## Negative Time Iteration

Lucid is based on a simple temporal logic. The time model follows from formalizing iteration as it appears in imperative programs with, say, while or for loops.

In this model there is a first or initial time point, and every time point has a unique successor. Imperative iterations normally terminate, so we should have only finitely many time points. Lucid avoids the complications of finite time domains by making everything at least notionally infinite, so that the domain of time points is the natural numbers with the usual order.

In temporal logic logicians have studied a huge variety of time domains. What do they mean in terms of iteration and how do we write iterative programs over nonstandard time domains?

## Programming With End-Of-Data

In the last post we introduced eod (end-of-data), a special sentinel value used to mark the end of a finite Lucid stream. Streams in Lucid are all formally infinite (non terminating) but we can use eod to represent finite streams as infinite ones filled with eod past a certain point. For example  the finite stream of the first five  primes is

2, 3, 5, 7, 9, eod, eod, eod, …

The input and output conventions are adjusted to interpret eod as termination. If the above stream is the output, the implementation will ‘print’ the first five values and terminate normally. If a user inputs the first five values, then terminates the input stream, this is not treated as an error. Instead, the ‘missing’ values are evaluated to eod if requested.

## Finite and Infinite in Lucid2D

Lucid2D is Lucid with both time and space dimensions, as discussed in my last blog post. Program variables denote intensions that can be thought of as streams of arrays. Infinite streams of infinite arrays.

“But wait!” I hear you say. “It’s nice to write elegant equations defining the infinite array of all primes but what about the everyday working world? What about the array of midterm marks of my class? It’s finite! How do you work with that in Lucid2D?”

## Multidimensional Dataflow

As I’ve already explained, Lucid can be understood as functional programming with an added time dimension. What about other dimensions? In particular, what about a space dimension?

The late Ed Ashcroft and I discovered this possibility when we tried to “add arrays” to Lucid. Initially, we intended Lucid to be a fairly conventional, general purpose language. So we considered various ‘features’ and tried to realize them in terms of expressions and equations.

## Choose Your Paradox – the downside of the Axiom of Choice

And He took the five loaves and the two fish, and looking up to heaven, He blessed and broke and gave the loaves to the disciples; and the disciples gave to the multitudes. So they all ate and were filled, and they took up twelve baskets full of the fragments that remained. – Matthew 14.

The logician Willard Quine defined a paradox as an “absurd” statement backed up by an argument.

The famous result of Banach and Tarski definitely counts as a paradox by this definition. They proved that it is possible to take a unit sphere (a ball with radius 1), divide it into five pieces, then by rotations and translations reassemble it into two unit spheres.

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## This Post is Meaningless – the Liar Paradox

The Liar Paradox is simple enough to explain – is the following statement true or false?

This statement is false.

If it’s true then it’s false, but if it’s false then it’s true  … nothing works.

In my not-so-humble opinion, most (maybe all) paradoxes are the last step in a proof by contradiction that some unstated assumption is false.

In this case, the assumption is that the above statement is meaningful – is either true or false. The assumption is false, the statement is meaningless. End of paradox.

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## Grammar, Go, and the Unconscious – the Power of Facts and Rules.

In the last post there was plenty about Gödel but not much about Grammar and Go-the-game. Time to pay my debt!

Basically I said that Gödel’s results proved that no fixed set of facts and rules can on their own form the basis of mathematical knowledge. I said that hard-earned experience is indispensable. That mathematics is ultimately an experimental science. (This is not the usual take on Gödel’s work.)