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Philomath

See also: LCF-style-NDMaxixeCardboard Prolog


Philomath is an LCF-style theorem prover written in ANSI C. It implements classical propositional logic inside a Natural Deduction system with labelled assumptions. For more information on this approach, see the LCF-style-ND repository.

How do I write a proof with this?

Create a file myproof.c:

#include "formula.h"
#include "proof.h"
int main(int argc, char **argv) {
    /* Proof of p -> p */
    return proves(
        impl_intro(1, suppose(var("p"), 1)),
        impl(var("p"), var("p"))
    );
}

Then compile and run it with:

./build-proof.sh myproof

And run the resulting executable:

./myproof.exe

If the exit code is 0, the proof is valid!

% echo $?
0

If you wish to build the proof with debugging output, you can pass the -D flag:

./build-proof.sh -D myproof

Limitations

LCF-style theorem proving relies on data abstraction, i.e. information hiding. In C, information hiding is accomplished by exposing, in a header file, only the fact that a struct exists, and not exposing its structure. Client code can then work with pointers to that struct without knowing what's contained in it.

It is however possible to subvert this mechanism. If your proof code does any of the following things, it is possible it is no longer a valid proof:

  • includes the file proof.c directly, rather than including proof.h
  • casts a struct proof value to some other type
  • casts a value of some other type to a struct proof

Fortunately, it is possible to statically analyze a C source file and confirm that it does none of these things, if such a level of assurance is desired.

TODO

Should really have more demo non-proofs, to check that it's not letting non-proofs pass themselves off as proofs.

Could stand to have a real suite of unit tests instead of just a handful of demo proofs than you can build and run manually.