Implement pattern matching

[fujidaiti]

It would be nice if we could control how the content in a file is converted to a dart object by giving a variable annotated with @EmbedLiteral a concrete type like Map<String, int>.

For example, suppose there is a JSON file such as:

// data.json
{ "apple": 0, "banana": 1, "orange": 2 }

Then we should be able to control how the above JSON object is converted into a Dart object by declaring the variable with a type:

// main.dart

@EmbedLiteral("./data.json")
// This variable should be a Map literal
const Map<String, int> dataAsMap = _$dataAsMap;

EmbedLiteral("./data.json")
// This variable should be a record literal
const ({ int apple, int banana, int orange }) dataAsRecord = _$dataAsRecord;

The generated code will look like this:

// main.g.dart
const _$dataAsMap = { "apple": 0, "banana": 1, "orange": 2 };
const _$dataAsRecord = ( apple: 0, banana: 1, orange: 2 );

Even better, this pattern matching allows you to partially extract data from the content file by specifying a desired structure of the record literal to be generated.

// another_main.dart

@EmbedLiteral("./data.json")
// takes only the `apple` and `banana` data from `data.json
const ({ int apple, int banana }) partialData = _$partialData;

This will generate the source:

// another_main.g.dart
const _$partialData = ( apple: 0, banana: 1 );

These features are useful to ensure that the generated objects always have the same structure, or in cases where the content is huge but only a portion of it is of interest.