[ghc-steering-committee] #380 GHC2021: MonadFailDesugaring

Simon Peyton Jones simonpj at microsoft.com
Mon Dec 28 09:52:05 UTC 2020

The user manual is pretty confusing. 

It says
* The extension is enabled by default
* The extension is temporary and will be deprecated in future

Those two seem to contradict.

Let's check.  I believe that, 
* Today, by default, MonadFailDesugaring is on; 
  that is, we desugar do-blocks to use MonadFail.fail
* That behaviour is what we want, permanently
* The extension -XMonadFailDesugaring and -XNoMonadFailDesugaring
  are already deprecated, but have not yet been removed.
* When we remove them, we'll continue to desugar do-blocks to us

We were due to remove them in 8.8, as the proposal says under "Transitional Strategy"

But we failed to do so.

I conclude: we don't need votes on this.  We should just execute on the proposal and remove the flags.  

I see that the proposal lists the following steps for GHC 8.2:
1. Remove -XMonadFail leaving its effects on at all times.
2. Remove fail from Monad
3. Instead, re-export Control.Monad.Fail.fail as Prelude.fail 
   and Control.Monad.fail
4. Control.Monad.Fail is now a redundant module that can be 
   considered deprecated.

We appear to have done (2) and (3), but not (1) or (4).  Would someone like to make a ticket to do them?



|  -----Original Message-----
|  From: ghc-steering-committee <ghc-steering-committee-
|  bounces at haskell.org> On Behalf Of Joachim Breitner
|  Sent: 24 December 2020 14:26
|  To: ghc-steering-committee at haskell.org
|  Subject: Re: [ghc-steering-committee] #380 GHC2021:
|  MonadFailDesugaring
|  Hi,
|  mostly a technical point, but just to avoid confusion
|  MonadFailDesugaring is already on by default, and according to the
|  docs, is supposed to be deprecated, _but making that behaviour
|  permanent_. (So, really, NoMonadFailDesugaring is what is being
|  deprecated).
|  I was previously confused by this and voted for "no", which is not
|  what my intention is.  I updated my vote to "yes".
|  This now has 7 votes and reached "barely out" status.
|  Dear Richard and Simon, Simon, Tom:
|  You currently have "maybe", "irrelevant" or "no" here. Do you indeed
|  want to deviate from the documented plan of getting rid of
|  NoMonadFailDesugaring?
|  Cheers,
|  Joachim
|  --
|  Joachim Breitner
|    mail at joachim-breitner.de
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