1. 17 Feb, 2016 1 commit
    • Ryan Scott's avatar
      Remove superfluous code when deriving Foldable/Traversable · a82956df
      Ryan Scott authored
      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
      a82956df