[Haskell-cafe] MRP, 3-year-support-window, and the non-requirement of CPP

Edward Kmett ekmett at gmail.com
Sat Oct 10 19:25:37 UTC 2015

The part of the MRP proposal that I actively care about because it fixes a
situation that *actually causes harm* is moving (>>) to the top level.


Right now (*>) and (>>) have different default definitions. This means that
code runs often with different asymptotics depending on which one you pick.
Folks often define one but not the other.

This means that the performance of mapM_ and traverse_ needlessly differ.
It means that we can't simply weaken the type constraint on mapM_ and
sequence_ to Applicative, it as a knock-on consequence it means we can't
migrate mapM and sequence out of Traversable to top level definitions and
thereby simply provide folks with more efficient parallelizable mapping
when they reach for the 'obvious tool'.

return itself lurking in the class doesn't matter to me all that much as it
doesn't break anybody's asymptotics and it already has a sensible
definition in terms of pure as a default, so effectively you can write code
as if MRP was already in effect today. It is a wart, but one that could be
burned off on however long a time table we want if we choose to proceed.


On Wed, Oct 7, 2015 at 5:13 PM, Mark Lentczner <mark.lentczner at gmail.com>

> On Wed, Oct 7, 2015 at 9:38 AM, Erik Hesselink <hesselink at gmail.com>
> wrote:
>> While I don't think it detracts from your argument, it seems you
>> misread the original proposal. At no point will it remove `return`
>> completely. It would be moved out of the `Monad` class and be made
>> into a top-level definition instead, so you would still be able to use
>> it.
> Then why bother?
> If you don't intend to regard code that uses "return" as old, out-dated,
> in need of updating, etc....
> If you don't intend to correct people on #haskell to use pure instead of
> return...
> If you don't tsk tsk all mentions of it in books....
> If you don't intend to actually deprecate it.
> Why bother?
> But seriously, why do you think that "you would still be able to use it"?
> That is true for only the simplest of code - and untrue for anyone who has
> a library that defines a Monad - or anyone who has a library that they want
> to keep "up to date". Do you really want to have a library where all your
> "how to use this" code has return in the examples? Shouldn't now be pure?
> Do I now need -XCPP just for Haddock? and my wiki page? And what gets shown
> in Hackage? This is just a nightmare for a huge number of libraries, and
> especially many commonly used ones.
> Why bother!
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