Introduce span with line context

lib/src/span.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library source_span.span;
 
 import 'location.dart';
 import 'span_mixin.dart';
 
 /// A class that describes a segment of source text.
@@ skipping 39 matching lines @@
   /// Formats [message] in a human-friendly way associated with this span.
   ///
   /// [color] may either be a [String], a [bool], or `null`. If it's a string,
   /// it indicates an ANSII terminal color escape that should be used to
   /// highlight the span's text. If it's `true`, it indicates that the text
   /// should be highlighted using the default color. If it's `false` or `null`,
   /// it indicates that the text shouldn't be highlighted.
   String message(String message, {color});
 }
 
+/// A class that describes a segment of source text with additional context.
+abstract class SourceSpanWithContext extends SourceSpan {

[nweiz, 2015/03/25 01:15:20]

Now that I think about it, it would be nice to have this in a separate library.

[Siggi Cherem (dart-lang), 2015/03/26 21:07:29]

Done.

+  /// The text of the line containing this span.
+  ///
+  /// If the span covers multiple lines, this should only contain the first.
+  final String context;
+
+  /// Creates a new span from [start] to [end] (exclusive) containing [text], in
+  /// the [context] line.
+  ///
+  /// [start] and [end] must have the same source URL and [start] must come
+  /// before [end]. [text] must have a number of characters equal to the
+  /// distance between [start] and [end]. [context] must contain [text], and
+  /// [text] should start at `start.column` within [context].
+  factory SourceSpanWithContext(
+      SourceLocation start, SourceLocation end, String text, String context) =
+      SourceSpanWithContextBase;
+}
+
 /// A base class for source spans with [start], [end], and [text] known at
 /// construction time.
 class SourceSpanBase extends SourceSpanMixin {
   final SourceLocation start;
   final SourceLocation end;
   final String text;
 
   SourceSpanBase(this.start, this.end, this.text) {
     if (end.sourceUrl != start.sourceUrl) {
       throw new ArgumentError("Source URLs \"${start.sourceUrl}\" and "
           " \"${end.sourceUrl}\" don't match.");
     } else if (end.offset < start.offset) {
       throw new ArgumentError('End $end must come after start $start.');
     } else if (text.length != start.distance(end)) {
       throw new ArgumentError('Text "$text" must be ${start.distance(end)} '
           'characters long.');
     }
   }
 }
+
+/// A base class for source spans with [start], [end], [text], and [context]
+/// known at construction time.
+class SourceSpanWithContextBase extends SourceSpanBase
+    implements SourceSpanWithContext {
+  /// The text of the line containing this span.
+  ///
+  /// If the span covers multiple lines, this should only contain the first.

[nweiz, 2015/03/25 01:10:39]

I just realized that this property is incorrect; the constructor will throw an
error if it's true, since [context] won't contain [text] anymore. Probably this
should actually contain *all* lines the span covers; this means
[FileSpan.context]'s behavior will have to be changed.

[Siggi Cherem (dart-lang), 2015/03/26 21:07:29]

I went with somewhere in between. I validate that context contains the text, and
that it begins at the appropriate column within a line. When emitting messages,
I emit preceding lines, but then I still truncate the span on the first line. We
can keep adding support for multi-line spans as a separate change

+  final String context;
+
+  /// Creates a new span from [start] to [end] (exclusive) containing [text], in
+  /// the [context] line.
+  ///
+  /// [start] and [end] must have the same source URL and [start] must come
+  /// before [end]. [text] must have a number of characters equal to the
+  /// distance between [start] and [end]. [context] must contain [text], and
+  /// [text] should start at `start.column` within [context].
+  SourceSpanWithContextBase(
+      SourceLocation start, SourceLocation end, String text, this.context)
+      : super(start, end, text) {
+    var index = context.indexOf(text);
+    if (index == -1) {
+      throw new ArgumentError(
+          'The context line "$context" must contain "$text".');
+    } else if (index != start.column) {
+      throw new ArgumentError('The span text "$text" must start at '
+          'column ${start.column + 1} in the context line "$context".');
+    }
+  }
+}