Variadics in patterns

[Kixiron]

I'm wanting to model uninterpreted functions in egg but I'm not sure how I'd do something akin to this:

(fun foo a b c)

Which would have the variant type of

enum Func {
    "fun" = Function(Symbol, Vec<Id>), // Or even just a `Vec<Id>` where the first is always a `Symbol`
}

The problem I forsee is trying to match on that, e.g.

(ne (fun ?f ?x ?y ?z) (fun ?f ?x ?y ?z)) => false

The problem is the trailing arguments, do you have to generate patterns for a set number of cases or is there some sort of ...?args notation?

[mwillsey]

There are no variadic patterns in egg right now. The difficulty would be that a variable could then refer to any number of eclass ids.

Could you try a cons-list representation? That should allow you to efficiently manipulate heads of lists.

[saulshanabrook]

Would you be open to a PR which added variadic support? The cons list approach does work, but is a bit more verbose and expensive.

[mwillsey]

At this moment, I'm not sure variadics carry their weight. They open up a ton of questions for patterns and e-matching (can you partially bind variadics), and I'm just not sure it's worth the complexity. We also have plans to move to a more efficient relational e-matching algorithm at some point, and that simply doesn't support variadics.

[saulshanabrook]

@mwillsey sounds good, thank you for the explanation!