[GHC] #11174: Traversable can't be derived for datatypes with unboxed arguments

[GHC]

#11174: Traversable can't be derived for datatypes with unboxed arguments
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        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:                    |
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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>
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