSDN

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

## Directory structure

    Thun
     |
     |-- LICENSE - GPLv3
     |-- README.md - this file
     |
     |-- archive
     |   |-- Joy-Programming.zip
     |   `-- README
     |
     |-- docs
     |   |-- Makefile - Generate https://joypy.osdn.io/ site.
     |   |-- notebooks - Jupyter Notebooks and supporting modules
     |   |-- reference - Docs for each function.
     |   |-- dep-graphs - Generated dependency graphs.
     |   `-- README - Table of Contents
     |
     `-- implementations
         |
         |-- Nim - interpreter
         |
         |-- Prolog - interpreter
         |            type inference
         |            work-in-progress compiler
         |
         |-- Python - interpreter
         |
         `-- defs.txt - common Joy definitions for all interpreters

## Documentation

### Jupyter Notebooks

### 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.)

## Installation

Clone the repo and follow the instructions in the individual `implementations` directories.

## Basics of Joy

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

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

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.

The dictionary associates symbols (strings) 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.

### 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](http://archive.vector.org.uk/art10000350

### 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 © 2014-2022 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/.