# Chaitin's construction

### From HaskellWiki

## Contents |

## 1 Introduction

Are there any real numbers which are defined exactly, but cannot be computed? This question leads us to exact real arithmetic, foundations of mathematics and computer science.

See Wikipedia article on Chaitin's construction, referring to e.g.

- Computing a Glimpse of Randomness (written by Cristian S. Calude, Michael J. Dinneen, and Chi-Kou Shu)
- Omega and why math has no TOEs (Gregory Chaitin).

## 2 Basing it on combinatory logic

Some more direct relatedness to functional programming: we can base Ω on combinatory logic (instead of a Turing machine).

### 2.1 Coding

See the prefix coding system described in Binary Lambda Calculus and Combinatory Logic (page 20) written by John Tromp:

of course, *c*, *d* are meta-variables, and also some other notations are changed slightly.

### 2.2 Decoding

Having seen this, decoding is rather straightforward. Let us represent it e.g with the following LL1 parser. Of course, we can build it on top of more sophisticated parser libraries (Parsec, arrow parsers)

#### 2.2.1 Decoding module

module Decode (clP) where import Parser (Parser, item) import CL (CL, k, s, apply) import CLExt ((>>^)) clP :: Parser Bool CL clP = item (bool applicationP baseP) applicationP :: Parser Bool CL applicationP = item (clP >>^ clP) baseP :: Parser Bool CL baseP = item (bool k s) kP, sP :: Parser Bool CL kP = return k sP = return s

#### 2.2.2 Combinatory logic term modules

##### 2.2.2.1 CL

module CL (CL, k, s, apply) where import Tree (Tree (Leaf, Branch)) import BaseSymbol (BaseSymbol, kay, ess) type CL = Tree BaseSymbol k, s :: CL k = Leaf kay s = Leaf ess apply :: CL -> CL -> CL apply = Branch

##### 2.2.2.2 CL extension

module CLExt ((>>^)) where import Control.Monad (Monad, liftM2) (>>^) :: liftM2 apply

##### 2.2.2.3 Base symbol

module BaseSymbol (BaseSymbol, kay, ess) where data BaseSymbol = K | S kay, ess :: BaseSymbol kay = K ess = S

#### 2.2.3 Utility modules

##### 2.2.3.1 Binary tree

module Tree where data Tree a = Leaf a | Branch (Tree a) (Tree a)

##### 2.2.3.2 Parser

module Parser (Parser, item) where type Parser token a = StateT [token] [] a item :: Parser a item = do token : tokens <- get put tokens return token

##### 2.2.3.3 Prelude extension

module PreludeExt (bool) where bool :: a -> a -> Bool -> a bool thenC elseC t = if t then thenC else elseC

### 2.3 Partial function approach

Now, Chaitin's construction will be here

where

- hnf
- should denote an unary predicate “has normal form” (“terminates”)
- dc
- should mean an operator “decode” (a function from finite bit sequences to combinatory logic terms)
- should denote the set of all finite bit sequences
- Dom
_{dc} - should denote the set of syntactically correct bit sequences (semantically, they may either terminate or diverge), i.e. the domain of the decoding function, i.e. the range of the coding function. Thus,
- “Absolute value”
- should mean the length of a bit sequence (not combinatory logic term evaluation!)

### 2.4 Total function approach

Here, dc is a partial function (from finite bit sequences). If this is confusing or annoying, then we can choose a more Haskell-like approach, making dc a total function:

dc :: [Bit] -> Maybe CL

then, Chaitin's construction will be

where should denote false truth value.

## 3 Related concepts

## 4 To do

Writing a program in Haskell -- or in combinatory logic:-) -- which could help in making conjectures on combinatory logic-based Chaitin's constructions. It would make only approximations, in a similar way that most Mandelbrot plotting softwares work: it would ask for a maximum limit of iterations.

chaitin --computation=cl --coding=tromp --limit-of-iterations=5000 --digits=10 --decimal