[rust-dev] "Instance Chains: Type Class Programming Without Overlapping Instances" and type classes in Rust

Tim Chevalier catamorphism at gmail.com
Mon Jan 9 17:36:58 PST 2012


Since I've been hibernating for a while, I'm not really familiar with
Rust's type class system, but after discussing it with Graydon a bit,
I would be curious to hear (probably from Marijn) about the
similarities and differences between it and instance chains as
described in this paper by some of my former colleagues:


The problem the paper addresses is in Haskell, where having multiple
instances in scope for the same class and type can cause unpredictable
behavior. (The paper explains the basic problem in more detail pretty
well.) It seems like there's an analogous issue in Rust when you
import multiple sets of methods for the same type class, which I
understand is handled with a compile failure. A difference is that
with instance chains, an instance is not a "first-class" thing that
you can import (like an item in Rust terminology) -- rather, you use
type-level if-then expressions to disambiguate which instances apply.

>From a research perspective, it would be good to compare/contrast what
Rust is doing with the instance chains approach, since I think they're
addressing some similar problems.


Tim Chevalier * http://catamorphism.org/ * Often in error, never in doubt
“I cannot hide my anger to spare you guilt, nor hurt feelings, nor
answering anger; for to do so insults and trivializes all our efforts.
Guilt is not a response to anger; it is a response to one’s own
actions or lack of action.” -- Audre Lorde

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