Ryan Scott authored
Summary: There are currently three datatypes that are exposed in `base` that have `Monoid` instances, but no `Semigroup` instances: * `IO` * `Event` (from `GHC.Event`) * `Lifetime` (from `GHC.Event`) (There is also `EventLifetime` in `GHC.Event.Internal`, but it is not exported directly, so I didn't bother with it.) Adding the `Semigroup` instances for these types directly in the modules in which they're defined resulted in some horrific import cycles, so I opted to take the easy approach of defining all of these instances in `Data.Semigroup`. (When `Semigroup` becomes a superclass of `Monoid`, these instances will have to be moved somehow.) Fixes #12464. Test Plan: It compiles Reviewers: hvr, ekmett, austin, bgamari Reviewed By: ekmett Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2588 GHC Trac Issues: #124648c6a3d68
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