Hello, I'm extending GHC's demand analysis to sum types, but _not_ recursive types. I was searching for a function that would tell me if a TyCon is recursive, so that I could analyse Maybes but avoid Lists. I found isRecursiveTyCon, but it's not actually for this purpose, there's actually a long note in typecheck/TcTyDecls.hs that explains why it's not fit for determining which type constructors are recursive in general. The key point from the note is "The "recursive" flag for algebraic data types is irrelevant (never consulted) for types with more than one constructor." It seems that because the strictness analyser never worked over sum types it never had to worry about recursive sum types and at some point the flag that says a data type is recursive broke for sum types without anyone knowing. For example the following mutually recursive data types have different return values from isRecursiveTyCon data OddNat = One | SuccO EvenNat data EvenNat = Zero | SuccE OddNat Does anyone know of the simplest way to test for this? Thanks for your time, Jose