[Haskell-cafe] Re: overlapping instances, selecting if type a does not belong to class?

Marc Weber marco-oweber at gmx.de
Mon Feb 26 20:29:55 EST 2007


> 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

> However, it seems that your particular problem can be solved with
> simpler means:
> > 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
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

	Thanks oleg!

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