Add flipped fmap

[wren ng thornton]

wren ng thornton wrote:
> Edward Kmett wrote:
>> <**> cannot be defined in terms of flip as it swaps the order of the 
>> effects as well.
> 
> But I thought:
> 
>     (<**>) = liftA2 (flip ($))
> 
> :)


If (<**>) were to have been defined as flip(liftA2(flip($))), then I 
would be supportive of it. My definition has the same perspicuous type 
as (<*>) but adding support for non-LtR ordering of effects/evaluation, 
which seems to be the goal of the (<**>) combinator. However, (<**>) as 
it is actually defined forces one to give up the applicative style in 
order to alter evaluation ordering. In so doing it does not come to 
support another style and therefore simply looks like a wart on the API. 
Someone might could make an argument for (<**>) supporting a sort of 
object-oriented style in terms of evaluation order, but that style is 
not otherwise supported by the Applicative combinators, and so I 
wouldn't find it convincing.

-- 
Live well,
~wren