Collections should offer consistent 'contains' membership predicates about motoko-base

[rossberg]

I think the design doc calls these has? Or is that different?

[matthewhammer]

Whatever the name, this operation needs to either be higher order, or only exist within APIs for containers whose elements have a known equal function for them.

Should the type vary depending on that situation?

[rossberg]

Maybe I was confused, has is meant to test key membership. Are you guys talking about value membership? Then maybe call that hasValue?

[matthewhammer]

I'm not sure about the name, but the common idea (across finite maps, sets and sequences) could be:

Does "this thing" exist in the collection, among "those things that I want to identify there"?

For map-like collections, that "identified thing" would be a key (regardless of its associated value in the map). For sets and sequences, it would be a value, since there are no keys and thus no other way to search or identify anything. In a map, however, the identity of values is the identity of their keys.

[matthewhammer]

FWIW, I would call the operation that I am describing contains, and have it return a Bool. It's not exactly has or hasValue, but it does one or the other, depending.

[rossberg]

@matthewhammer, I'd assume that for a map you generally want both, so they should be distinguished by coherent naming conventions.

[matthewhammer]

Agreed, you'd want both has and hasValue.

I guess you're saying that you don't also want to alias has for maps with the additional name contains (my proposal to satisfy this issue); if not, I understand. It seems strange/gross.

Not sure how to resolve the issue as stated then.

[rossberg]

Still not sure what's actually being proposed. Is it

• ask for key (has, should already present everywhere AFAIK)
• ask for value (hasValue, would need equality function, I suppose)
• ask for value based on predicate (essentially, Option.isSome o find)
• ask for entry based on predicate (essentially, Option.isSome o findEntry)
• several of the above?

Can somebody clarify?