[Haskell-cafe] Monad of no `return` Proposal (MRP): Moving `return` out of `Monad`

Johan Tibell johan.tibell at gmail.com
Mon Oct 5 15:12:38 UTC 2015

Perhaps we should weigh the +1 and -1s in this thread with the number of
lines of Haskell written by the voter? ;)

On Mon, Oct 5, 2015 at 5:09 PM, Gershom B <gershomb at gmail.com> wrote:

> On October 5, 2015 at 10:59:35 AM, Bryan O'Sullivan (bos at serpentine.com)
> wrote:
> > I would like to suggest that the bar for breaking all existing
> libraries, books, papers,
> > and lecture notes should be very high; and that the benefit associated
> with such a breaking
> > change should be correspondingly huge.
> >
> My understanding of the argument here, which seems to make sense to me, is
> that the AMP already introduced a significant breaking change with regards
> to monads. Books and lecture notes have already not caught up to this, by
> and large. Hence, by introducing a further change, which _completes_ the
> general AMP project, then by the time books and lecture notes are all
> updated, they will be able to tell a much nicer story than the current one?
> As for libraries, it has been pointed out, I believe, that without CPP one
> can write instances compatible with AMP, and also with AMP + MRP. One can
> also write code, sans CPP, compatible with pre- and post- AMP.
> So the reason for choosing to not do MRP simultaneous with AMP was
> precisely to allow a gradual migration path where, sans CPP, people could
> write code compatible with the last three versions of GHC, as the general
> criteria has been.
> So without arguing the necessity or not, I just want to weigh in with a
> technical opinion that if this goes through, my _estimation_ is that there
> will be a smooth and relatively painless migration period, the sky will not
> fall, good teaching material will remain good, those libraries that bitrot
> will tend to do so for a variety of reasons more significant than this, etc.
> It is totally reasonable to have a discussion on whether this change is
> worth it at all. But let’s not overestimate the cost of it just to further
> tip the scales :-)
> —gershom
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