Specifying Function Contract

[mrufsvold]

The thread Case Study: Method Invalidations caused by Pkg.jl with Julia 1.11 ended up answering a problem here:

@ChrisRackauckas

I think a nice example of that (::Any == ::Any)::Bool which is perfectly fine in Base as the only return type from == in Base, but then any symbolic representation of code turns == into something symbolic and every code with a type-unstable == recompiles due to Symbolics existing. The problem is, if == “should” only ever return a Boolean (which I don’t think should be the case, there’s other counter examples), then we should get an error for violating the rule.

Specifically here, given the importance of Symbolics in the Julia ecosystem, the meaning of function names doesn’t always match the mechanical use of those functions. Setting a contract that requires == to return a Bool would be problematic for them.

I am certainly no Core Developer, so I don’t pretend to know the tradeoffs for serving different use cases. But I do wonder if it would be possible to use Cassette.jl to transform all (F)(args...) where F to (F)(SymbolicCall(), args...) that could have its own contract, so that “normal” use cases could benefit from the correctness checks that function contracts could provide.