Well it might not be ambiguous. Consider
instance C Int b where...
Simon
| -----Original Message-----
| From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]
| On Behalf Of Ross Paterson
| Sent: 11 December 2006 12:41
| To: [email protected]
| Subject: Re: [Haskell] GHC Error question
|
| 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
|
| _______________________________________________
| Glasgow-haskell-users mailing list
| [email protected]
| http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
_______________________________________________
Glasgow-haskell-users mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
