Destructuring assignment

[glaebhoerl]

Given

struct Point { x: int, y: int }
fn returns_point(...) -> Point { ... }
fn returns_tuple(...) -> (int, int) { ... }

it would be nice to be able to do things like

let a; let b;
(a, b) = returns_tuple(...);
let c; let d;
Point { x: c, y: d } = returns_point(...);

and not just in lets, as we currently allow.

Perhaps even:

let my_point: Point;
(my_point.x, my_point.y) = returns_tuple(...);

(Most use cases would likely involve mut variables; but those examples would be longer.)

Related issues from the rust repo: rust-lang/rust#10174 rust-lang/rust#12138

[sbditto85]

Not sure the best way to indicate a vote in the affirmative, but 👍 +1 for this

[MattWindsor91]

Not sure how rust's RFC process goes, but I assume this needs to be written up in the appropriate RFC format first? I like it, mind.

[arthurprs]

EDIT: not so fond of it anymore

[bstrie]

@glaebhoerl, how do you expect this to be done? It seems to me that it would require the ability for patterns to appear in arbitrary positions, which strikes me as completely infeasible.

[glaebhoerl]

@bstrie I don't have any plans myself. There was some discussion of this elsewhere, possibly on the rust repository issues - I think the idea might've been that we could take the intersection of the pattern and expression grammars?

[bstrie]

Assuming that we took the easy route and made this apply only to assignments, we'd also need to take our grammar from LL(k) to LL(infinity). I also don't think that an arbitrarily restricted pattern grammar will make the language easier to read and understand. Finally, the only time when this feature would be useful is when you can't use a new let binding because of scope, in which case the current workaround is to use a temporary. I'm not currently convinced that the gain is worth the cost.

[DavidJFelix]

👍 I've found myself wanting this from time to time, especially in reducing repetition in match statements or normal assignment. Right now I'm using small purpose-built functions instead of this. I haven't considered if it would be possible to abuse a feature like this easily or not.

[tstorch]

I would be thrilled if this would be implemented! Here is a small example why:

Currently in libcore/str/mod.rs the function maximal_suffix looks like this:

fn maximal_suffix(arr: &[u8], reversed: bool) -> (uint, uint) {
    let mut left = -1; // Corresponds to i in the paper
    let mut right = 0; // Corresponds to j in the paper
    let mut offset = 1; // Corresponds to k in the paper
    let mut period = 1; // Corresponds to p in the paper

    while right + offset < arr.len() {
        let a;
        let b;
        if reversed {
            a = arr[left + offset];
            b = arr[right + offset];
        } else {
            a = arr[right + offset];
            b = arr[left + offset];
        }
        if a < b {
            // Suffix is smaller, period is entire prefix so far.
            right += offset;
            offset = 1;
            period = right - left;
        } else if a == b {
            // Advance through repetition of the current period.
            if offset == period {
                right += offset;
                offset = 1;
            } else {
                offset += 1;
            }
        } else {
            // Suffix is larger, start over from current location.
            left = right;
            right += 1;
            offset = 1;
            period = 1;
        }
    }
    (left + 1, period)
}

This could easily look like this:

fn maximal_suffix(arr: &[u8], reversed: bool) -> (uint, uint) {
    let mut left = -1; // Corresponds to i in the paper
    let mut right = 0; // Corresponds to j in the paper
    let mut offset = 1; // Corresponds to k in the paper
    let mut period = 1; // Corresponds to p in the paper

    while right + offset < arr.len() {
        let a;
        let b;
        if reversed {
            a = arr[left + offset];
            b = arr[right + offset];
        } else {
            a = arr[right + offset];
            b = arr[left + offset];
        };
        // Here is the interesting part
        (left, right, offset, period) =
            if a < b {
                // Suffix is smaller, period is entire prefix so far.
                (left, right + offset, 1, right - left)
            } else if a == b {
                // Advance through repetition of the current period.
                if offset == period {
                    (left, right + offset, 1, period)
                } else {
                    (left, right, offset + 1, period)
                }
            } else {
                // Suffix is larger, start over from current location.
                (right, right + 1, 1, 1)
            };
        // end intereseting part
    }
    (left + 1, period)
}

If we apply, what is currently possible this would be the result:

fn maximal_suffix(arr: &[u8], reversed: bool) -> (uint, uint) {
    // Corresponds to (i, j, k, p) in the paper
    let (mut left, mut right, mut offset, mut period) = (-1, 0, 1, 1);

    while right + offset < arr.len() {
        let (a, b) =
            if reversed {
                (arr[left + offset], arr[right + offset])
            } else {
                (arr[right + offset], arr[left + offset])
            };
        (left, right, offset, period) =
            if a < b {
                // Suffix is smaller, period is entire prefix so far.
                (left, right + offset, 1, right - left)
            } else if a == b {
                // Advance through repetition of the current period.
                if offset == period {
                    (left, right + offset, 1, period)
                } else {
                    (left, right, offset + 1, period)
                }
            } else {
                // Suffix is larger, start over from current location.
                (right, right + 1, 1, 1)
            };
    }
    (left + 1, period)
}

This is easily more readble and I guess readbility of code is a major contribution to code safety and attracts more people to the language and projects written in that laguage.

[bombless]

It doesn't feel right...
If you insist, I think this looks better:

introduce a, b;
let (a, b) = returns_tuple(...);
introduce c, d;
let Point { x: c, y: d } = returns_point(...);

Still doesn't feel right, but looks more reasonable.

[DavidJFelix]

So already @bombless this clashes for me as introduce would then become the longest word in rust.

[bombless]

@DavidJFelix I don't know, I'd say -1 for this assignment idea.
And maybe change introduce to intro will make you feel better.

[DavidJFelix]

@bombless, a bit but not much. The point of "let" isn't to offer assignment, it's to introduce the variable. Assignment is done with an assignment operator, "=", If we use both the "=" and let for assignment, it becomes redundant. This is why you see:

let mut x: uint;
...
x = 123456789;

the point of this issue is that "let" allows us to unravel tuple-packed variables as we declare them and also set their value in one assignment, rather than multiple assignments; but later throughout the program, the assignment operator ceases to do this unraveling and must be done for each variable.

[taralx]

So there's two ways to do this. With a desugaring pass (easier) or by actually extending the implementation of ExprAssign in the typechecker and translation. The former works, but I suspect it doesn't produce as nice a set of error messages when types don't match.

Thoughts?

[carllerche]

I am 👍 for this too