Ross Paterson wrote:

...

To avoid needing a non-H'98 instance of Functor for (->).

I'm puzzled as to why nhc98 lacks this instance in this module, given the import chain

Distribution.Simple.Command -> Distribution.ParseUtils -> Data.Tree -> Control.Applicative -> Control.Monad.Instances

nhc98 has a performance hack to avoid dumping large numbers of instance decls into .hi interface files. For any given instance, if both the class and the type are defined in the Prelude, then the instance can be omitted from the interface file (for any module except the Prelude itself), under the assumption that it will already be available by default via the Prelude, which is implicitly imported everywhere anyway. This assumption is entirely reasonable for H'98. But non-H'98 modules that add new instances for Prelude types and classes break the assumption. There is a workaround: essentially every non-Prelude module that is intended to define or re-export such non-standard instances, can tell the compiler to pretend it is building the Prelude, and so should include all instances in the interface. The flag is {-# OPTIONS_NHC98 --prelude #-}. The awkward part is that you need to add that line to _every_ module in the chain you gave above. Regards, Malcolm