What would be nice if programming languages provided some support for such exhaustive testing[1].

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!).

Now it may be that applying this to anything more complex than stacks won't work well & it won't be perfect but I thought this was worth experimenting with. This would be like functional testing of all the nuts and bolts and components that go in an airplane. The airplane may still fall apart but that would be a "composition" error!

[1] There are "proof assisant" or formal spec languages such as TLA+, Coq, Isabelle etc. but they don't get used much by the average programmer. I want something more retail!

On May 23, 2024, at 1:52 PM, Rob Pike <robpike@gmail.com> wrote:

The semantic distinction is important but the end result is very similar. "Fuzzing" as it is now called (for no reason I can intuit) tries to get to the troublesome cases faster by a sort of depth-first search, but exhaustive will always beat it for value. Our exhaustive tester for bitblt, first done by John Reiser if I remember right, set the stage for my own thinking about how you properly test something.

-rob

On Thu, May 23, 2024 at 11:49 PM Douglas McIlroy <douglas.mcilroy@dartmouth.edu> wrote:
> Doug McIlroy was generating random regular expressions

Actually not. I exhaustively (within limits) tested an RE recognizer without knowingly generating any RE either mechanically or by hand.

The trick: From recursive equations (easily derived from the grammar of REs), I counted how many REs exist up to various limits on token counts, Then I generated all strings that satisfied those limits, turned the recognizer loose on them and counted how many it accepted. Any disagreement of counts revealed the existence (but not any symptom) of bugs.

Unlike most diagnostic techniques, this scheme produces a certificate of (very high odds on) correctness over a representative subdomain. The scheme also agnostically checks behavior on bad inputs as well as good. It does not, however, provide a stress test of a recognizer's capacity limits. And its exponential nature limits its applicability to rather small domains. (REs have only 5 distinct kinds of token.)

Doug