Add more metadata to non-programmatic barback exceptions.

pkg/barback/lib/src/errors.dart

Index: pkg/barback/lib/src/errors.dart
diff --git a/pkg/barback/lib/src/errors.dart b/pkg/barback/lib/src/errors.dart
index 9ebca8e608ab57273230922ffd5d3fe3334d64d3..8e624da7cefbd458788a78a5e7246d90f74a244e 100644
--- a/pkg/barback/lib/src/errors.dart
+++ b/pkg/barback/lib/src/errors.dart
@@ -7,7 +7,10 @@ library barback.errors;
 import 'dart:async';
 import 'dart:io';
 
+import 'package:stack_trace/stack_trace.dart';
+
 import 'asset_id.dart';
+import 'transformer.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
@@ -18,32 +21,92 @@ class AssetNotFoundException implements Exception {
   String toString() => "Could not find asset $id.";
 }
 
-/// Error thrown when two transformers both output an asset with [id].
-class AssetCollisionException implements Exception {
+/// The interface for exceptions from the barback graph or its transformers.
+///
+/// These exceptions are never produced by programming errors in barback.
+abstract class BarbackException implements Exception {}
+
+/// Error thrown when two or more transformers both output an asset with [id].
+class AssetCollisionException implements BarbackException {
+  /// All the transforms that output an asset with [id].
+  final Set<TransformInfo> transforms;
   final AssetId id;
 
-  AssetCollisionException(this.id);
+  AssetCollisionException(Iterable<TransformInfo> transforms, this.id)
+      : transforms = new Set.from(transforms);
 
   String toString() => "Got collision on asset $id.";
 }
 
 /// Error thrown when a transformer requests an input [id] which cannot be
 /// found.
-class MissingInputException implements Exception {
+class MissingInputException implements BarbackException {
+  /// The transform that requested [id].
+  final TransformInfo transform;
   final AssetId id;
 
-  MissingInputException(this.id);
+  MissingInputException(this.transform, this.id);
 
   String toString() => "Missing input $id.";
 }
 
 /// Error thrown when a transformer outputs an asset with the wrong package
 /// name.

[Bob Nystrom, 2013/08/13 00:04:29]

"with the wrong..." -> "to a different package than the primary input's."

[nweiz, 2013/08/13 19:15:11]

Done.

-class InvalidOutputException implements Exception {
-  final String package;
+class InvalidOutputException implements BarbackException {
+  /// The transform that output the asset.
+  final TransformInfo transform;
+  final AssetId id;
+
+  InvalidOutputException(this.transform, this.id);
+
+  String toString() => "Invalid output $id: must be in package "
+      "${transform.primaryId.package}.";

[Bob Nystrom, 2013/08/13 00:04:29]

Can you explain in the error message *why* it must be this and mention the
primary input ID?

[nweiz, 2013/08/13 19:15:11]

I've added data about the transforms to all the error messages.

+}
+
+/// Error wrapping an exception thrown by a transform.
+class TransformerException implements BarbackException {
+  /// The transform that threw the exception.
+  final TransformInfo transform;
+
+  /// The wrapped exception.
+  final error;
+
+  TransformerException(this.transform, this.error);
+}
+
+/// Error thrown when a source asset [id] fails to load.
+///
+/// This can be thrown either because the source asset was expected to exist and
+/// did not or because reading it failed somehow.
+class AssetLoadException implements BarbackException {
   final AssetId id;
 
-  InvalidOutputException(this.package, this.id);
+  /// The wrapped exception.
+  final error;
+
+  AssetLoadException(this.id, this.error);
+
+  String toString() => "Failed to load source asset $id: $error\n"
+      "${new Trace.from(getAttachedStackTrace(error)).terse}";
+}
+
+/// Information about a single transform in the barback graph.
+///
+/// A transform is a [transformer] as it's run on a single asset, its
+/// [primaryId].

[Bob Nystrom, 2013/08/13 00:04:29]

This is worded a bit strangely. How about:

Identifies a single transformation in the barback graph.

A transformation is uniquely identified by the ID of its primary input, and the
transformer that is applied to it.

[nweiz, 2013/08/13 19:15:11]

Done.

+class TransformInfo {
+  /// The transformer that's run for this transform.
+  final Transformer transformer;
+
+  /// The id of this transform's primary asset.
+  final AssetId primaryId;
+
+  TransformInfo(this.transformer, this.primaryId);
+
+  bool operator==(other) =>
+      other is TransformInfo &&
+      other.transformer == transformer &&
+      other.primaryId == primaryId;
 
-  String toString() => "Invalid output $id: must be in package $package.";
+  int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 }