Another Gypsy user here...

For KSOS-11 the kernel was described in SPECIAL - as a set of axioms and theorems. There was no actual connection between the formal specification in SPECIAL and the Modula code.

Some of the critical user-space code for a trusted downgrade program, to bridge data from higher levels of classification to lower, was written in Gypsy. I visited UT Austin and Dr Good(?)'s team to learn it, IIRC. Gypsy was considered better in that the specification was tied to the executable through the pre/post conditions - and the better support for semi-automated theorem proving.




On Sat, May 25, 2024 at 10:18 AM Paul Winalski <paul.winalski@gmail.com> wrote:
On Fri, May 24, 2024 at 8:18 PM Bakul Shah via TUHS <tuhs@tuhs.org> wrote:

At one point I had suggested turning Go's Interface type to something like Guttag style abstract data types in that relevant axioms are specified right in the interface definition. The idea was that any concrete type that implements that interface must satisfy its axioms. Even if the compiler ignored these axioms, one can write a support program that can generate a set of comprehensive tests based on these axioms. [Right now a type "implementing" an interface only needs to have a set of methods that exactly match the interface methods but nothing more] The underlying idea is that each type is in essence a constraint on what values an instance of that type can take. So adding such axioms simply tightens (& documents) these constraints. Just the process of coming up with such axioms can improve the design (sor of like test driven design but better!).

At one point I worked with a programming language called Gypsy that implemented this concept.  Each routine had a prefix that specified axioms on the routine's parameters and outputs.  The rest of Gypsy was a conventional procedural language but the semantics were carefully chosen to allow for automated proof of correctness.  I wrote a formal specification for the DECnet session layer protocol (DECnet's equivalent of TCP) in Gypsy.  I turned up a subtle bug in the prose version of the protocol specification in the process.

-Paul W.