Add configurable error comparison

Author: cdwort

lib/scientist/experiment.rb

@@ -121,6 +121,16 @@ def compare(*args, &block)
     @_scientist_comparator = block
   end
 
+  # A block which compares two experimental errors.
+  #
+  # The block must take two arguments, the control Error and a candidate Error,
+  # and return true or false.
+  #
+  # Returns the block.
+  def error_compare(*args, &block)
+    @_scientist_error_comparator = block
+  end
+
   # A Symbol-keyed Hash of extra experiment data.
   def context(context = nil)
     @_scientist_context ||= {}
@@ -164,13 +174,9 @@ def name
     "experiment"
   end
 
-  # Internal: compare two observations, using the configured compare block if present.
+  # Internal: compare two observations, using the configured compare and error_compare lambdas if present.
   def observations_are_equivalent?(a, b)
-    if @_scientist_comparator
-      a.equivalent_to?(b, &@_scientist_comparator)
-    else
-      a.equivalent_to? b
-    end
+    a.equivalent_to? b, @_scientist_comparator, @_scientist_error_comparator
   rescue StandardError => ex
     raised :compare, ex
     false

lib/scientist/observation.rb

@@ -47,25 +47,34 @@ def cleaned_value
 
   # Is this observation equivalent to another?
   #
-  # other      - the other Observation in question
-  # comparator - an optional comparison block. This observation's value and the
-  #              other observation's value are yielded to this to determine
-  #              their equivalency. Block should return true/false.
+  # other            - the other Observation in question
+  # comparator       - an optional comparison proc. This observation's value and the
+  #                    other observation's value are passed to this to determine
+  #                    their equivalency. Proc should return true/false.
+  # error_comparator - an optional comparison proc. This observation's Error and the
+  #                    other observation's Error are passed to this to determine
+  #                    their equivalency. Proc should return true/false.
   #
   # Returns true if:
   #
   # * The values of the observation are equal (using `==`)
   # * The values of the observations are equal according to a comparison
-  #   block, if given
+  #   prod, if given
+  # * The exceptions raised by the obeservations are equal according to the
+  #   error comparison proc, if given.
   # * Both observations raised an exception with the same class and message.
   #
   # Returns false otherwise.
-  def equivalent_to?(other, &comparator)
+  def equivalent_to?(other, comparator=nil, error_comparator=nil)
     return false unless other.is_a?(Scientist::Observation)
 
     if raised? || other.raised?
-      return other.exception.class == exception.class &&
-        other.exception.message == exception.message
+      if error_comparator
+        error_comparator.call(exception, other.exception)
+      else
+        return other.exception.class == exception.class &&
+          other.exception.message == exception.message
+      end
     end
 
     if comparator

test/scientist/experiment_test.rb

@@ -180,6 +180,18 @@ def @ex.publish(result)
     assert @ex.published_result.matched?
   end
 
+  it "compares errors with an error comparator block if provided" do
+    @ex.error_compare { |a, b| a.class == b.class }
+    @ex.use { raise "foo" }
+    @ex.try { raise "bar" }
+
+    resulting_error = assert_raises RuntimeError do
+      @ex.run
+    end
+    assert_equal "foo", resulting_error.message
+    assert @ex.published_result.matched?
+  end
+
   it "knows how to compare two experiments" do
     a = Scientist::Observation.new(@ex, "a") { 1 }
     b = Scientist::Observation.new(@ex, "b") { 2 }

test/scientist/observation_test.rb

@@ -92,7 +92,7 @@
     refute x.equivalent_to?(y)
   end
 
-  it "compares values using a comparator block" do
+  it "compares values using a comparator proc" do
     a = Scientist::Observation.new("test", @experiment) { 1 }
     b = Scientist::Observation.new("test", @experiment) { "1" }
 
@@ -108,6 +108,25 @@
     assert_equal [a.value, b.value], yielded
   end
 
+  it "compares exceptions using an error comparator proc" do
+    x = Scientist::Observation.new("test", @experiment) { raise FirstError, "error" }
+    y = Scientist::Observation.new("test", @experiment) { raise SecondError, "error" }
+    z = Scientist::Observation.new("test", @experiment) { raise FirstError, "ERROR" }
+
+    refute x.equivalent_to?(z)
+    refute x.equivalent_to?(y)
+
+    compare_on_class = -> (error, other_error) {
+      error.class == other_error.class
+    }
+    compare_on_message = -> (error, other_error) {
+      error.message == other_error.message
+    }
+
+    assert x.equivalent_to?(z, nil, compare_on_class)
+    assert x.equivalent_to?(y, nil, compare_on_message)
+  end
+
   describe "#cleaned_value" do
     it "returns the observation's value by default" do
       a = Scientist::Observation.new("test", @experiment) { 1 }