Hello, I got a question about what the form of types in Haskell are allowed. Specifically, what are the requirements of the context of a type? The paper Typing Haskell in Haskell required the type to be in the form P => t, where P to be in head-normal form or in other words shouldn't be something like Num Bool. Now what about multi-parameter classes? The paper Type classes: an exploration of the design space listed several potential choices but didn't say exactly what. The Haskell 2010 report covered single-parameter type classes only. Given the following definition. class Collects ce e | ce -> e where empty :: ce insert :: e -> ce -> ce insert2 c = insert True (insert False c) The function insert2 has the following type insert2 :: Collects ce Bool => ce -> ce This shows that the definition of head-normal forms for multi-parameter class differs from that for single-parameter classes. Is there any definite specification about the context of Haskell types? Thank you, Sheng