On Fri, Feb 12, 2010 at 2:11 PM, Andrew Coppin
OK, well in that case, I'm utterly puzzled as to why both forms exist in the first place. If TFs don't allow you to do anything that can't be done with ATs, why have them?
My head hurts...
I think the question is the reverse -- why do ATs exist when you can do everything with the more general Type Families? This is the answer from the GHC documentation: "Type families appear in two flavours: (1) they can be defined on the toplevel or (2) they can appear inside type classes (in which case they are known as associated type synonyms). The former is the more general variant, as it lacks the requirement for the type-indices to coincide with the class parameters. However, the latter can lead to more clearly structured code and compiler warnings if some type instances were - possibly accidentally - omitted." http://www.haskell.org/haskellwiki/GHC/Indexed_types#Detailed_definition_of_...