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
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