#11174: Traversable can't be derived for datatypes with unboxed arguments -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: RyanGlScott Type: bug | Status: closed Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 7.10.2 Resolution: fixed | Keywords: Generics Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D1908 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"a82956df5b34175410e0feb9e2febe7d39b60b49/ghc" a82956d/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="a82956df5b34175410e0feb9e2febe7d39b60b49" Remove superfluous code when deriving Foldable/Traversable Currently, `-XDeriveFoldable` and `-XDeriveTraversable` generate unnecessary `mempty` and `pure` expressions when it traverses of an argument of a constructor whose type does not mention the last type parameter. Not only is this inefficient, but it prevents `Traversable` from being derivable for datatypes with unlifted arguments (see Trac #11174). The solution to this problem is to adopt a slight change to the algorithms for `-XDeriveFoldable` and `-XDeriveTraversable`, which is described in [this wiki page](https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/DeriveFu nctor#Proposal:alternativestrategyforderivingFoldableandTraversable). The wiki page also describes why we don't apply the same changes to the algorithm for `-XDeriveFunctor`. This is techincally a breaking change for users of `-XDeriveFoldable` and `-XDeriveTraversable`, since if someone was using a law-breaking `Monoid` instance with a derived `Foldable` instance (i.e., one where `x <> mempty` does not equal `x`) or a law-breaking `Applicative` instance with a derived `Traversable` instance, then the new generated code could result in different behavior. I suspect the number of scenarios like this is very small, and the onus really should be on those users to fix up their `Monoid`/`Applicative` instances. Fixes #11174. Test Plan: ./validate Reviewers: hvr, simonpj, austin, bgamari Reviewed By: simonpj, bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1908 GHC Trac Issues: #11174 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11174#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler