[commit: ghc] wip/ghc-8.0-det: Remove some varSetElems in dsCmdStmt (0234bfa)

[git at git.haskell.org]

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

On branch  : wip/ghc-8.0-det
Link       : http://ghc.haskell.org/trac/ghc/changeset/0234bfa6c15c3f49f60c288c3a29399105450ca0/ghc

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

commit 0234bfa6c15c3f49f60c288c3a29399105450ca0
Author: Bartosz Nitka <niteria at gmail.com>
Date:   Wed May 11 07:47:15 2016 -0700

    Remove some varSetElems in dsCmdStmt
    
    varSetElems introduces unnecessary determinism and it's easy to
    preserve determinism here.
    
    Test Plan: ./validate
    
    Reviewers: goldfire, simonmar, austin, bgamari, simonpj
    
    Reviewed By: simonpj
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D2195
    
    GHC Trac Issues: #4012
    
    (cherry picked from commit 0e719885f53e20f2e14a94b32d858b47b516a8fc)


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0234bfa6c15c3f49f60c288c3a29399105450ca0
 compiler/deSugar/DsArrows.hs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/compiler/deSugar/DsArrows.hs b/compiler/deSugar/DsArrows.hs
index ea10b74..cdf839a 100644
--- a/compiler/deSugar/DsArrows.hs
+++ b/compiler/deSugar/DsArrows.hs
@@ -48,6 +48,7 @@ import VarSet
 import SrcLoc
 import ListSetOps( assocDefault )
 import Data.List
+import Util
 
 data DsCmdEnv = DsCmdEnv {
         arr_id, compose_id, first_id, app_id, choice_id, loop_id :: CoreExpr
@@ -786,7 +787,7 @@ dsCmdStmt ids local_vars out_ids (BindStmt pat cmd _ _ _) env_ids = do
     (core_cmd, fv_cmd, env_ids1) <- dsfixCmd ids local_vars unitTy pat_ty cmd
     let pat_vars = mkVarSet (collectPatBinders pat)
     let
-        env_ids2 = varSetElems (mkVarSet out_ids `minusVarSet` pat_vars)
+        env_ids2 = filterOut (`elemVarSet` pat_vars) out_ids
         env_ty2 = mkBigCoreVarTupTy env_ids2
 
     -- multiplexing function
@@ -863,8 +864,9 @@ dsCmdStmt ids local_vars out_ids
                  , recS_later_rets = later_rets, recS_rec_rets = rec_rets })
         env_ids = do
     let
-        env2_id_set = mkVarSet out_ids `minusVarSet` mkVarSet later_ids
-        env2_ids = varSetElems env2_id_set
+        later_ids_set = mkVarSet later_ids
+        env2_ids = filterOut (`elemVarSet` later_ids_set) out_ids
+        env2_id_set = mkVarSet env2_ids
         env2_ty = mkBigCoreVarTupTy env2_ids
 
     -- post_loop_fn = \((later_ids),(env2_ids)) -> (out_ids)