Re: Is it time to start deprecating FunDeps?

Ultimately this is the wrong forum for this discussion as neither type equalities nor functional dependencies are in Haskell' at this time. On Wed, May 1, 2013 at 7:04 PM, AntC wrote:

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Martin Sulzmann writes:

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On Wed, May 1, 2013 at 11:13 AM, AntC wrote:

I want to replace FD's with Equality Constraints.

Ok, that's the bit I've missed, but then I argue that you can't fully encode FDs.

Consider again the 'Sum' example.

In the FD world:

Sum x y z1, Sum x y z2 ==> z1 ~ z2

enforced by

Sum x y z | x y -> z

I'm still not sure you've 'got' it. The class has 2 FD's. Oleg put:

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class Sum x y z | x y -> z, x z -> y

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In my TF encoding, we find a similar derivation step

SumF1 x y ~ z1, SumF1 x y ~ z2 ==> z1 ~ z2

But you haven't captured the FD from {result, arg1} -> arg2. In the TF example you first posted, you expressed that with an explicit equality constraint. (I won't repeat yours, because it wasn't to do with Peano Arith.)

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So, you're asking can we translate/express FDs using GHC intermediate language with plain type equations only?

No, not asking, but stating; and not talking about the intermediate language, but the surface language.

Could I respectfully suggest you re-read the OP.

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