Destructuring

[lrhn]

Destructuring is now parts of the patterns feature, and tracked by the specification of that (#546).
No further updates are expected to this issue, it's kept open only as a representation of the status of that particular part of pattern matching.

---

Dart could have some way to easily "destructure" composite values.

Example proposal from the DEP process: https://github.com/DirectMyFile/dep-destructuring (by @kaendfinger).

[andreashaese]

For more inspiration, I'll add some Swift examples.

Tuples & tuple desctructuring

let tuple = (1, "A")
let (x, y) = tuple

Tuple dot notation

let triple = (1, 2, 3)
let (a, b) = (triple.0, triple.2) // a = 1, b = 3

Binding with pattern matching

switch tuple {
case let (x, y) where x > 0 && y == "A":
    print(x, y)
    
case let z:
    print(z)
}

Binding with for loops

let dictionary = [1: "one", 2: "two", 3: "three"]

for (key, value) in dictionary {
    print(key, value)
}

Wildcards

for (_, value) in dictionary {
    print(value)
}

switch triple {
case let (a, _, _):
    print(a)
}

[segfaultmedaddy]

BTW it will allow a sweet feature like swapping data without necessity of temporal variable, e.g.:

Currently:

int a = 1;
int b = 2;

int tmp = a;
a = b;
b = tmp;

with destructuring:

int a = 1;
int b = 2;

[a, b] = [b, a];

a; // 2
b; // 1

[andreashaese]

Yup, that's covered in the proposal.

[thomasio101]

The proposal only details destructuring in the context of assignment, such as in variable assignments/declarations, or in loops. But I'm seeing quite a potential here for using destructuring in the context of function/constructor/accessor arguments, as is the case in Python.

Examples

These examples have all been written in Python.

Iterable unpacking

def my_function(x, y):
  print(x)
  print(y)

my_tuple = (1, 2)

my_function(*my_tuple)

This prints the following;

1
2

Mapping unpacking

def my_function(x, y):
  print(x)
  print(y)

my_dictionary = {'x':1, 'y':2}

my_function(**my_dictionary)

This prints the following;

1
2

It also works in reverse;

my_dictionary = {'y':2, 'x':1}

my_function (**my_dictionary)

This will print the following

1
2

NOTE See the Python specifications for a more comprehensive look at the "unpacking" functionality

Issue

The problem that this concept poses, is the proposal's lack of an operator like the * or ** operators from Python. This lack of operators means there's no distinguishment between destructuring a value to parameters and passing the value itself as an argument.

[thomasio101]

.. several posts irrelevant to the issue of destructuring - deleted by the author

[thomasio101]

@tatumizer Here is the repo concerning the tests

[thomasio101]

@tatumizer I've already published a test on the repo which eliminates the possibility of a significant performance differential between positional, named and mixed arguments in the verbose method.

I will also be doing other performance tests in the repository, but maybe it would be interesting if you open an issue on there with your test's source code? That way I can also provide some feedback and possibly replicate the test my self to compare the results.

EDIT

• In case you read over the link to source code in my report. Here it is.
• I've also made some code to generate Markdown tables, you can find it here - I guess it might come in useful for visualizing your own results.

[thomasio101]

I was aware of that, my report's hypothesis states that I did not expect any significant difference between named and positional arguments in verbose code.

The reason I tested it was just to make sure we weren't going off a wild guess.

[thomasio101]

That sounds like a good idea, anyways, I'm going to make a generic API for running tests, although I need to see if the overhead is not to much (maybe I could over-engineer this a bit with a web panel with fancy graphs and the ability to monitor tests remotely :)......)

[lrhn]

Lists are not tuples, so I would't use var [a, b] = ["Hello world", 5]; for tuples. The expression on the right-hand side is an existing Dart expression with type List<Object>.
Doing list deconstruction this way is possible (and risks throwing if the list expression is not a literal and ends up not having two elements).

For a tuple-based syntax like var (a, b) = ("Hello world", 5); I'd expect the RHS to have type String * int and infer String for a and int for b.

[lrhn]

I won't put my money on any specific tuple type syntax yet, too many options are open. I intended String * int as the semantic type here.

The point of tuples, maybe even named tuples like (1, "yes", reserve: "table") is that a tuple has a static type, so we can spread it in a parameter list and know statically which slots it will apply to, and whether it matches the required parameter count and types.

You can spread a list in a list, or a map in a map. You can't just spread a map or list in a parameter list because it's a different structure with different requirements (like a per-element type), but a tuple can be an abstraction with the same structure as a parameter list, so it would make sense to spread a tuple in another tuple, or in a parameter list.

(Maybe (String a, int b) p = someTupleOperation(); print(p == (a, b)); // true).