[Question] Is it possible to compute the intersection of two generic HLists in stable Rust?

[Only1Loatheb]

Hi,
Is it possible to compute the intersection of two generic HLists of unknown types at compile time in stable Rust?
I expect it to work similarly to Plucker or Sculptor, but instead of having Remainder associated type it would have Intersection associated type.
Just like in Sculptor, the user shouldn't have to specify the exact Indices type parameter.
I saw a related github issue [Question] Treating an HList like a set/map?, but I didn't see the HList intersection operation discussed there.
HList intersection would be useful for my proof-of-concept process definition library brrricks.
Thank you for your fantastic work on Frunk, and thank you in advance for taking the time to answer my question!

[lloydmeta]

two generic HLists of unknown types

Not exactly sure what this means but I'll assume this isn't saying this means "arbitrary-but-compile-time-known" Hlists....otherwise the answer is a flat no :D

That aside..intuitively it feels doable: just starting with some very basic cases (horrid pseudocode):

1. Starting: let firstHList = hlist![ a: A, remainder ] and let secondHList = hlist![ ... ]: intersection is

• if you can pluck A from the secondHList and get back remainderOfSecondHListAfterPlucking: intersection is hlist![ A ] + firstHList.tail.intersect(remainderOfSecondHListAfterPlucking)
• if you can't : intersection is HNil + firstHList.tail.intersect(secondHList)

2. Exit condition: hlist![ ] and hlist![ ... ] : intersection is hlist![ ]

I think the problem really is how to split the cases in (1) : Plucking must work or it won't compile - theres is no option to say "does not exist" (in a way, this is I think what this comment is saying #187 (comment)).

So, I think the same situation would apply to this (that post is asking for a union, this is asking for an intersection). That said, I think the only way to know for sure is to bang your head at it 😆 ....

[Only1Loatheb]

I think I will just assign an identifier to a HList element.
Example here: https://github.com/Only1Loatheb/hlist-intersection-assuming-manual-id-assignment/blob/master/src/main.rs

[ExpHP]

Fascinating! I wonder if that works because of something special about const type parameters, or if this would work for peano integers as well?

[Only1Loatheb]

All necessary operations can be implemented for Peano integers. I opted for the library solution for convenience.
equals
prepend if
filter

It could even work with frunk chars if an equals operation that returns a bool defined ad hoc or typenum bit is implemented.