Improve FoldError formatting to show original exception messages

Fixes #65

Thanks to @RonnyPfannschmidt for the report!

Author: radix

effect/fold.py

@@ -17,10 +17,12 @@ def __init__(self, accumulator, wrapped_exception):
         self.wrapped_exception = wrapped_exception
 
     def __str__(self):
-        tb_lines = traceback.format_tb(self.wrapped_exception[2])
+        tb_lines = traceback.format_exception(*self.wrapped_exception)
         tb = ''.join(tb_lines)
-        return "FoldError(%r, %r) -> ORIGINAL TRACEBACK FOLLOWS\n%s" % (
-            self.accumulator, self.wrapped_exception, tb)
+        st = (
+            "<FoldError after accumulating %r> Original traceback follows:\n%s"
+            % (self.accumulator, tb))
+        return st.rstrip('\n')
 
 
 def fold_effect(f, initial, effects):

effect/test_fold.py

@@ -63,6 +63,22 @@ def test_fold_effect_errors():
     assert excinfo.value.wrapped_exception[0] is ZeroDivisionError
     assert str(excinfo.value.wrapped_exception[1]) == 'foo'
 
+def test_fold_effect_str():
+    effs = [Effect('a'), Effect(Error(ZeroDivisionError('foo'))), Effect('c')]
+
+    dispatcher = SequenceDispatcher([
+        ('a', lambda i: 'Ei'),
+    ])
+
+    eff = fold_effect(operator.add, 'Nil', effs)
+
+    with dispatcher.consume():
+        with raises(FoldError) as excinfo:
+            sync_perform(_base_and(dispatcher), eff)
+    assert str(excinfo.value).startswith(
+        "<FoldError after accumulating 'NilEi'> Original traceback follows:\n")
+    assert str(excinfo.value).endswith('ZeroDivisionError: foo')
+
 
 def test_sequence():
     """Collects each Effectful result into a list."""