add "destructure variable declaration" assist(s) #52025

pq · 2023-04-12T21:23:32Z

Particularly for records, a destructuring assist would be handy and I expect commonly used but it would be interesting to explore lists, maps and objects too.

Records

For example,

(int, int) f() => (0,1);

m() {
  var r = f();
}

could become

(int, int) f() => (0,1);

m() {
  var (f1, f2) =  f();
}

And,

point() => (x: 1, y: 2);

z() {
  var r = point();
}

might become:

point() => (x: 1, y: 2);

z() {
  var (:x, :y) = point();
}

It gets interesting if the record declaration is accessed.

For example, I'd expect no assist in this case:

(int, int) f() => (0,1);

m() {
  var r = f();
  print(r);
}

but,

(int, int) f() => (0,1);

m() {
  var r = f();
  print(r.$1);
  print(r.$2);
}

could reasonably become:

(int, int) f() => (0,1);

m() {
  var (f1, f2) = f();
  print(f1);
  print(f2);
}

Maps

Map<String,int>  f() => {'x': 1};

m() {
  var r = f();
}

could become

Map<String,int>  f() => {'x': 1};

m() {
  var {} =  f();
}

(But note, they'll need to do something to {} since map patterns can't be empty.)

Lists

List<int> f() => [];

m() {
  var r = f();
}

could become

List<int> f() => [];

m() {
  var [] = f();
}

(Or var [...] = f();)

Objects

In a simple case we might take something like

class A {
  int a;
  A(this.a);
}

f() => A(1);

m() {
  var o = f();
}

to:

class A {
  int a;
  A(this.a);
}

f() => A(1);

m() {
  var A(:a) = f();
}

but this doesn't scale if A has a lot of fields.

Maybe better to start w/

var A() = f();

and possibly go better if fields are accessed.

class A {
  int a, b, c;
  A(this.a, this.b, this.c);
}

f() => A(1, 2, 3);

m() {
  var o = f();
  print('${o.a} ${o.b}');
}

could become

class A {
  int a, b, c;
  A(this.a, this.b, this.c);
}

f() => A(1, 2, 3);

m() {
  var A(:a, :b) = f();
  print('$a $b');
}

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bwilkerson · 2023-04-13T13:45:18Z

I'm not convinced that the non-record cases carry their weight, but I do agree that an assist to destructure a record is worth considering. If we add this, we should focus on records for a first cut and then consider the others as enhancements to the base functionality.

pq · 2023-04-13T18:25:16Z

/fyi @leafpetersen

First, basic cut. See: #52025 Change-Id: I3388f5ec96a45211669e3ac8a5975ff39feea4bb Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/295381 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Phil Quitslund <pquitslund@google.com>

pq · 2023-04-14T21:16:45Z

Basic records support landed w/ cae0bfa.

There's an open question of whether a field name suggestion should be a linked edit (https://dart-review.googlesource.com/c/sdk/+/295381/comment/6c626001_ad74fd62/) which I'll return to. Also, I wonder if we should add wildcards as additional suggestions for positional fields?

That is, we might add _ after $1 here:

@bwilkerson?

bwilkerson · 2023-04-17T14:28:41Z

I like the idea of offering a wildcard pattern here, it's a reasonable choice.

We should also consider suggesting wildcard patterns in code completion.

stereotype441 · 2023-04-17T18:32:18Z

I like this idea, especially for record and object patterns. I'm not quite as convinced by the map and list use cases.

Is there a way that we could make it available even if there's no pre-existing variable declaration? I.e. if I have:

class A {
  int a;
  A(this.a);
}

f() => A(1);

m() {
  f();
}

it would be nice if I could put the cursor on the f(); statement and trigger an assist that changes it to var A(:a) = f();

pq · 2023-04-17T18:40:14Z

Is there a way that we could make it available even if there's no pre-existing variable declaration?

Currently the expectation is that you'd do this in two steps, introducing the local variable first.

I do like the idea of supporting this in one go though. Maybe we could add something like "Assign value to a new destructured pattern variable" to the already existing assist?

(Word-smithing welcome.)

(FWIW @bwilkerson, @scheglov: this is another example of micro vs. macro edits as discussed earlier today.)

pq · 2023-04-17T19:11:23Z

After some thinking and an informal poll of folks, I'd like to go ahead with a simple conversion for Object patterns that takes

class A {
  int a;
  A(this.a);
}

f() => A(1);

m() {
  var o = f();
}

and produces

class A {
  int a;
  A(this.a);
}

f() => A(1);

m() {
  var A() = f();
}

with the caret inside the new declaration (A( < caret > )) assuming that folks will use code completion to fill in the fields they care about. (Note that completion improvements are being tracked in: #52058.)

See: #52025 Change-Id: I298c7f5b82b47b8cb02ca1a401e0658aab8f40a3 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/295680 Commit-Queue: Phil Quitslund <pquitslund@google.com> Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

See: #52025 Change-Id: I6dc5d2f0c8391d8ad719c23b47aadeac9a8ab969 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/295722 Reviewed-by: Brian Wilkerson <brianwilkerson@google.com> Commit-Queue: Phil Quitslund <pquitslund@google.com>

pq added legacy-area-analyzer Use area-devexp instead. P2 A bug or feature request we're likely to work on devexp-assist Issues with analysis server assists labels Apr 12, 2023

pq self-assigned this Apr 12, 2023

pq added the dart-model-language-patterns Issues with analyzer's support for the patterns language feature label Apr 17, 2023

This was referenced Apr 17, 2023

assist(s) to convert from if-else statements to switches (and back) #52068

Closed

if-case conversion assists #52069

Closed

srawlins added the type-enhancement A request for a change that isn't a bug label Mar 18, 2024

bwilkerson added area-devexp For issues related to the analysis server, IDE support, linter, `dart fix`, and diagnostic messages. and removed legacy-area-analyzer Use area-devexp instead. labels Feb 21, 2025

bwilkerson removed the dart-model-language-patterns Issues with analyzer's support for the patterns language feature label Mar 13, 2025

pq removed their assignment Apr 14, 2025

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Records

Maps

Lists

Objects