MonadFail decisions

Edward Kmett ekmett at
Fri Oct 16 16:22:47 UTC 2015

The current intention is to go ahead with MonadFail.

It sounds like we'll need to delay the warnings themselves until around 8.4.

We can add them, but not turn them on by default in the short term. This
has the knock-on effect of delaying the whole plan a release or two, but
otherwise the plan is very actionable.

A lot of the opposition comes from fear that we 'might do anything at any
time'. If we're up front about what is coming and give sufficient notice
and the ability for folks to maintain a reasonably wide backwards
compatibility window without needing to dip into CPP or suppress warnings
them most of those fears go away.


On Fri, Oct 16, 2015 at 12:09 PM, David Luposchainsky <
dluposchainsky at> wrote:

> On 13.10.2015 16:29, Simon Peyton Jones wrote:
> > Also, David, did our conversation at HX help you get un-stuck?
> Hi Simon,
> yes, it was definitely a good pointer. On the other hand, I found the
> Haskell
> Exchange to be quite a frustrating event with respect to current events:
> there
> was a load of very loud, but in my opinion very wrong, categorical
> opposition to
> breaking changes in general.
> I spent quite a bit of time worrying about MonadFail in the past, but
> right now
> I'd like to wait for a "tentative yes" from the CLC before I keep going,
> because
> I'm really not sure the mob is going to make me throw away my patch.
> Granted, a
> lot of the discussion is about MRP, but many of the points brought up
> there are
> equally valid against the MFP.
> David
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