#12464: Add `instance Semigroup a => Semigroup (IO a)` -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: feature request | Status: patch Priority: normal | Milestone: Component: Core Libraries | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D2588 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ryan Scott <ryan.gl.scott@…>): In [changeset:"8c6a3d68c0301bb985aa2a462936bbcf7584ae9c/ghc" 8c6a3d6/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="8c6a3d68c0301bb985aa2a462936bbcf7584ae9c" Add missing Semigroup instances for Monoidal datatypes in base 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: #12464 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12464#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler