[ANNOUNCE] GHC 8.4.1-rc1 available
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,
>> 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
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