-
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