consider a new lint: `use_switch_expressions`

[pq]

We might consider a lint to encourage the use of switch-expressions.

BAD

Color shiftHue(Color color) {
  switch (color) {
    case Color.red:
      return Color.orange;
    case Color.blue:
      return Color.purple;
    case Color.green:
      throw 'Green is bad';
    case Color.yellow:
      return Color.green;
  }
}

GOOD

Color shiftHue(Color color) {
  return switch (color) {
    Color.red => Color.orange,
    Color.orange => Color.yellow,
    Color.yellow => Color.green,
    Color.green => throw 'Green is bad'
    Color.blue => Color.purple,
    Color.purple => Color.red
  };
}

[bwilkerson]

Not sure what the rule here would be. Would it look for switch statements where every case returns a value and there's no code following the case (other, perhaps, than a return statement to return the default when there's no default case)? Are there other patterns you have in mind?

[pq]

Yep. That's the pattern I have in mind. Maybe a bit specific but I do see a bunch of candidates for conversion in the SDK.

Here are a few as food for thought.

Future<MacroExecutor> start(SerializationMode serializationMode,
    CommunicationChannel communicationChannel, String program,
    [List<String> arguments = const []]) {
  switch (communicationChannel) {
    case CommunicationChannel.stdio:
      return _SingleProcessMacroExecutor.startWithStdio(
          serializationMode, program, arguments);
    case CommunicationChannel.socket:
      return _SingleProcessMacroExecutor.startWithSocket(
          serializationMode, program, arguments);
  }
}

int firstQuoteLength(String first, Quote quote) {
  switch (quote) {
    case Quote.Single:
    case Quote.Double:
      return 1;

    case Quote.MultiLineSingle:
    case Quote.MultiLineDouble:
      return lengthOfOptionalWhitespacePrefix(first, /* start = */ 3);

    case Quote.RawSingle:
    case Quote.RawDouble:
      return 2;

    case Quote.RawMultiLineSingle:
    case Quote.RawMultiLineDouble:
      return lengthOfOptionalWhitespacePrefix(first, /* start = */ 4);
  }
}

ErrorToken buildUnexpectedCharacterToken(int character, int charOffset) {
  if (character < 0x1f) {
    return new AsciiControlCharacterToken(character, charOffset);
  }
  switch (character) {
    case unicodeReplacementCharacter:
      return new EncodingErrorToken(charOffset);

    /// See [General Punctuation]
    /// (http://www.unicode.org/charts/PDF/U2000.pdf).
    case 0x00A0: // No-break space.
    case 0x1680: // Ogham space mark.
    case 0x180E: // Mongolian vowel separator.
    case 0x2000: // En quad.
    case 0x2001: // Em quad.
    case 0x2002: // En space.
    case 0x2003: // Em space.
    case 0x2004: // Three-per-em space.
    case 0x2005: // Four-per-em space.
    case 0x2006: // Six-per-em space.
    case 0x2007: // Figure space.
    case 0x2008: // Punctuation space.
    case 0x2009: // Thin space.
    case 0x200A: // Hair space.
    case 0x200B: // Zero width space.
    case 0x2028: // Line separator.
    case 0x2029: // Paragraph separator.
    case 0x202F: // Narrow no-break space.
    case 0x205F: // Medium mathematical space.
    case 0x3000: // Ideographic space.
    case 0xFEFF: // Zero width no-break space.
      return new NonAsciiWhitespaceToken(character, charOffset);

    default:
      return new NonAsciiIdentifierToken(character, charOffset);
  }
}

  static String idToString(Id id, String value) {
    switch (id.kind) {
      case IdKind.member:
        MemberId elementId = id as MemberId;
        return '$memberPrefix${elementId.name}:$value';
      case IdKind.cls:
        ClassId classId = id as ClassId;
        return '$classPrefix${classId.name}:$value';
      case IdKind.library:
        return '$libraryPrefix$value';
      case IdKind.node:
        return value;
      case IdKind.invoke:
        return '$invokePrefix$value';
      case IdKind.update:
        return '$updatePrefix$value';
      case IdKind.iterator:
        return '$iteratorPrefix$value';
      case IdKind.current:
        return '$currentPrefix$value';
      case IdKind.moveNext:
        return '$moveNextPrefix$value';
      case IdKind.implicitAs:
        return '$implicitAsPrefix$value';
      case IdKind.stmt:
        return '$stmtPrefix$value';
      case IdKind.error:
        return '$errorPrefix$value';
    }
  }

