Hi,
(I've attached the full code for this problem)
First I'll explain the problem description, I have two class ClassA and
ClassB, the former has two parameters and the latter has one. The second
parameter of ClassA is constrained by ClassB.
class ClassB a where
class ClassB b => ClassA a b where
Because I wish to effectively pass the context of ClassA around, I need
to create a pair of dictionary types (as in Restricted Data Types in
Haskell, Hughes 99), one to represent ClassA (DictClassA) and one to
represent ClassB (DictClassB). DictClassA also contains a term of type
DictClassB since ClassA is a subclass of ClassB. I should then be able
to call all the functions of ClassB via the appropriate term of
DictClassA, like so (assuming we want to use func2);
*Test> func2D (classBD (dict::DictClassA Int String)) "hello"
"bye"
So far so good, but now suppose I want Class A to have the further
constraint
class (Data (DictClassA a) b, ClassB b) => ClassA a b where
(so as to make ClassA a subclass of Data)
If we now try and do
*Test> func2D (classBD (dict::DictClassA Int String)) "hello"
We go into an infinite loop. Why? The expression still type-checks ok
and I can't see what it is trying to do. All the functions of ClassA can
be accessed ok, but not ClassB.
*Test> funcD ((dict::DictClassA Int String)) "hello" 5
"hello"
Is it something to do with ClassB only having one parameter?
I'm running GHC 20041231.
-Si.
--
Simon David Foster