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Thanks, this was much easier to understand than the original article. What will I use this for? Usually for the “T” to be useful it needs to have some interface

by brap 4y ago

Thanks, this was much easier to understand than the original article. What will I use this for? Usually for the “T” to be useful it needs to have some interface. In Java I’d just use Collection<Integer> or something more abstract.

mrkeen 4y ago
T keeps its type info statically. So if you put a Set in, you can get a Set out. Casting to Collection loses the static type info, so you couldn't write foo(bar(set)) if bar returned a Collection but foo accepted a Set.
orblivion 4y ago
I forget what exactly a GADT is (even though I've used Haskell a bit before). But just going by the linked example from Reddit, you could make a generic `map` function that worked on any container type. You'd have to implement the map function separately for each type. But then you could make a bunch of utility functions that you'd implement once that use the generic `map` under the hood, and all of those container types would be able to use those utility functions. Not a Rust guy but I think that's the idea.
- kibwen 4y ago
  Just so that we're clear on acronyms for everyone out there, "GADT" is a computer science term for "generalized algebraic data type", which has no relation to "GAT", which is just Rust terminology for generics in associated types (even in Rust, I doubt that anyone will bother to refer to "GAT" in the coming years, rather than just lumping them in with the concept of associated types in general; there's really no need to teach them separately).
- ghostwriter 4y ago
  > you could make a generic `map` function that worked on any container type. not only container types, it can be any type that has a consistent context of evaluation for the final value being calculated: "doing async IO", "doing optional presence", "doing potentially erroneous extraction", and so on. In that sense, "doing multiplicity of values" is the context expressing a container type you mentioned as an example.

by brap 4y ago

mrkeen 4y ago

T keeps its type info statically. So if you put a Set in, you can get a Set out. Casting to Collection loses the static type info, so you couldn't write foo(bar(set)) if bar returned a Collection but foo accepted a Set.

orblivion 4y ago

I forget what exactly a GADT is (even though I've used Haskell a bit before). But just going by the linked example from Reddit, you could make a generic `map` function that worked on any container type. You'd have to implement the map function separately for each type. But then you could make a bunch of utility functions that you'd implement once that use the generic `map` under the hood, and all of those container types would be able to use those utility functions. Not a Rust guy but I think that's the idea.

kibwen 4y ago
Just so that we're clear on acronyms for everyone out there, "GADT" is a computer science term for "generalized algebraic data type", which has no relation to "GAT", which is just Rust terminology for generics in associated types (even in Rust, I doubt that anyone will bother to refer to "GAT" in the coming years, rather than just lumping them in with the concept of associated types in general; there's really no need to teach them separately).
ghostwriter 4y ago
> you could make a generic `map` function that worked on any container type. not only container types, it can be any type that has a consistent context of evaluation for the final value being calculated: "doing async IO", "doing optional presence", "doing potentially erroneous extraction", and so on. In that sense, "doing multiplicity of values" is the context expressing a container type you mentioned as an example.