Discussion: Arities of <*> and *> for Monad instances

[David Feuer]

http://hackage.haskell.org/package/base-4.7.0.1/docs/Control-Applicative.html
says that

    The other methods have the following default definitions,
    which may be overridden with equivalent specialized
    implementations:

    u *> v = pure (const id) <*> u <*> v


and

    If f is also a Monad, it should satisfy

    ...
    (<*>) = ap


The (potential) trouble is that these have higher arities than is always
natural. For example, it would seem reasonable to say

    (<*>) Just f = fmap f
    (<*>) Nothing = const Nothing

and to replace the default definition of (*>) like so:

    (*>) a1 = (<*>) (id <$ a1)

but these are not strictly equivalent because they have arity 1 instead of
arity 2. Would such definitions be be better in some cases? If so, should
we weaken the rules a bit?
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