Add extra information to FormatException.

sdk/lib/core/exceptions.dart

Index: sdk/lib/core/exceptions.dart
diff --git a/sdk/lib/core/exceptions.dart b/sdk/lib/core/exceptions.dart
index 800a6529ab209a109d82c40a115d020f15734e16..bccc8b2f0b8724adfa2b348cd8a65dd2ba8332e6 100644
--- a/sdk/lib/core/exceptions.dart
+++ b/sdk/lib/core/exceptions.dart
@@ -44,13 +44,122 @@ class FormatException implements Exception {
    * A message describing the format error.
    */
   final String message;
+  /**
+   * The source that caused the error.

[nweiz, 2014/07/14 19:33:50]

The documentation and name doesn't make it clear at all that this is supposed to
be the source text. I assumed that it was the URL or path to the source.

[Lasse Reichstein Nielsen, 2014/07/15 08:55:59]

I'll try to make it more explicit, including the words "The actual source .."

+   *
+   * This is usually a [String], but can be other types too. If it is a string,
+   * parts of it may be included in the [toString] message.
+   *
+   * May also be `null` if omitted.
+   */
+  final source;

[nweiz, 2014/07/14 19:33:49]

It would be a lot more useful to consumers of this class if this had a mandated
type, especially since [toString] completely ignores it if it's anything other
than a String.

[Lasse Reichstein Nielsen, 2014/07/15 08:55:59]

I don't want to mandate a type. It will almost always be a String, but you
should also be able to use this exception when parsing a byte-buffer.

A FormatException should never be unexpected, so the receiver would know which
input was given to the function throwing it. If that function is parsing byte
lists, the source will be that byte list, and the caller can decide what to do
with it.

+  /**
+   * The position in source where the error was detected.

[nweiz, 2014/07/14 19:33:49]

Please be clearer about what you mean by "position". I assume it's "characters
from the beginning of the source"?

For what it's worth, the source map package refers to this as "offset" rather
than "position".

[Lasse Reichstein Nielsen, 2014/07/15 08:55:59]

I like "offset", so I changed it everywhere.
Also tried to explain what it means (if source is a string, it's the index of a
code unit).

+   *
+   * May be omitted. If present, [source] should also be present.
+   */
+  final int position;
 
   /**
    * Creates a new FormatException with an optional error [message].
+   *
+   * Optionally also supply the [source] that had the incorrect format, and
+   * even the [position] in the format where this was detected.
    */
-  const FormatException([this.message = ""]);
+  const FormatException([this.message = "", this.source, this.position]);
 
-  String toString() => "FormatException: $message";
+  /**
+   * Returns a description of the format exception.
+   *
+   * The description always contains the [message].
+   * If [source] was provided, the description will contain (at least a part of)
+   * the source.
+   * If [position] is also provided, the part of the source included will
+   * contain that position, and the position will be marked.
+   *
+   * If the source contains a line break before position, only the line
+   * containing position will be included, and its line number will also be
+   * part of the description. Line and character offsets are 1-based.
+   */
+  String toString() {
+    String report = "FormatException";
+    if (message != null && message.isNotEmpty) {
+      report = "$report: $message";
+    }
+    int position = this.position;
+    if (source is! String) {
+      if (position != null) {
+        report += " (at position $position)";
+      }
+      return report;
+    }
+    if (position != null && (position < 0 || position > source.length)) {
+      position = null;
+    }
+    // Source is string and position is null or valid.
+    if (position == null) {
+      String source = this.source;
+      if (source.length > 78) {
+        source = source.substring(0, 75) + "...";
+      }
+      return "$report\n$source";
+    }
+    int lineNum = 1;
+    int lineStart = 0;
+    bool lastWasCR;
+    for (int i = 0; i < position; i++) {
+      int char = source.codeUnitAt(i);
+      if (char == 0x0a) {
+        if (lineStart != i || !lastWasCR) {
+          lineNum++;
+        }
+        lineStart = i + 1;
+        lastWasCR = false;
+      } else if (char == 0x0d) {
+        lineNum++;
+        lineStart = i + 1;
+        lastWasCR = true;
+      }
+    }
+    if (lineNum > 1) {
+      report += " (at line $lineNum, character ${position - lineStart + 1})\n";
+    } else {
+      report += " (at character ${position + 1})\n";
+    }
+    int lineEnd = source.length;
+    for (int i = position; i < source.length; i++) {
+      int char = source.codeUnitAt(i);
+      if (char == 0x0a || char == 0x0d) {
+        lineEnd = i;
+        break;
+      }
+    }
+    int length = lineEnd - lineStart;
+    int start = lineStart;
+    int end = lineEnd;
+    String prefix = "";
+    String postfix = "";
+    if (length > 78) {
+      // Can't show entire line. Try to anchor at the nearest end, if
+      // one is within reach.
+      int index = position - lineStart;
+      if (index < 75) {
+        end = start + 75;
+        postfix = "...";
+      } else if (end - position < 75) {
+        start = end - 75;
+        prefix = "...";
+      } else {
+        // Neither end is near, just pick an area around the position.
+        start = position - 36;
+        end = position + 36;
+        prefix = postfix = "...";
+      }
+    }
+    String slice = source.substring(start, end);
+    int markOffset = position - start + prefix.length;
+    return "$report$prefix$slice$postfix\n${" " * markOffset}^\n";
+  }
 }
 
 class IntegerDivisionByZeroException implements Exception {