Rust for C++ programmers – part 2: control flow

Re:Rust for C++ programmers – part 2: control flow by bjz_ on Sunday April 20, 2014 @09:18AM (#7616689)

Showing let bindings with their explicit type declarations might help:

    let mut x: int = 2;
    let x: &mut int = &mut 2;
    let (mut x, ref y, &z): (int, int, &int) = (1, 2, &3);

The pattern binding goes to the left of the `:`, and the input type goes to the right. Also note that you can pattern match on types too:

    let (mut x, ref y, &z): (_, int, &_) = (1, 2, &3);

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Re:Rust for C++ programmers – part 2: control flow by acqq on Sunday April 20, 2014 @10:08AM (#7616802)

Thanks, if _ is allowed as the type, why are explicit types written at all?

Is the last example equivalent to

    let (mut x, ref y, &z): (_, _, &_) = (1, 2, &3);

and can we go even further

    let (mut x, ref y, &z) = (1, 2, &3);

If we can, when do we actually need specifying the types? To do the type conversion or what?

Probably because I don't know Haskell (sorry) but I still imagine x y and z as names and even mut, ref and & as the "access" specifications, which define somehow the "type" of x y and z and not the type the thing accessed with the given name.

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Re:Rust for C++ programmers – part 2: control flow by bjz_ on Sunday April 20, 2014 @11:18AM (#7617025)

I might not have been clear enough. I was just trying to show how the current syntax fits in with `:`, and why your suggestions for `: mut` would not fit with that. All of these are valid Rust code:

    let (mut x, ref y, &z): (int, int, &int) = (1, 2, &3);
    let (mut x, ref y, &z): (_, _, &_) = (1, 2, &3);
    let (mut x, ref y, &z) = (1, 2, &3);

The types are still the same in each case. No conversions or dynamic typing is happening in any of them - they are all statically typed.

Rust, like Haskell and ML, uses an advanced form of type reconstruction called Hindley-Milner[0] type inference. This is a constraint solving algorithm that can deduce types from very little information. Sometimes you do need to add some annotations though to help the inference along a little, but that is very rare (although more common in Rust than in Haskell or ML).

The grammar could be expressed as:

    'let' <pattern> (':' <type>)? '=' <expr>

Where `<type>` is the type of the expression to the right of the `=`. As I said before: in practice it is very rare to explicitly annotate a `let` binding.

[0]: http://en.wikipedia.org/wiki/Hindley-Milner

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