[Haskell-cafe] The mother of all functors/monads/categories

[Max Bolingbroke]

On 27 June 2010 14:30, Roman Leshchinskiy <rl at cse.unsw.edu.au> wrote:
> In that case, isn't D just the dictionary for (X d) and a value of type (d a)? I.e., couldn't we always define it as:
>
> data D d a where { D :: X d => d a -> D d a }

I wondered about this, but how would you write e.g. the "return"
method for Codensity? You don't have an input dictionary to work with.

Cheers,
Max