Re: [Haskell-cafe] Questions about the Functor class and it's use in "Data types à la carte"

Dominic Steinitz wrote:

This would give

= \x -> bot x

and by eta reduction

This is the point: eta does not hold if seq exists. undefined `seq` 1 == undefined (\x -> undefined x) `seq` 1 == 1 The "(.) does not form a category" argument should be something like: id . undefined == (\x -> id (undefined x)) /= undefined where the last inequation is due to the presence of seq. That is, without seq, there is no way to distinguish between undefined and (const undefined), so you could use a semantic domain where they coincide. In that case, eta does hold. Regards, Zun.