Kill some unnecessary varSetElems

When you do `varSetElems (tyCoVarsOfType x)` it's equivalent to
`tyCoVarsOfTypeList x`.

Why? If you look at the implementation:
```
tyCoVarsOfTypeList ty = runFVList $ tyCoVarsOfTypeAcc ty
tyCoVarsOfType ty = runFVSet $ tyCoVarsOfTypeAcc ty
```
they use the same helper function. The helper function returns a
deterministically ordered list and a set. The only difference
between the two is which part of the result they take. It is redundant
to take the set and then immediately convert it to a list.

This helps with determinism and we eventually want to replace the uses
of `varSetElems` with functions that don't leak the values of uniques.
This change gets rid of some instances that are easy to kill.

I chose not to annotate every place where I got rid of `varSetElems`
with a comment about non-determinism, because once we get rid of
`varSetElems` it will not be possible to do the wrong thing.

Test Plan: ./validate

Reviewers: goldfire, austin, simonmar, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2115

GHC Trac Issues: #4012

1 files changed, 1 insertions, 1 deletions

diff --git a/compiler/deSugar/DsArrows.hs b/compiler/deSugar/DsArrows.hs
index 1738a5d8ba..ea10b7424e 100644
--- a/compiler/deSugar/DsArrows.hs
+++ b/compiler/deSugar/DsArrows.hs
@@ -937,7 +937,7 @@ dsRecCmd ids local_vars stmts later_ids later_rets rec_ids rec_rets = do
     core_rec_rets <- mapM dsExpr rec_rets
     let
         -- possibly polymorphic version of vars of later_ids and rec_ids
-        out_ids = varSetElems (unionVarSets (map exprFreeIds (core_later_rets ++ core_rec_rets)))
+        out_ids = exprsFreeIdsList (core_later_rets ++ core_rec_rets)
         out_ty = mkBigCoreVarTupTy out_ids
 
         later_tuple = mkBigCoreTup core_later_rets