Support `switch` in Collection Literals

[rubenferreira97]

Dart supports if and for in collection literals, and even if-case. However, there's currently no support for switch inside collection literals, which limits expressiveness and leads to repeated expressions.

Example (today, using if-case):

final map = {
  for (final pair in pairs.split(';'))
    if (pair.split('=') case [final key]) key: '',
    else if (pair.split('=') case [final key, final value]) key: value,
};

This repeats split('=') and is harder to maintain (e.g. add cases).

Proposed (with switch support and implicit variables):

final map = {
  for (final pair in pairs.split(';'))
    switch (pair.split('=')) {
      case [final key]: key: '',
      case [final key, final value]: key: value,
    }
};

I believe switch in this context shouldn't require exhaustiveness—unmatched cases could simply be skipped, just like with if. If exhaustiveness is needed, a switch expression could be used instead.

I also understand that this level of logic might be considered too much for collection literals, but personally I've encountered cases where it would be useful. I'm filing this issue to open up discussion and gather feedback.

[rrousselGit]

It is supported. But you used switch statements, yet you should use switch expressions.

The syntax is a little bit different:

final map = {
  for (final pair in ''.split(';'))
    ...switch (pair.split('=')) {
      [final key] => {key: ''},
      [final key, final value] => {key: value},
      [] => {},
    },
};

[rubenferreira97]

Ye, I also found this workaround, but:

1. The syntax feels heavier than it needs to be.
2. I'm not sure if creating multiple intermediate sets/maps and spreading them is optimized.
3. You need a default case (to spread nothing? 😅).

[mmcdon20]

See also: #2124

[rubenferreira97]

@mmcdon20 Thanks for pointing me to that issue. GitHub search can be really frustrating at times.

If we could inline a map entry (like 'key': 'value' in if statements), this might solve my problems for exhaustive cases:

enum Color { red, blue, yellow }

void main() {
  final color = Color.red;
  
  // Works fine
  final list = [
    switch (color) {
      Color.red => 'red',
      Color.blue => 'blue',
      Color.yellow => 'yellow',
    },
  ];

  // Works fine
  final set = {
    switch (color) {
      Color.red => 'red',
      Color.blue => 'blue',
      Color.yellow => 'yellow',
    },
  };

  // Error
  final map = {
    switch (color) {
      Color.red => 'a': 'red',
      Color.blue => 'b': 'blue',
      Color.yellow => 'c': 'yellow',
    },
  };
}

However, for non-exhaustive cases, there's no real solution because we have to be exhaustive in switch expressions and can't skip values:

final list = [
  for (final color in colors)
    switch (color) {
      Color.red => 'red',
      Color.blue => 'blue',
      _ => continue,
    },
];

[mateusfccp]

2. I'm not sure if creating multiple intermediate sets and spreading them is optimized.

It shouldn't be a problem. If you feel like this is a bottleneck try to identify if it is not being optimized, in which case you can open a bug report in the sdk repository.

[munificent]

@mmcdon20 is right, #2124 is the main issue for this request.