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, Elm, Nim, OCaml, Prolog, and Scheme).
Joy is:
The best source 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.
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] and
p0 [abs] ii <=
p1 [<>] [pop !-] or
then [ !-] [[++]] [[--]] ifte dip
else [pop !-] [--] [++] ifte
It might seem unreadable but with familiarity it becomes as legible as any other notation.
The Thun specification document describes the Thun dialect, however most of the documentation is in the form of Jupyter Notebooks that go into more detail.
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 (It's usually out of date, I'm working on it.)
The original Joy has several datatypes (such as strings and sets) but the Thun dialect currently only uses four:
true and false.[ and ] brackets.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 ]).
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
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.
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.
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".)
From here it kinda falls apart...
TODO
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
This is the basis set of functions, the rest of functions in the Thun dialect of Joy are defined in terms of these:
branch
dip
i
loop
clear
concat
cons
dup
first
pop
rest
stack
swaack
swap
truthy
inscribe
+ - * / %
< > >= <= != <> =
lshift rshift
Copyright © 2014 - 2023 Simon Forman
This file is part of Thun
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