128 lines
3.6 KiB
Markdown
128 lines
3.6 KiB
Markdown
# Belatuk Combinator
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[](https://pub.dartlang.org/packages/belatuk_combinator)
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[](https://dart.dev/null-safety)
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[](https://github.com/dart-backend/belatuk-common-utilities/packages/combinator/LICENSE)
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Packrat parser combinators that support static typing, generics, file spans, memoization, and more.
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**RECOMMENDED:**
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Check `example/` for examples.
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The examples contain examples of using:
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* Generic typing
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* Reading `FileSpan` from `ParseResult`
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* More...
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## Basic Usage
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```dart
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void main() {
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// Parse a Pattern (usually String or RegExp).
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var foo = match('foo');
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var number = match(RegExp(r'[0-9]+'), errorMessage: 'Expected a number.');
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// Set a value.
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var numWithValue = number.map((r) => int.parse(r.span.text));
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// Expect a pattern, or nothing.
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var optional = numWithValue.opt();
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// Expect a pattern zero or more times.
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var star = optional.star();
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// Expect one or more times.
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var plus = optional.plus();
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// Expect an arbitrary number of times.
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var threeTimes = optional.times(3);
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// Expect a sequence of patterns.
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var doraTheExplorer = chain([
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match('Dora').space(),
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match('the').space(),
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match('Explorer').space(),
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]);
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// Choose exactly one of a set of patterns, whichever
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// appears first.
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var alt = any([
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match('1'),
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match('11'),
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match('111'),
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]);
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// Choose the *longest* match for any of the given alternatives.
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var alt2 = longest([
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match('1'),
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match('11'),
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match('111'),
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]);
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// Friendly operators
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var fooOrNumber = foo | number;
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var fooAndNumber = foo & number;
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var notFoo = ~foo;
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}
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```
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## Error Messages
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Parsers without descriptive error messages can lead to frustrating dead-ends
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for end-users. Fortunately, `belatuk_combinator` is built with error handling in mind.
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```dart
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void main(Parser parser) {
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// Append an arbitrary error message to a parser if it is not matched.
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var withError = parser.error(errorMessage: 'Hey!!! Wrong!!!');
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// You can also set the severity of an error.
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var asHint = parser.error(severity: SyntaxErrorSeverity.hint);
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// Constructs like `any`, `chain`, and `longest` support this as well.
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var foo = longest([
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parser.error(errorMessage: 'foo'),
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parser.error(errorMessage: 'bar')
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], errorMessage: 'Expected a "foo" or a "bar"');
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// If multiple errors are present at one location,
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// it can create a lot of noise.
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//
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// Use `foldErrors` to only take one error at a given location.
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var lessNoise = parser.foldErrors();
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}
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```
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## Whitespaces
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Handling optional whitespace is dead-easy:
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```dart
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void main(Parser parser) {
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var optionalSpace = parser.space();
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}
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```
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## For Programming Languages
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`belatuk_combinator` was conceived to make writing parsers for complex grammars easier,
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namely programming languages. Thus, there are functions built-in to make common constructs
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easier:
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```dart
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void main(Parser parser) {
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var array = parser
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.separatedByComma()
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.surroundedBySquareBrackets(defaultValue: []);
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var braces = parser.surroundedByCurlyBraces();
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var sep = parser.separatedBy(match('!').space());
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}
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```
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## Differences between this and Petitparser
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* `belatuk_combinator` makes extensive use of Dart's dynamic typing
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* `belatuk_combinator` supports detailed error messages (with configurable severity)
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* `belatuk_combinator` keeps track of locations (ex. `line 1: 3`)
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