Hi, I'd like to know what are the typing rules used in Haskell (98 is ok). Specifically, I'd like to know what makes let i = \x -> x in (i True, i 1) legal, and not let a = 1 in (a + (1 :: Int), a + (1.0 :: Float)) Is it correct that polymorphic functions can be used polymorphically (in multiple places) while non-functions receive a monomorphic type ? Also, I'd like to know why id id True is permitted but not (\f -> f f True) id Is it possible to change the later expression so it type checks ? Are there any type inference algorithms that would accept it ? I'm asking this because I have implemented a simple type inference algorithm following PLAI and I can infer the type for id id True (because I use fresh type variable names for id whenever I apply it before unification) and have an occur check fail for the later expression, just like in haskell. Thanks, Thu