[commit: ghc] master: Refactor TcRnMonad.mapAndRecoverM (66f0056)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/66f0056ae1279c3149053aa600c7fe09575212b1/ghc

>---------------------------------------------------------------

commit 66f0056ae1279c3149053aa600c7fe09575212b1
Author: Simon Jakobi <simon.jakobi at gmail.com>
Date:   Thu Nov 22 11:51:53 2018 -0500

    Refactor TcRnMonad.mapAndRecoverM
    
    This version doesn't require the 'reverse' step after the monadic
    fold.
    
    Test Plan: ./validate
    
    Reviewers: bgamari, tdammers
    
    Reviewed By: tdammers
    
    Subscribers: monoidal, rwbarton, carter
    
    Differential Revision: https://phabricator.haskell.org/D5343


>---------------------------------------------------------------

66f0056ae1279c3149053aa600c7fe09575212b1
 compiler/typecheck/TcRnMonad.hs | 2 +-
 compiler/utils/Maybes.hs        | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/compiler/typecheck/TcRnMonad.hs b/compiler/typecheck/TcRnMonad.hs
index eb5a63a..a033bc4 100644
--- a/compiler/typecheck/TcRnMonad.hs
+++ b/compiler/typecheck/TcRnMonad.hs
@@ -990,7 +990,7 @@ recoverM recover thing
 -- | Drop elements of the input that fail, so the result
 -- list can be shorter than the argument list
 mapAndRecoverM :: (a -> TcRn b) -> [a] -> TcRn [b]
-mapAndRecoverM f = fmap reverse . foldAndRecoverM (\xs x -> (:xs) <$> f x ) []
+mapAndRecoverM f = mapMaybeM (fmap rightToMaybe . try_m . f)
 
 -- | The accumulator is not updated if the action fails
 foldAndRecoverM :: (b -> a -> TcRn b) -> b -> [a] -> TcRn b
diff --git a/compiler/utils/Maybes.hs b/compiler/utils/Maybes.hs
index 3a139a5..14bc46b 100644
--- a/compiler/utils/Maybes.hs
+++ b/compiler/utils/Maybes.hs
@@ -18,6 +18,7 @@ module Maybes (
         firstJust, firstJusts,
         whenIsJust,
         expectJust,
+        rightToMaybe,
 
         -- * MaybeT
         MaybeT(..), liftMaybeT, tryMaybeT
@@ -62,6 +63,10 @@ whenIsJust Nothing  _ = return ()
 orElse :: Maybe a -> a -> a
 orElse = flip fromMaybe
 
+rightToMaybe :: Either a b -> Maybe b
+rightToMaybe (Left _)  = Nothing
+rightToMaybe (Right x) = Just x
+
 {-
 ************************************************************************
 *                                                                      *