Corey O'Connor wrote: > On Tue, Jan 27, 2009 at 4:51 AM, <o...@okmij.org> wrote: >> Doug McIlroy wrote: >>> A fragment of an attempt to make pairs serve as complex numbers, >>> using ghc/hugs extensions: >>> >>> instance Num a => Num (a,a) where >>> (x,y) * (u,v) = (x*u-y*v, x*v+y*u) >> The recent versions of GHC have a nifty equality constraint, so the >> code can be written simply > > I'm confused on why > instance Num a => Num (a, a) where > > is not equivalent to > instance (Num a, Num b, a ~ b) => Num (a, b) where > > I don't know the details of the type inference algorithm. What am I > missing to understand why they are not the same?
Type inference doesn't backtrack, and it starts by giving different expressions different free type variables. So it doesn't initially "know" that the two numeric literals in the tuple have the same type, and so the instance ... => Num (a, a) doesn't apply - after all, there might be some different instance where the two tuple elements are of different types. In contrast Num (a, b) will always apply, and only later do we discover that we've forced a = b by selecting it. But that's fine as there's no possibility of another Num instance for tuples (ignoring overlapping instances for now). Ganesh ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ============================================================================== _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
