Re: [Haskell-cafe] Re: Eta-expansion and existentials (or: types destroy my laziness)

On 22 October 2010 12:03, Dan Doel wrote:

 data Mu f = In { out :: f (Mu f) }

 instance Show (f (Mu f)) => Show (Mu f) where    show = show . out

Is that an example of a value recursive dictionary?

Assuming the Show (f (Mu f)) instance uses the (Mu f) one, AFAIK this should indeed build a loopy dictionary. I think this extension was motivated by "Scrap your Boilerplate with Class" - see section 5 of http://research.microsoft.com/en-us/um/people/simonpj/papers/hmap/gmap3.pdf. Cheers, Max