Introduce span with line context

lib/src/span.dart

Index: lib/src/span.dart
diff --git a/lib/src/span.dart b/lib/src/span.dart
index 9f150482c6da354adbbad9753a12a306b4bf432c..6f9013e55dc3b582ae01205ace1976110e557db9 100644
--- a/lib/src/span.dart
+++ b/lib/src/span.dart
@@ -57,6 +57,25 @@ abstract class SourceSpan implements Comparable<SourceSpan> {
   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 {
@@ -76,3 +95,33 @@ class SourceSpanBase extends SourceSpanMixin {
     }
   }
 }
+
+/// 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".');
+    }
+  }
+}