[ANNOUNCE] GHC 8.4.1-rc1 available

Ben Gamari ben at well-typed.com
Wed Mar 7 15:55:21 UTC 2018

Kazu Yamamoto (山本和彦) <kazu at iij.ad.jp> writes:

> Hello Ben,
Hi Kazu,

>> As always, do let us know if you encounter any trouble in the course of
>> testing. Thanks for your help!
> I tried GHC 8.4.1rc1 to understand how SemigroupMonoid and MonadFail
> proposals work.
> GHC 8.4.1rc1 surely detects Monoid data types if they are not
> instances of Semigroup. However, GHC users guide still says that both
> -Wcompat and -Wsemigroup are disabled by default. Should we update the
> document or am I missing something?
What you are seeing isn't a warning, it's an error.
8.4.1 throws errors for Monoids missing Semigroup instances not because
of -Wsemigroup but rather because Semigroup is now a superclass of

-Wsemigroup will likely be deprecated in a future release now since
we've reached phase 2 of the Semigroup/Monoid proposal.

> GHC 8.4.1rc does not find Monad data types which define "fail" even
> with -Wcompat. Now I understand that
> -Wnoncanonical-monadfail-instances is necessary. Why doesn't -Wcampat
> include -Wnoncanonical-monadfail-instances?

My understanding is that the MonadFail proposal is in a bit of a state
of limbo, so its preparatory warning flag hasn't yet been added to


- Ben
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 487 bytes
Desc: not available
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20180307/044a703d/attachment.sig>

More information about the ghc-devs mailing list