On Wed, 14 Nov 2012, Tyson Whitehead wrote:
On November 14, 2012 15:28:06 Henning Thielemann wrote:
Yes, that would work. It is only annoying that for liftA3 I do not need 'pure', at all. Thus we may note in the record that Data.Map is an example where <*> and liftAn make sense (for n>0) but 'pure' (i.e. Pointed) cannot be defined.
Sorry for jumping in in the middle without have really followed the conversation, but, assuming you want to define (<*>) without pure, would you also happen to want to define join without return?
That is, is it also sensibly a non-Pointed Monad?
TMap implements Applicative and Monad instances corresponding to the Reader monad. That is, join m ! k == m!k!k. This would not work with a plain Data.Map, since (m!k!k) may not exist.