What is the correct term for _ in a type hint?

问题

In type hints in Rust it is possible to use partial types in annotations like this:

let myvec: Vec<_> = vec![1, 2, 3];

What is the correct terminology for the underscore in the partial type annotation? I'm interested in both the Rust terminology as well as more academic type theory terminology.

回答1:

I was able to find a piece of official documentation where the underscore is named in the context of patterns, but I doubt it's a "strict" name:

Patterns consist of some combination of literals, destructured arrays or enum constructors, structs and tuples, variable binding specifications, wildcards (..), and placeholders (_).

The Book provides the following description in the glossary:

_: "ignored" pattern binding (see Patterns (Ignoring bindings)). Also used to make integer-literals readable (see Reference (Integer literals)).

I was not able to find a definition pointing specifically to partial type annotations, but I think "placeholder" (or "type placeholder", depending on the context) would not be ambiguous.

回答2:

After some digging it seems that Vec<_> is consistently called a partial type (so in let x: Vec<_> we have a partial type annotation, while Fn(String) -> _ would be a partial type signature) but the _ in this context is varyingly called either a type wildcard or a type placeholder, and _ in the type grammar can be read as the token for "infer this type" (at the time of the PR mentioned below, TyInfer internally in the compiler).

Some interesting reading:

• Partial type signatures in Haskell
• The pull request which added _ to the Rust type grammar
• Intermingled parameter lists - Niko Matsakis' blog post in which he proposes to "Introduce _ as a notation for an unspecified lifetime or type"

Interesting detail from the PR:

let x: _ = 5;
let x    = 5;

The two lines above are equivalent, and both parsed as variable x with type TyInfer.

回答3:

In the compiler, it seems to be called Infer (in syntax::ast, rustc::hir, and rustc::ty)

I think this naming is somewhat reasonable, because these _s are replaced with fresh (type) inference variables before doing Hindley-Milner-like type inference.

来源：https://stackoverflow.com/questions/45033804/what-is-the-correct-term-for-in-a-type-hint