Language feature: flat tuple as function arguments

[swfsql]

Optionally flatten a tuple as arguments themselves when fed into a function.

This suggestion is tangent to other two suggestions:

• Support flattening of tuples #2138 - Support flattening of tuples
• Language feature: named returns #2638 (comment)

So, for examaple:

fn some_fn(a: A, b: B) -> () {
  // ()
} 

let tuple = (A, B);
some_fn(tuple);

And:

(0..10)
  .map(|_| (A, B))
  .map(some_fn)
  .collect::<Vec<_>>();

would be possible/accepted.

---

I'm not sure if this could be reached with generics or some other way, but I believe it cannot.

The closest I could get with generics was:

struct A;
struct B;

fn call1<F, T1, FR>(tuple: (F, T1,)) -> FR
    where F: FnOnce(T1) -> FR {
    let function = tuple.0;
    function(tuple.1)
}

fn call2<F, T1, T2, FR>(tuple: (F, T1, T2)) -> FR
    where F: FnOnce(T1, T2) -> FR {
    let function = tuple.0;
    function(tuple.1, tuple.2)
}

fn some_fn(_a: A, _b: B) -> () {
    println!("hi");
    ()
}

fn main() -> () {

    let tuple = (A, B);
    some_fn(tuple.0, tuple.1);
    
    (0..2)
        .map(|_| (some_fn, A, B))
        .map(call2)
        .collect::<Vec<_>>();
}

But it would require inserting the function into the tuple beforehand, calling a call function (and also call the right one).. which is quite off from this feature request.

---

Another approach would be to wrap the original function, unflattening it:

// ...

fn some_fn_unflat((a, b): (A, B)) -> () {
    some_fn(a, b)
}

fn main() -> () {
    let tuple = (A, B);
    some_fn(tuple.0, tuple.1);
    
    (0..2)
        .map(|_| (A, B))
        .map(some_fn_unflat)
        .collect::<Vec<_>>();
}

Which is quite approachable. I originally didn't think about this, a macro could generate those wrapped functions.

I guess I'll be closing then lol sorry - but if someone would consider it to be appropriate as a standard approach, please feel free to reopen it :P

[swfsql]

If anyone is interested,
I've uploaded an initial crate with the mentioned syntax:

• https://crates.io/crates/loosen
• https://crates.io/crates/loosen_map