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

[Tom Schrijvers]

> 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 at cs.kuleuven.be