[Haskell-cafe] Re: Unique functor instance

Janis Voigtlaender voigt at tcs.inf.tu-dresden.de
Tue Nov 25 09:09:51 EST 2008

DavidA wrote:
> I suspect that the answer to the question is, yes, you can have different 
> Functor instances. All you need is a sum-product type that it's possible to 
> interpret as two different abstractions, leading to two different Functor 
> instances.

The sum-product types are exactly the "not-too-exotic" types to which my
proof applies. So as long as extensional equivalence means Haskell
equivalence, and not some "modulo an interpretation" equivalence (like
considering two lists equivalent if they contain the same elements but
in potentially different order), the answer is no, one cannot have
different funtor instances.

Ciao, Janis.

Dr. Janis Voigtlaender
mailto:voigt at tcs.inf.tu-dresden.de

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