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I have seen Vec<_> a couple of times already. For example:

let a = "line1\r\nline2\nline3";
println!("{:?}", a.lines().collect::<Vec<_>>());

But what does that 'uncertain face' <_> mean?

I'm used to a typename in angle brackets, but what type can that be? The only meaning of underscore that I'm aware of is from Python as a name for an unused variable.

kmdreko
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Amomum
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1 Answers1

41

It means "Rust compiler, infer what type goes into the Vec". And it is indeed analogous to the unused variable in Python (and in Rust itself), in that it represents a placeholder for a type, like it can represent a placeholder for a variable name.

You can find an explanation in The Rust Programming Language chapter about iterator consumers:

Using a _ will let you provide a partial hint:

let one_to_one_hundred = (1..101).collect::<Vec<_>>(); This says "Collect into a Vec<T>, please, but infer what the T is for me." _ is sometimes called a "type placeholder" for this reason.

Shepmaster
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Paolo Falabella
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    Thanks! For some reason generics chapter in the docs doesn't say anything about parameter type inference. – Amomum Dec 18 '15 at 21:45
  • Is there any documentation somewhere specifically about the `_` syntax? I can't find any. I only find text where it is used when iterators are explained. – Lii Aug 12 '17 at 17:36
  • @Lii I think that's the only place it's mentioned right now. The issue on github for documenting the feature has been closed adding that paragraph to the iterators section: https://github.com/rust-lang/rust/pull/22293 – Paolo Falabella Aug 15 '17 at 12:07