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by simonask 106 days ago
I would also have liked to see some motivational examples, but I think the most interesting upside of an effect system is composability.

Rust is actually really unique among imperative languages in its general composability - things just compose really well across most language features.

The big missing pieces for composability are higher-kinded types (where you could be generic over Option, Result, etc.), and effects (where you could be generic over async-fn, const-fn, hypothetical nopanic-fn, etc.)

The former becomes obvious with the amount of interface duplication between types like Option and Result. The latter becomes obvious with the number of variants of certain functions that essentially do the same thing but with a different color.
3 comments
zozbot234 106 days ago
Some uses of higher-kinded types (though not all of them) can be addressed by leveraging Generic Associated Types (GAT).

Part of the problems is that the "things just compose really well" point becomes gradually less and less applicable as you involve the lower-level features Rust must be concerned with. Abstractions start to become very leaky and it's not clear how best to patch things up without a large increase in complexity. A lot of foundational PL research is needed to address this sensibly.

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pjmlp 105 days ago
Not so much composability on async Rust.
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gf000 105 days ago
> Rust is actually really unique among imperative languages in its general composability

Can you compare it to some other imperative language? Because I really don't see anything particularly notable in Rust that would give it this property.

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simonask 105 days ago
I’m mainly comparing to the progeny of C, where the biggest difference is the fact that almost everything is an expression in Rust.

No need for ternary operators. C# unsafe blocks can only appear as statements (so you cannot delegate from a safe to an unsafe constructor, e.g.). C++ cannot return from the middle of an expression.

A related aspect is the type system, which composes with expressions in really interesting ways, so things like constant array sizes can be inferred.

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