On Jul 21, 2010, at 11:25 AM, Victor Nazarov wrote:
1) UndecidableInstances are required for this code to compile. Is it possible to avoid this language extension? What is the reason for it to be required?
The rule "No type variable has more occurrences in the assertion than in the head" is violated with "instance Show (f (Mu f)) => Show (Mu f)". The head becomes larger, so ghc cannot tell that checking the assertions will terminate.
2) app, var, abs functions. app function is defined differently for every example. Is it possible to perform some automatic lifting so that app will work for any combination of ApplicativeF? Can I use Functor class or should I define my own type-class for this purpose? Is it possible at all?
Yes, it is possible, see the section "automating injections" in the Data Types A La Carte paper. greetings, Sjoerd Visscher