Re: Add flipped fmap

On Wed, Apr 07, 2010 at 04:12:24PM +0200, Bas van Dijk wrote:

On Wed, Apr 7, 2010 at 3:58 PM, Felipe Lessa wrote:

...

Simmetry, as the ticket says.

Indeed, I would like to emphasize that my main reason for adding a flipped fmap is symmetry. I find it hard to explain why <$$> shouldn't be in this list:

(<$>) ∷ Functor f ⇒ (α → β) → (f α → f β) (<$$>) ∷ Functor f ⇒ f α → (α → β) → f β

(<*>) ∷ Applicative f ⇒ f (α → β) → (f α → f β) (<**>) ∷ Applicative f ⇒ f α → f (α → β) → f β

(>>=) ∷ Monad f ⇒ f α → (α → f β) → f β (=<<) ∷ Monad f ⇒ (α → f β) → (f α → f β)

Let me point out that (<**>) is NOT defined as flip (<*>)! In fact it is defined as (<**>) = liftA2 (flip ($)) which is quite different, since it performs effects in the opposite order from flip (<*>). So arguing for (<$$>) as something parallel to (<**>) is a bit misleading. I don't know whether this lends support to one argument or another; I'm personally fairly neutral on the proposal, but I thought this was important to point out. -Brent