Add flipped fmap

[Johan Tibell]

Hi!

On Wed, Apr 7, 2010 at 3:58 PM, Felipe Lessa <felipe.lessa at gmail.com> wrote:
> On Wed, Apr 07, 2010 at 03:33:51PM +0200, Johan Tibell wrote:
>> Yet, if we add all the function compositions that some group of
>> people use in their code our APIs get much more complicated.
>
> I completely agree, but I think you're overreacting to this
> proposal (no offense intended, please :]).  I don't see a
> tendency of adding every combinator to the libraries, and I don't
> think this proposal will change anything.

I'm certainly trying to make a general argument that's larger than
this proposal and the important part I want to get across is the
criteria used to decide what functions to add to the (base) libraries.

In particular I want us to establish that the the following arguments
aren't strong enough:

- X people use this composition a lot so we should add it.

- We added something like this before (e.g. added flipped versions of
some operators) so doing it again is fine.

Here are some good reasons to add something to an API:

- X is not expressible using the current API (or this very
difficult/error prone to express).

- X is cannot be written efficiently (in terms of run-time
performance/space usage) using the current API.

- The API exports function X but what it really should have exported
was combinators Y and Z (that e.g. make the API more
general/composable/etc).

Cheers,
Johan