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Chaitin's construction

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m (→Decoding: Changing term ``arrow parsers'' into a link to Arrow#Parser)
(Not so important details come to a new Chaitin's construction/Parser page. Parsers are not necessary here, a good generator is enough)
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Having seen this, decoding is rather straightforward.
Having seen this, decoding is rather straightforward.
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Let us describe the seen language with a LL(1) grammar, and let us make use of the lack of backtracking, lack of look-ahead, when deciding which parser approach to use.
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[[/Parser|Here is a parser]] for illustration, but it serves only didactical purposes: it will not be used in the final implementation, because a good term geerator makes parsing superfluous at this task.
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Some notes about the used parser library: I shall use the didactical approach read in paper [http://www.cs.nott.ac.uk/~gmh/bib.html#monparsing Monadic Parser Combinators] (written by [http://www.cs.nott.ac.uk/Department/Staff/gmh/ Graham Hutton] and Erik Meier). The optimalisations described in the paper are avoided here. Of course, we can make optimalisations, or choose sophisticated parser libraries (Parsec, [[Arrow#Parser|arrow parsers]]). A pro for this simpler parser: it may be easier to augment it with other monad transformers. But, I think, the task does not require such ability. So the real pro for it is that it looks more didactical for me. Of couse, it may be inefficient at many other tasks, but I hope, the LL(1) grammar will not raise huge problems.
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==== Decoding module ====
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<haskell>
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module Decode (clP) where
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import Parser (Parser, item)
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import CL (CL, k, s, apply)
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import CLExt ((>>@))
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import PreludeExt (bool)
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clP :: Parser Bool CL
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clP = item >>= bool applicationP baseP
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applicationP :: Parser Bool CL
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applicationP = clP >>@ clP
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baseP :: Parser Bool CL
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baseP = item >>= bool k s
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kP, sP :: Parser Bool CL
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kP = return k
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sP = return s
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</haskell>
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==== Combinatory logic term modules ====
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===== CL =====
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<haskell>
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module CL (CL, k, s, apply) where
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import Tree (Tree (Leaf, Branch))
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import BaseSymbol (BaseSymbol, kay, ess)
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type CL = Tree BaseSymbol
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k, s :: CL
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k = Leaf kay
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s = Leaf ess
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apply :: CL -> CL -> CL
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apply = Branch
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</haskell>
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===== CL extension =====
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<haskell>
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module CLExt ((>>@)) where
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import CL (CL, apply)
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import Control.Monad (Monad, liftM2)
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(>>@) :: Monad m => m CL -> m CL -> m CL
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(>>@) = liftM2 apply
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</haskell>
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===== Base symbol =====
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<haskell>
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module BaseSymbol (BaseSymbol, kay, ess) where
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data BaseSymbol = K | S
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kay, ess :: BaseSymbol
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kay = K
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ess = S
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</haskell>
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==== Utility modules ====
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===== Binary tree =====
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<haskell>
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module Tree (Tree (Leaf, Branch)) where
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data Tree a = Leaf a | Branch (Tree a) (Tree a)
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</haskell>
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===== Parser =====
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<haskell>
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module Parser (Parser, runParser, item) where
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import Control.Monad.State (StateT, runStateT, get, put)
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type Parser token a = StateT [token] [] a
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runParser :: Parser token a -> [token] -> [(a, [token])]
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runParser = runStateT
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item :: Parser token token
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item = do
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token : tokens <- get
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put tokens
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return token
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</haskell>
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===== Prelude extension =====
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<haskell>
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module PreludeExt (bool) where
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bool :: a -> a -> Bool -> a
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bool thenC elseC t = if t then thenC else elseC
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</haskell>
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=== Approach based on decoding with partial function ===
=== Approach based on decoding with partial function ===

Revision as of 12:41, 4 August 2006

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.

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:

\widehat{\mathbf S} \equiv 00
\widehat{\mathbf K} \equiv 01
\widehat{\left(x y\right)} \equiv 1 \widehat x \widehat y

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. Here is a parser for illustration, but it serves only didactical purposes: it will not be used in the final implementation, because a good term geerator makes parsing superfluous at this task.

2.3 Approach based on decoding with partial function

Now, Chaitin's construction will be here

\sum_{p\in \mathrm{Dom}_\mathrm{dc},\;\mathrm{hnf}\left(\mathrm{dc}\;p\right)} 2^{-\left|p\right|}

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)
2\!\;^{*}
should denote the set of all finite bit sequences
Domdc
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, \left\{00, 01, 1\;00\;00, 1\;00\;01, 1\;01\;00, 1\;01\;01, \dots\right\} = \mathrm{Dom}_{\mathrm{dc}} = \mathrm{Rng}_{\widehat\ }
“Absolute value”
should mean the length of a bit sequence (not combinatory logic term evaluation!)

2.4 Approach based on decoding with total function

Seen above, dc was a partial function (from finite bit sequences). We can implement it e.g. as 

dc :: [Bit] -> CL
dc = fst . head . runParser clP
where the use of
head
reveals that it is a partial function (of course, because not every bit sequence is a correct coding of a CL-term).

If this is confusing or annoying, then we can choose a more Haskell-like approach, making dc a total function: 

dc :: [Bit] -> Maybe CL
 dc = fst . head . runParser (neverfailing clP)

where 

neverfailing :: MonadPlus m => m a -> m (Maybe a)
 neverfailing p = liftM Just p `mplus` return Nothing

then, Chaitin's construction will be

\sum_{p\in 2^*,\;\mathrm{maybe}\;\downarrow\;\mathrm{hnf}\;\left(\mathrm{dc}\;p\right)} 2^{-\left|p\right|}

where \downarrow 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
Retrieved from "http://www.haskell.org/haskellwiki/Chaitin%27s_construction"

Category: Theoretical foundations