Chaitin's construction
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).
Basing it on combinatory logic
Some more direct relatedness to functional programming: we can base on combinatory logic (instead of a Turing machine).
Coding
See the prefix coding system described in Binary Lambda Calculus and Combinatory Logic (page 20) written by John Tromp:
of course, , are meta-variables, and also some other notations are changed slightly.
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.
Approach based on decoding with partial function
Now, Chaitin's construction will be here
where
- should denote an unary predicate “has normal form” (“terminates”)
- should mean an operator “decode” (a function from finite bit sequences to combinatory logic terms)
- should denote the set of all finite bit sequences
- 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!)
Approach based on decoding with total function
Seen above, 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 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
where should denote false truth value.
Related concepts
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