Re: Proxy, new Typeable, and type-level equality

Note the eq lib and the type-eq/(:~:) GADT-based approach are interchangeable. You can upgrade a Leibnizian equality to a type equality by applying the Leibnizian substitution to an a :~: a. lens also has a notion of an Equality family at the bottom of the type semilattice for lens-like constructions, which is effectively a naked Leibnizian equality sans newtype wrapper. The only reason eq exists is to showcase this approach, but in practice I recommend just using the GADT, modulo mutterings about the name. :) That said those lowerings are particularly useful, if subtle -- Oleg wrote the first version of them, which I simplified to the form in that lib and they provide functionality that is classically not derivable for Leibnizian equality. Sent from my iPhone On Apr 4, 2013, at 3:01 AM, Erik Hesselink wrote:

On Wed, Apr 3, 2013 at 6:08 PM, Richard Eisenberg wrote:

...

Comments? Thoughts?

Edward Kmett 'eq' library uses a different definition of equality, but can also be an inspiration for useful functions. Particularly, it includes:

lower :: (f a :~: f b) -> a :~: b

Another question is where all this is going to live? In a separate library? Or in base? And should it really be in a GHC namespace? The functionality is not bound to GHC. Perhaps something in data would be more appropriate.

Erik

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