In a project I'm working on, one data type I've defined is this: data FilterIS = FilterIS { source :: (InputStream s) => s, filter :: Filter } which, to me, just means it holds any instance of the InputStream class and a Filter value. Sure, says ghci, fine by me. However, if I try to do anything with that datatype, even something as simple as newFilterIS = FilterIS I get the following error, or something very like it: Stream.hs:96: Inferred type is less polymorphic than expected Quantified type variable `s' escapes Expected type: s -> t Inferred type: (forall s1. (InputStream s1) => s1) -> (forall fs. (InputStream fs) => fs -> IO Word8) -> FilterIS In the definition of `newFilterIS': FilterIS I've gotten used to having to spend a while figuring out what error messages mean, but it bugs me that there seems to be some problem with the type that's "brought out" by just making a synonym for the constructor. I know there's nothing wrong with the line where I define a synonym; there's practically nothing there to *be* wrong, so the problem has to be in the type... and yet the compiler didn't catch it until I added that synonym line. What's up? Abe