Type inference was a mistake (borretti.me)

submitted 7 months ago by SuperFola@programming.dev to c/programming_languages@programming.dev

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[-] BatmanAoD@programming.dev 5 points 7 months ago* (last edited 7 months ago)

The thing about Rust's type inference that seems wild to anyone who hasn't seen Hindley-Milner/ML style type systems before is that it's "bidirectional" (in quotes because that's not a proper type theory term as far as I know). The type of the left-side of an assignment can determine the type (and behavior!) of the right side. For instance, this is ambiguous:

let foo = [("a", 1), ("b", 2)].into_iter().collect();

The expression creates an iterator over the (letter, number) pairs, and collect() stores the elements in a newly created container. But which container type? Here are two valid variants:

let foo: Vec<_> = [("a", 1), ("b", 2)].into_iter().collect();

This creates a vector with items ("a", 1) and ("b", 2).

let foo: HashMap<_, _> = [("a", 1), ("b", 2)].into_iter().collect();

This creates a mapping where "a" and "b" are keys, and 1 and 2 are the corresponding values.

Playground link in case you'd like to mess with this concept: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=76f999f4db600415643b0c58c19c69b7

this post was submitted on 06 Apr 2024

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