[Haskell] GHC Error question

[Ross Paterson]

On Mon, Dec 11, 2006 at 12:16:06PM +0000, Simon Peyton-Jones wrote:
> [...] Just to summarise, the difficulty is this:
>         I have a dictionary of type (C a b1)
>         I need a dictionary of type (C a b2)
>         There is no functional dependency between C's parameters
> 
> PS: the complete program is this:
>         class C a b where
>                 op :: a -> a
> 
>         f :: C a b => a -> a
>         f x = op x

That raises a point I'd wondered about.  GHC requires only that each
type variable in the context be reachable from the type via a chain
of assertions:

http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html#type-restrictions

What's the rationale for that, rather than calling types like the above
type of f ambiguous?

The examples given in the User's Guide involve functional dependencies,
albeit obscured by superclasses