Well. The instance declaration is like this: instance FwdSig Native instance FwdSig Def instance FwdSig d => Sig d instance Sig XXX Neither Native nor Def are direct instances of Sig. And this XXX, it is an instance of Sig, not a FwdSig. As you can see, there's no real overlapping at all if we take the "FwdSig d =>" as a precondition for "Sig d". However, ghc seems disregard the "FwdSig d =>" piece and insists that the "Sig d" and "Sig XXX" forms an overlapping. This confuses me and the only reason I can figure out is that ghc does not attempt to resolve ambiguity based on "=>", which, I guess, could make the implementation simpler. Brandon Michael Moore <brandon@its.caltech.edu>@haskell.org on 03/11/2004 06:56:41 PM Sent by: haskell-bounces@haskell.org To: Ben_Yu@asc.aon.com cc: haskell@haskell.org, oleg@pobox.com Subject: Re: [Haskell] What is the best way to write adapters? On Thu, 11 Mar 2004 Ben_Yu@asc.aon.com wrote:
Thanks! Oleg.
This works and it looks nice!
And now, my code can be like:
class FwdSig d where (forall a. Sig a => a -> w) -> d -> w
All the types that supports such forwarding are instances of FwdSig.
My Def type is:
instance FwdSig Def where fwd f (ClassDef c) = f c fwd f (ProtDef p) = f p
instance Sig Def where getName = fwd getName getMethods = fwd getMethods ...
My Native type is:
instance FwdSig Native where fwd f (NativeSignature s) = f s fwd f (NativeProtocol p) = f p
instance Sig Native where getName = fwd getName getMethods = fwd getMethods ...
Many annoying forwarding functions are gone.
The only thing that I hope to be better is this "getXXX = fwd getXXX"
piece
of code. Is it possible to reuse the same piece of code for both Native and Def and any other possible types?
I'm not as handy with the type system as Oleg, but I can help out here. The problem with your new instance is that if the compiler is trying to see if Native is an instance of Sig, it can start with the declaration Sig Native, or the declarations FwdSig a => Sig a, both of which could potentially derive an instance of Sig Native. Additionally passing the -fallow-overlapping-instances flag will permit you to compile a program where instances overlap like this, and will select the most specific matching instance (looking only at the head). Your code is fine. Brandon
Inspired by your generic code, I wrote such thing:
instance FwdSig d => Sig d where getName = fwd getName getMethods = fwd getMethods ...
However, my ghc complains about the use of "Sig d".
I followed its recommendation and put -fallow-undecidable-instances flag with the surprise that the "FwdSig d=>Sig d" instance declaration conflicts with my other "instance Sig XXX" declarations.
Surely this is not a serious problem, I can live with repeating the "getXXX=fwd getXXX" several times. Just curious about how further this can go.
Ben.
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