Name: Kenneth A. Endfinger
Email: [email protected]
GitHub Username: kaendfinger
Allow values to be destructured on assignment.
Problems to solve:
- Assigning multiple variables from objects and lists can often be cumbersome and look rather ugly. One way to solve this is to allow developers to assign multiple variables in one go, in a simple and easy to understand manner.
class Point {
final int x;
final int y;
Point(this.x, this.y);
}
var {x, y} = new Point(5, 1);
assert(x == 5);
assert(y == 1);var [x, y] = [5, 2];
assert(x == 5);
assert(y == 2);var [a, b] = [1, 2];
[a, b] = [b, a];
assert(a == 2);
assert(b == 1);var [String a, int b] = ["Hello World", 5];
assert(a == "Hello World");
assert(b == 5);target.[a, b] = [1, 2];Destructuring should be allowed in any kind of assignment. Identifiers should be separated by a , with optional whitespace.
The following is what should happen when evaluating a destructuring assignment.
The value on the right should be evaluated, and the values should be assigned to the respective variables left to right.
The result of the expression should always be the right hand side of the expression.
It is a static warning if an variable identifier does not correspond to a property of the right hand side, unless the right hand side is marked as a proxy.
Ordering of the identifiers does not affect the outcome of the assignment. For instance: {a, b} = value is the same as {b, a} = value.
Starting with the first identifier, iterate through the identifiers with each identifier i and access the property on right with the name i.
In other words:
for (var identifier in identifiers) {
variables.set(identifier, getField(right, identifier));
}It is a runtime error if the number of identifiers on the left hand side is out of range of the right hand side list.
Starting with i = 0, for each identifier, assign the variable to (right)[i], then increment i by 1.
In other words:
for (var i = 0; i < identifiers.length; i++) {
variables.set(identifiers[i], right[i]);
}The behavior of the destructuring should be equivalent to the given code that does not use this proposal's syntax.
/// With this Proposal
var [a, b] = [1, 2];
/// Without this Proposal
var tmp = [1, 2];
var a = tmp[0];
var b = tmp[1];/// With this Proposal
var [String a, int b] = ["Hello", 5];
/// Without this Proposal
var tmp = ["Hello", 5];
String a = tmp[0];
int b = tmp[1];class Point {
final int x;
final int y;
Point(this.x, this.y);
}
/// With this Proposal
var {x, y} = new Point(5, 1);
/// Without this Proposal
var tmp = new Point(5, 1);
var x = tmp.x;
var y = tmp.y;class Point {
final int x;
final int y;
Point(this.x, this.y);
}
/// With this Proposal
var {int x, int y} = new Point(5, 1);
/// Without this Proposal
var tmp = new Point(5, 1);
int x = tmp.x;
int y = tmp.y;No alternatives have been proposed yet.
- This must also work in for-in loops:
for(var {a, b} in pairs) {
print("$a:$b");
}- Compound Assignments are allowed only in non-declarative assignments.
When compound assignment is used, the compound operator is applied to each variable like so:
For every variable
v, the proper destructuring is done up until it is time to assign to the variable. Before this occurs, the compound operator is applied to the variable. For example,[a, b] |= [true, false], would evaluate and destructure the right hand side. The variables are then assigned like this,a |= true,b |= false.
The changes described above should be incorporated into the specification.
No working implementation is available, although one can experience this syntax in practice by using languages that have this feature already.
TODO
TC52, the Ecma technical committee working on evolving the open Dart standard, operates under a royalty-free patent policy, RFPP (PDF). This means if the proposal graduates to being sent to TC52, you will have to sign the Ecma TC52 external contributor form and submit it to Ecma.