Is it possible to unpack a tuple into function arguments?

Question

If I want to unpack a tuple and pass it as arguments is there a way to do this:

//Does not compile
fn main() {
    let tuple = (10, Vec::new());
    foo(tuple);
}
fn foo(a: i32, b: Vec<i32>) {
    //Does stuff.
}

Instead of having to do this:

fn main() {
    let tuple = (10, Vec::new());
    foo(tuple.0, tuple.1);
}
fn foo(a: i32, b: Vec<i32>) {
    //Does stuff.
}

score 24 · Accepted Answer · answered Oct 05 '16 at 15:57

24

On a nightly compiler:

#![feature(fn_traits)]

fn main() {
    let tuple = (10, Vec::new());
    std::ops::Fn::call(&foo, tuple);
}
fn foo(a: i32, b: Vec<i32>) {
}

There is AFAIK no stable way to do that.

answered Oct 05 '16 at 15:57

mcarton

1

But there is, see my answer. – ljedrz Oct 05 '16 at 17:00
11

@ljedrz I assumed he did not want/could not change the function signature. – mcarton Oct 05 '16 at 17:06
1

In that case yes, I wouldn't see any other way either. – ljedrz Oct 05 '16 at 17:09

ljedrz · Answer 2 · 2016-10-05T16:57:40.027

21

There is a way, using the magic of pattern matching:

fn main() {
    let tuple = (10, Vec::new());
    foo(tuple);
}

fn foo((a, b): (i32, Vec<i32>)) {
    // do stuff
}

As with let bindings, function arguments are irrefutable patterns, so any pattern that is valid in a let binding is also valid as an argument.

So you can specify an argument like:

(a, b): (i32, Vec<i32>)

just like you would in a let statement.

edited Oct 05 '16 at 16:57

answered Oct 05 '16 at 16:48

ljedrz

7

can you elaborate about this a little bit more? Isn't it then like a calling a function with a single argument which happened to be a tuple? – piotao Jul 23 '20 at 22:28
1

@piotao Yes, `foo` is a one argument function that takes a tuple. It just so happens that the tuple is immediately pattern-matched and unpacked in the function parameters. – Colonel Thirty Two Jun 24 '21 at 16:26

score 10 · Answer 3 · answered Oct 05 '16 at 15:45

10

let (a, b) = (10, Vec::new());
foo(a, b);

answered Oct 05 '16 at 15:45

Lee

13

I upvoted, but this answer would be better if it explicitly answered the question rather than just offering alternative code. – trent Oct 05 '16 at 15:57

3 Answers3