Comment by verdagon | Hacker News Reader

Comment by verdagon | Hacker News Reader

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I'm glad you brought inheritance up! This rule has solved it nicely in my experience: if a struct is linear, then it can only inherit linear interfaces. And implementation inheritance works because it's really just a combination of interface inheritance + composition + method forwarding, none of which seem to have any particular trouble with linear types.

Even if a language really wanted to have linear types in reference-counted objects, I don't think we'd see cycles in practice. Generally, for a reference-counted object to contain a linear type, we'd need exactly one reference to be the special "linear reference" (think a linear flavor of unique_ptr) which has ultimate responsibility for "consuming" (albeit not deleting) the contents. Making a unique_ptr cycle is much harder than making a regular RC cycle, so I'm not too worried, even if it is theoretically possible.

Re: trivial moves, I'm not sure what you mean. The system works through the entire program, not just local variables. Also, the `onException` method would also handle panics. Hope that helps!

As for inheritance - I'm increasingly convinced that it is a useful idea to have an explicit `Object` (or `BaseObject`, to deal with languages where primitives are special) class at the root of the hierarchy, rather than relying on some nebulous type-checker-only `Top` type (sometimes called `Any`, though that usually has additional footguns). One thing I do think is a mistake in languages that have an `Object` class is that it is often not abstract. In particular it is often used for sentinel-like objects (with no contents - the lack of a name is annoying), to say nothing of Javascript's mess. Is "top-most interface" really a sufficient concept? As much as I've come to love late-defined traits (among others, no more PIMPL), I'm not convinced we can really afford to stop thinking in terms of an OO tree, or even to give up on multiple inheritance

You don't need RC to have a cycle - it already occurs with a pair of structs that refer to each other using `mut Option[Unique[T]]`. Rust seems to have settled for "document it aggressively, and otherwise ignore it" ... yet code very similar to this is actually very useful, so (with a few extra layers of nesting) it often does appear spontaneously in the wild. But the whole point of linear types is that you can no longer just say "leaks are okay I guess".

Moving even a small 64KB buffer isn't actually trivial even if your type system thinks it is by composition, but most linear-ish type systems seem to aggressively use destructuring (possibly via pattern-matching) which relies on moving the members. Not to mention all the other actually-useful things C++ move constructors can do (update peer objects, for example). While "move-by-default" is a definite winner, "trivial-move-only" is not. The question of "what exactly happens so I can move out of a language-defined Option[T], leaving None ... vs how can a user-defined variant type choose to do something similar" is significantly complicated by nontrivial moves.

> Inheritance is no longer a simple part of the language. And trust me, you do want inheritance.

I don't miss it.

> languages where primitives are special

Isn't the object-primitive separation a design flaw? (e.g. c++ can have lists of int, but Java cannot. In Java, sometimes a boolean has two values and sometimes 3, etc.)

> root of the hierarchy

Why a hierarchy? All beginner Java devs at some point attempt to put their domain objects into some kind of hierarchy before realising the futility of it. Especially in game dev.

Sometimes I want to operate on objects which can quack(), and sometimes I want to operate on objects which can toBytes(), and sometimes I want to operate on objects which can do both those those things. But why must they have a common ancestor? Why can't they simply exist in disjoint families?

> I'm increasingly convinced that it is a useful idea to have an explicit `Object` (or `BaseObject`, to deal with languages where primitives are special) class at the root

This is one of those things I dislike about Java. Object has a bunch of included behaviours that I do not want. equals() and hashCode() have the wrong default behaviour, and toString() isn't much better. I would much prefer the compiler to block me if I try to equate two things which haven't defined equality, rather than slip in its own implementation.

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