Wow.
That said, it is quite possible in Haskell to achieve genuine class-based dispatch, with backtracking if necessary: http://pobox.com/~oleg/ftp/Haskell/poly2.txt Thanks for digging this up. I'll have to reread it tomorrow. I wasn't able to find the definition of AllOf(But): quote type Nums = Int :*: Integer :*: AllOf Fractionals :*: HNil type Ords = Bool :*: Char :*: AllOf Nums :*: HNil type Eqs = AllOf (TypeCl OpenEqs) :*: AllOfBut Ords Fractionals :*: HNil
instance (HList a) => HListAppendArbitrary a HNil a where hAppendArbitrary a _ = a instance (HList a, HList b, HList c) => HListAppendArbitrary a (HCons b d) c where hAppendArbitrary a b = hAppend a b -- or use HCons with recursion You are right. But it's more complicated in the sense that it needs two instance declarations compared to one (called much boilerplate in the
However, it seems that your particular problem can be solved with simpler means: article) ...
Incidentally, the code in your previous messages contained -fallow-incoherent-instances. I'd like to caution against using this extension. I agree. It was a stupid >>perhaps this will make it compile<< try and error approach.
Thanks oleg! Marc