Re: [Haskell-cafe] Scope of type variables in associated types

21 May 2007

      ...
Ok, thanks for the clarification. One final question then, how could I
rewrite the following in associated types:
class OneStep a b | a -> b
instance OneStep (Cons v t) t
class TwoStep a b | a -> b
instance (OneStep a b, OneStep b c) => TwoStep a c
If the fundep and b is dropped then I get:
class OneStep a
   data OS a :: *
instance OneStep (Cons v t)
   data OS (Cons v t) = t <<<< data constructor missing !!!
class TwoStep a
   data TS a :: *
instance (OneStep a, OneStep b) => TwoStep a
This last one can't be right, as I can't express the fact that the OS in
"OneStep a" provides the link with "OneStep b". So is there a way to
achieve this sort of chaining with associated types?
You'd have to write
...
class OneStep a
   data OS a :: *
instance OneStep (Cons v t)
   data OS (Cons v t) = OSC t
class TwoStep a
   data TS a :: *
instance (OneStep a, OneStep (OS a)) => TwoStep a where
   type TS a = TSC (OS (OS a))
which seems rather awkward with all these additional data type 
constructors.

You'd be better off with associated type synonyms:
...
class OneStep a
   type OS a :: *
instance OneStep (Cons v t)
   type OS (Cons v t) = t
class TwoStep a
   type TS a :: *
instance (OneStep a, OneStep (OS a)) => TwoStep a
   type TS a = OS (OS a)
which are currently under development.

--
Tom Schrijvers

Department of Computer Science
K.U. Leuven
Celestijnenlaan 200A
B-3001 Heverlee
Belgium

tel: +32 16 327544
e-mail: tom.schrijvers@cs.kuleuven.be