The absolutely critical thing is to have checked every alternative when dealing with a sum type value. This is hard to do with macros, though not impossible (b

The absolutely critical thing is to have checked every alternative when dealing with a sum type value. This is hard to do with macros, though not impossible (basically you'd need a macro to start a matching context and which introduces a variable in which to keep track of all the alternatives checked, then you need to arrange for the end of the matching context to check that all alternatives were checked).