I wrote:
...fmapDefault and foldMapDefault... cannot be used as methods in superclasses, since the superclasses must already exist before these functions can be defined.
Ross Paterson wrote:
The intention is that if you only want to define Traversable, you can fill in the required superclass instances by defining
instance Functor F where fmap = fmapDefault
instance Foldable F where foldMap = foldMapDefault
instance Traversable F where traverse = ...
That should work fine.
Cool, I didn't know that works. On the other hand, in general, that mechanism sounds a bit dangerous. It could lead to infinite recursion that would be very difficult to debug in the case of a complex class dependency graph, couldn't it?
However the fmap definition won't work if you defined sequenceA instead of traverse, so the documentation of fmapDefault needs to be corrected to say that.
Case in point. OK, how about this: In the documentation for class Traversable, change (`fmapDefault`) to (see `fmapDefault`), similarly for foldMapDefault. Documentation for fmapDefault: -- | This function should be equivalent to `fmap` in -- the `Functor` superclass instance. If you do not -- already have a `Functor` instance, you can use -- this function to define one: -- -- > instance Functor T where -- > fmap = fmapDefault -- -- Note, however, that this will lead to infinite -- recursion if you did not provide an explicit -- implementation of the `traverse` method. Documentation for foldMapDefault: -- | This function should be equivalent to `foldMap` in -- the `Foldable` superclass instance. If you do not -- already have a `Foldable` instance, you -- can use this function to define one: -- -- > instance Functor T where -- > fmap = fmapDefault Thanks, Yitz