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0 pointsdevit1y ago0 comments

You can make the struct generic on the type to have a field being an impl Trait type.

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I know. Sometimes that can work. But that choice will ripple out to cause all sorts of other complications throughout your codebase. Now you have a Struct<T>. How do you pass one of these structs as a parameter to another function? Do you need to make that function generic over all T? How do you embed that into another struct, in turn? Do you put generic arguments everywhere? And so on.

Fundamentally, I don't want my type to be generic over all implementations. I want a concrete type. I want the type returned by one specific, often private, function.

But, nope. Not today. Maybe with TAIT, whenever that ships.

FridgeSeal1y ago

Am I right in understanding: you want some private, internal structure to be generic over some types, but you don’t want any wrapping structs to carry around that generic?

Could this problem be reformulated to use associated types, rather than full generics?

josephg1y ago

No. I have a function which returns impl Trait. The most common culprit for this is impl Iterator. (For example, a function does a map, filter or fold over something else and returns the resulting iterator).

I want to make another struct which slowly consumes that impl Iterator. So, the second struct is struct MyStruct { iter: (iterator returned from that function above) }.

Unfortunately, the iterator type doesn’t have a name. So currently that means it’s impossible to put this iterator in a struct.

This makes anything returned via impl Iterator (and impl Future and so on) second class citizens in rust. You are very limited in the ways you can use these objects compared to normal values.

My code is filled with hand written iterators that I could construct much more easily with map/filter/fold and friends. But I make them the long way simply so the resulting type has a name. The current situation is very silly.

j / k navigate · click thread line to collapse

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josephg1y ago

Fundamentally, I don't want my type to be generic over all implementations. I want a concrete type. I want the type returned by one specific, often private, function.

But, nope. Not today. Maybe with TAIT, whenever that ships.

FridgeSeal1y ago

Am I right in understanding: you want some private, internal structure to be generic over some types, but you don’t want any wrapping structs to carry around that generic?

Could this problem be reformulated to use associated types, rather than full generics?

josephg1y ago

I want to make another struct which slowly consumes that impl Iterator. So, the second struct is struct MyStruct { iter: (iterator returned from that function above) }.

Unfortunately, the iterator type doesn’t have a name. So currently that means it’s impossible to put this iterator in a struct.

This makes anything returned via impl Iterator (and impl Future and so on) second class citizens in rust. You are very limited in the ways you can use these objects compared to normal values.

j / k navigate · click thread line to collapse