On Tue, 2009-01-27 at 10:05 -0800, 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
The head of this instance is Num (a, a), while > is not equivalent to > instance (Num a, Num b, a ~ b) => Num (a, b) where the head of this instance is Num (a, b) So if GHC doesn't already have an equation a ~ b in scope, it'll skip the first instance during instance search, but find the second one (incidentally introducing the equation a ~ b as above). So GHC is more aggressive about using the second instance. (Or, to put it another way: instance selection ignores the context on the instance. Or, to put it another way: the context on the instance is part of the *output* of instance selection, not the input). jcc _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
