Aha! The first reply that actually provides an answer to my question. Many thanks. Could you say any more about this, or perhaps provide an example? Its har

Aha! The first reply that actually provides an answer to my question. Many thanks. Could you say any more about this, or perhaps provide an example? Its hard to know what you mean. EDIT: Attempting to answer my own question, perhaps it occurs when you define a product type with a constructor data Foo = Foo bar baz and then you want to add a new field to Foo data Foo = Foo bar baz quux All your explicit uses of Foo in construction and pattern matching have now broken.