A question concerning functional dependencies
I wanted to write a small package implementing vector spaces, etc. A part of it is class Module v s where (*) :: s-v-v defining the multiplication of a vector by a scalar: w = a*v Now, as in many other circumstances, concrete vectors are based on concrete scalars, and I defined really: class Module v s | v-s . One typical instance of vectors is the orthodox functional construction instance Num s = Module (v-s) s where (s*f) x = s * (f x) and such tests: u = 2.5 * sin; res = u 3.14 pass without tears. But I wanted also that operators of the type (b-s) - (b-s), for example: inver f = recip . f . recip be vectors. So: instance ...= Module ((v-s)-(v-s)) s where (s*op) f = s*(op f) But GHCi yells that two instances in view of the functional dependency declared are in conflict. Since I believe that I do not really understand fundeps in Haskell, and this is not a GHC 'feature' only, I send this query to the haskell list. I don't see this conflict. I could remove the fundep, but then I have other (smaller, but a bit annoying) problems, so I want to keep it, if only for my own instruction. Good people, help, please. Why v [-s] cannot coexist in this context with ((v-s)-v) [-s] Of course all extensions, including overlapping instances are on. Jerzy Karczmarczuk Caen, France ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: A question concerning functional dependencies
At 2002-09-02 02:46, Jerzy Karczmarczuk wrote: class Module v s | v-s . ... instance Num s = Module (v-s) s ... instance ...= Module ((v-s)-(v-s)) s ... But GHCi yells that two instances in view of the functional dependency declared are in conflict. GHCi is correct. Bear in mind GHC ignores contexts in instance declarations when calculating conflicts. This is unfortunate, but apparently fixing it would be hard. ((v-s)-(v-s)) is a substitution instance of (v-s). So from the first instance, instance ...= Module (v-s) s GHC can derive instance ...= Module ((v-s)-(v-s)) (v-s) ...which conflicts with your second instance per the fundep: instance ...= Module ((v-s)-(v-s)) s The solution of course is to declare new datatypes: newtype Vector v s = MkVector (v-s) newtype Operator v s = MkOperator ((v-s)-(v-s)) etc. -- Ashley Yakeley, Seattle WA ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
Re: A question concerning functional dependencies
On Mon, Sep 02, 2002 at 03:11:58AM -0700, Ashley Yakeley wrote: At 2002-09-02 02:46, Jerzy Karczmarczuk wrote: class Module v s | v-s . ... instance Num s = Module (v-s) s ... instance ...= Module ((v-s)-(v-s)) s ... But GHCi yells that two instances in view of the functional dependency declared are in conflict. GHCi is correct. Bear in mind GHC ignores contexts in instance declarations when calculating conflicts. This is unfortunate, but apparently fixing it would be hard. Even taking contexts into account, these two might still conflict: if you had instance Num ((v-s)-v) for some values of v and s, you would get a conflict. GHC (and Hugs) check for potential conflicts like this unless you explicitly allow overlapping instances. --Dylan msg11456/pgp0.pgp Description: PGP signature
Re: A question concerning functional dependencies
At 2002-09-02 07:47, Dylan Thurston wrote: GHC (and Hugs) check for potential conflicts like this unless you explicitly allow overlapping instances. AFAIK, even with overlapping instances allowed, GHC will still complain if there's a fundep. See http://sourceforge.net/tracker/?func=detailaid=441389group_id=8032atid= 108032. -- Ashley Yakeley, Seattle WA ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