protocol.ElementKind protocolElementKind(DeclarationKind kind) {
  switch (kind) {
    case DeclarationKind.CLASS:
      return protocol.ElementKind.CLASS;
    case DeclarationKind.CLASS_TYPE_ALIAS:
      return protocol.ElementKind.CLASS_TYPE_ALIAS;
    case DeclarationKind.CONSTRUCTOR:
      return protocol.ElementKind.CONSTRUCTOR;
    case DeclarationKind.ENUM:
      return protocol.ElementKind.ENUM;
    case DeclarationKind.ENUM_CONSTANT:
      return protocol.ElementKind.ENUM_CONSTANT;
    case DeclarationKind.EXTENSION:
      return protocol.ElementKind.EXTENSION;
    case DeclarationKind.FIELD:
      return protocol.ElementKind.FIELD;
    case DeclarationKind.FUNCTION:
      return protocol.ElementKind.FUNCTION;
    case DeclarationKind.FUNCTION_TYPE_ALIAS:
      return protocol.ElementKind.TYPE_ALIAS;
    case DeclarationKind.GETTER:
      return protocol.ElementKind.GETTER;
    case DeclarationKind.METHOD:
      return protocol.ElementKind.METHOD;
    case DeclarationKind.MIXIN:
      return protocol.ElementKind.MIXIN;
    case DeclarationKind.SETTER:
      return protocol.ElementKind.SETTER;
    case DeclarationKind.TYPE_ALIAS:
      return protocol.ElementKind.TYPE_ALIAS;
    case DeclarationKind.VARIABLE:
      return protocol.ElementKind.TOP_LEVEL_VARIABLE;
  }
}

protocol.ElementKind getElementKind(search.DeclarationKind kind) {
      switch (kind) {
        case search.DeclarationKind.CLASS:
          return protocol.ElementKind.CLASS;
        case search.DeclarationKind.CLASS_TYPE_ALIAS:
          return protocol.ElementKind.CLASS_TYPE_ALIAS;
        case search.DeclarationKind.CONSTRUCTOR:
          return protocol.ElementKind.CONSTRUCTOR;
        case search.DeclarationKind.ENUM:
          return protocol.ElementKind.ENUM;
        case search.DeclarationKind.ENUM_CONSTANT:
          return protocol.ElementKind.ENUM_CONSTANT;
        case search.DeclarationKind.FIELD:
          return protocol.ElementKind.FIELD;
        case search.DeclarationKind.FUNCTION:
          return protocol.ElementKind.FUNCTION;
        case search.DeclarationKind.FUNCTION_TYPE_ALIAS:
          return protocol.ElementKind.FUNCTION_TYPE_ALIAS;
        case search.DeclarationKind.GETTER:
          return protocol.ElementKind.GETTER;
        case search.DeclarationKind.METHOD:
          return protocol.ElementKind.METHOD;
        case search.DeclarationKind.MIXIN:
          return protocol.ElementKind.MIXIN;
        case search.DeclarationKind.SETTER:
          return protocol.ElementKind.SETTER;
        case search.DeclarationKind.TYPE_ALIAS:
          return protocol.ElementKind.TYPE_ALIAS;
        case search.DeclarationKind.VARIABLE:
          return protocol.ElementKind.TOP_LEVEL_VARIABLE;
        default:
          return protocol.ElementKind.CLASS;
      }
    }

  WatchEventType _convertChangeType(watcher.ChangeType type) {
    switch (type) {
      case watcher.ChangeType.ADD:
        return WatchEventType.ADD;
      case watcher.ChangeType.MODIFY:
        return WatchEventType.MODIFY;
      case watcher.ChangeType.REMOVE:
        return WatchEventType.REMOVE;
      default:
        throw StateError('Unknown change type: $type');
    }
  }

  DartType instantiate({
    required NullabilitySuffix nullabilitySuffix,
  }) {
    switch (nullabilitySuffix) {
      case NullabilitySuffix.question:
        return NeverTypeImpl.instanceNullable;
      case NullabilitySuffix.star:
        return NeverTypeImpl.instanceLegacy;
      case NullabilitySuffix.none:
        return NeverTypeImpl.instance;
    }
  }

  @override
  TypeImpl withNullability(NullabilitySuffix nullabilitySuffix) {
    switch (nullabilitySuffix) {
      case NullabilitySuffix.question:
        return instanceNullable;
      case NullabilitySuffix.star:
        return instanceLegacy;
      case NullabilitySuffix.none:
        return instance;
    }
  }

  factory Variance._fromEncoding(int encoding) {
    switch (encoding) {
      case 0:
        return unrelated;
      case 1:
        return covariant;
      case 2:
        return contravariant;
      case 3:
        return invariant;
    }
    throw ArgumentError('Invalid encoding for variance: $encoding');
  }

[pq]

/fyi @munificent

[munificent]

I like it! Not sure if it makes sense to always encourage switch expressions, since those could potentially look weird if they get huge, but it's worth experimenting with.

Another pattern to recognize might be a variable declaration immediately followed by a switch statement where every case initializes it.

[pq]

It'd be good to get a bit more feedback on this proposal.

/fyi @eernstg @lrhn @bwilkerson @stereotype441 @srawlins

[bwilkerson]

Not sure if it makes sense to always encourage switch expressions, ... but it's worth experimenting with.

Sounds like it might be too early for a lint rule, based on the lack of information about whether and when users would want to be using an expression.

But that won't prevent us from having an assist in place. And implementing the assist might provide some insights about what usage patterns the lint should or shouldn't report.

[pq]

As per a conversation w/ @kevmoo and @jacob314, some good motivation in flutter/flutter#125930.