Add flipped fmap

[wren ng thornton]

Edward Kmett wrote:
> I think in many ways the strict left to right effect evaluation order is
> what makes Applicative code readable.

I was going to mention that as the potential downside. However, I'd 
interpret my version in the same vein as ($!). That is: evaluate the 
argument, evaluate the function, invoke function on argument. The ($!) 
combinator violates the standard call-by-need ordering, just as 
flip(<**>) violates the standard left-to-right ordering, but people 
don't generally find ($!) unintuitive...

Of course, both are CPS transformations for the sake of controlling 
order of evaluation. The ($!) and flip(<**>) versions allow this 
transformation to be implicit by retaining the usual direct-style 
ordering of application; whereas flip($!) and (<**>) make the CPS 
ordering of application explicit. Reasoning in CPS doesn't bother me, 
but I think it's more elegant to be locally consistent about which style 
is being used.

-- 
Live well,
~wren