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dart / dart_style / b6070b4007d466b36fd6c68d8f6e22bc217e8dd9
commitb6070b4007d466b36fd6c68d8f6e22bc217e8dd9[log] [tgz]
authorBob Nystrom <rnystrom@google.com>Wed Jan 31 14:43:43 2024 -0800
committerGitHub <noreply@github.com>Wed Jan 31 14:43:43 2024 -0800
tree5b2203f6acfd6f2629afca6e1cf12515acd937c6
parent398e8c84717028bee0763a8b882c263e6e97c210 [diff]
Prune solutions derived from invalid ones. (#1372)

Prune solutions derived from invalid ones.

A solution is invalid if a child piece writes a newline at a point in
time where a parent disallows newlines.

When that happens, the parent's may be explicitly bound to a state by
the solution, or it may still be in an unbound state (and thus using
the default). If unbound, then even though this solution is invalid, it
may lead to later valid ones by binding that parent piece to a different
state.

But once a solution binds a piece to a state, all solutions derived
from that state keep the same binding. So if we hit an invalid solution
and the piece whose newline constraint was already violated, we can
discard that solution and all possible solutions derived from it.

This skips an entire potentially huge subtree from the solution space
being explored.

In an example giant deeply nested function call from the benchmark,
before this optimization, the solver tries 329,057 solutions before
finding a winning one. With this optimization, it only takes 5,405.
3 files changed
tree: 5b2203f6acfd6f2629afca6e1cf12515acd937c6
  1. .github/
  2. benchmark/
  3. bin/
  4. dist/
  5. example/
  6. lib/
  7. test/
  8. tool/
  9. .gitignore
  10. analysis_options.yaml
  11. AUTHORS
  12. CHANGELOG.md
  13. LICENSE
  14. pubspec.yaml
  15. README.md
README.md

The dart_style package defines an automatic, opinionated formatter for Dart code. It replaces the whitespace in your program with what it deems to be the best formatting for it. Resulting code should follow the Dart style guide but, moreso, should look nice to most human readers, most of the time.

The formatter handles indentation, inline whitespace, and (by far the most difficult) intelligent line wrapping. It has no problems with nested collections, function expressions, long argument lists, or otherwise tricky code.

The formatter turns code like this:

// BEFORE formatting
if (tag=='style'||tag=='script'&&(type==null||type == TYPE_JS
      ||type==TYPE_DART)||
  tag=='link'&&(rel=='stylesheet'||rel=='import')) {}

into:

// AFTER formatting
if (tag == 'style' ||
  tag == 'script' &&
      (type == null || type == TYPE_JS || type == TYPE_DART) ||
  tag == 'link' && (rel == 'stylesheet' || rel == 'import')) {}

The formatter will never break your code—you can safely invoke it automatically from build and presubmit scripts.

Style fixes

The formatter can also apply non-whitespace changes to make your code consistently idiomatic. You must opt into these by passing either --fix which applies all style fixes, or any of the --fix--prefixed flags to apply specific fixes.

For example, running with --fix-named-default-separator changes this:

greet(String name, {String title: "Captain"}) {
  print("Greetings, $title $name!");
}

into:

greet(String name, {String title = "Captain"}) {
  print("Greetings, $title $name!");
}

Using the formatter

The formatter is part of the unified dart developer tool included in the Dart SDK, so most users get it directly from there. That has the latest version of the formatter that was available when the SDK was released.

IDEs and editors that support Dart usually provide easy ways to run the formatter. For example, in WebStorm you can right-click a .dart file and then choose Reformat with Dart Style.

Here's a simple example of using the formatter on the command line:

$ dart format test.dart

This command formats the test.dart file and writes the result to the file.

dart format takes a list of paths, which can point to directories or files. If the path is a directory, it processes every .dart file in that directory or any of its subdirectories.

By default, it formats each file and write the formatting changes to the files. If you pass --output show, it prints the formatted code to stdout.

You may pass a -l option to control the width of the page that it wraps lines to fit within, but you're strongly encouraged to keep the default line length of 80 columns.

Validating files

If you want to use the formatter in something like a presubmit script or commit hook, you can pass flags to omit writing formatting changes to disk and to update the exit code to indicate success/failure:

$ dart format --output=none --set-exit-if-changed .

Running other versions of the formatter CLI command

If you need to run a different version of the formatter, you can globally activate the package from the dart_style package on pub.dev:

$ pub global activate dart_style
$ pub global run dart_style:format ...

Using the dart_style API

The package also exposes a single dart_style library containing a programmatic API for formatting code. Simple usage looks like this:

import 'package:dart_style/dart_style.dart';

main() {
  final formatter = DartFormatter();

  try {
    print(formatter.format("""
    library an_entire_compilation_unit;

    class SomeClass {}
    """));

    print(formatter.formatStatement("aSingle(statement);"));
  } on FormatterException catch (ex) {
    print(ex);
  }
}

Other resources

  • Before sending an email, see if you are asking a frequently asked question.

  • Before filing a bug, or if you want to understand how work on the formatter is managed, see how we track issues.