## Posts

Showing posts from February, 2008

### The Co-Natural Numbers in Haskell and Coq

I was playing around with the Peano numerals in Haskell (as I often do), because I remember briefly reading about a generalisation of exponentiation in a paper once when I didn't have time to read it. I decided I'd quickly look at it again now that I was in a mood to procrastinate.

>data Nat = Z | S Nat deriving Show This data type is usually identified with the peano numerals. In fact as we will see, in haskell it isn't quite the natural numbers. We take Z to mean 0 and take (Sn Z) or (S (S (S ... Z))) n times, to mean n. It's basically a unary number system where the length of the string gives us the number.

It is pretty straightforward to write the basic arithmetic operations on the peano numerals.

> add x Z = x > add x (S y)= S (add x y)> mult x Z = Z > mult x (S y)= add x (mult x y)> expt x Z =(S Z)> expt x (S y)= mult x (expt x y)Here we can see an extension of this series fairly easily. The only real question we might have is what to do …

### Verifying the Monad Laws with Supercompilation

A while ago I wrote a post about how one can use coq to make a proper monad module. I was just thinking today that it would be nice to have a tool for haskell that would allow one to write down conjectures and discharge them automatically with a theorem prover. Supercompilation makes a nice clean theorem prover for haskell since one could express the equations of interest in haskell itself. Below is an example of the list monad, and then the 3 monad laws written as conj1,conj2 and conj3. I prove the first law by manual supercompilation. The next two are left as an exercise for the interested reader.

equal xs ys = case xs of [] -> case ys of [] -> True _ -> False (x:xs') -> case ys of [] -> False (y:ys') -> case (eq x y) of False -> False …

### Proof by Program Transformation

Program transformation is a sort of meta-compilation where the source and target languages are the same. In this sense it can be seen as an automorphism on the set of programs in a language.

As was pointed out by Turchin, it is sometimes possible to prove properties of programs simply by performing a program transformation. Basically a program, and a program which verifies it's behaviour (a predicate) can be replaced with a truth value if the program transformation is sufficiently powerful. An obvious example of course, which can be calculated using partial evaluation would be something like (max (< 3) (1::2::1::nil)). In fact very complex properties can be proved automatically by Distillation, a particularly powerful program transformation system.

In my research I've been working with the μ-Calculus, which is a type of temporal logic that is very powerful (LTL, CTL, CTL* can all be embedded in the μ-Calculus) and good for describing reactive systems. It uses a notion…