What is the point of 'fmapDefault' in 'Data.Traversable'?

Question

I'm looking at the documentation for Data.Traversable and came across fmapDefault - https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base/Data-Traversable.html#g:3

fmapDefault :: Traversable t => (a -> b) -> t a -> t b

The documentation states that -

This function may be used as a value for fmap in a Functor instance, provided that traverse is defined.

So presumably it can be used to derive an fmap for a Traversable instance. However, Traversable has Functor as a superclass.

class (Functor t, Foldable t) => Traversable t where
    ...

So you cannot define a Traversable instance without defining the Functor instance first! And wherever you have a Traversable, you have access to an fmap, which is equivalent to (and perhaps more efficient than) fmapDefault.

So where would one use fmapDefault, instead of the much more familiar fmap?

Answer 1

It allows you to write

data Foo a = ...

instance Functor Foo where -- we do define the functor instance, but we “cheat”
  fmap = fmapDefault       -- by using `Traversable` in its implementation!

instance Traversable Foo where
  traverse = ...           -- only do this manually.

That said, I don't think this is really sensible. Functor instances are usually trivial to do by hand, and the obvious implementation will indeed likely be more efficient than a Traversable derived one. Usually, the instance can in fact be created automatically:

{-# LANGUAGE DeriveFunctor #-}

data Foo a = ...
       deriving (Functor)