Hello, I was just looking at the details of how contexts on datatypes work in Haskell'98 and I noticed the following. The report states that the context on a particular constructor should contain all those predicates that mention only variables that are in the fields of the constructor. This wording does not scale very well to mult-paramater type classes, and indeed Hugs (20050308) and GHC (6.4.1) do different things. Here is an example:
class C a b
data C a b => T a b = Mk a
According to Hugs, Mk :: a -> T a b, which follows the report: the constarint is not added because it mentions a variable, b, which is not in the fields of Mk. According to GHC, however, Mk :: C a b => a -> T a b, which also makes sense if we interpret the report to mean: "all those predicates that constrain a variable in the fields of the constructor". I know that contexts on data types are not a very popular part of the Haskell design, but it would be nice to be consistent. To me it seems that GHC's interpretation is the correct one. -Iavor PS: Appologies if this is fixed in later versions of Hugs, I don't have a newer version at the moment to try it out.