#13328: Foldable, Functor, and Traversable deriving handle phantom types badly -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.1 Resolution: | Keywords: deriving-perf Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by David Feuer <David.Feuer@…>): In [changeset:"69f070d8e4d6043937e3405675ac911448bfcb44/ghc" 69f070d/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="69f070d8e4d6043937e3405675ac911448bfcb44" Deriving for phantom and empty types Make `Functor`, `Foldable`, and `Traversable` take advantage of the case where the type parameter is phantom. In this case, * `fmap _ = coerce` * `foldMap _ _ = mempty` * `traverse _ x = pure (coerce x)` For the sake of consistency and especially simplicity, make other types with no data constructors behave the same: * `fmap _ x = case x of` * `foldMap _ _ = mempty` * `traverse _ x = pure (case x of)` Similarly, for `Generic`, * `to x = case x of` * `from x = case x of` Give all derived methods for types without constructors appropriate arities. For example, ``` compare _ _ = error ... ``` rather than ``` compare = error ... ``` Fixes #13117 and #13328 Reviewers: austin, bgamari, RyanGlScott Reviewed By: RyanGlScott Subscribers: ekmett, RyanGlScott, rwbarton, thomie Differential Revision: https://phabricator.haskell.org/D3374 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:12> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler