HscTypes: Use foldl' instead of foldr

In this case we are building a map, for which `foldl'` is much better
suited. This has a small but consistent impact on compiler allocations,
```
        -1 s.d.                -----          -0.161%
        +1 s.d.                -----          -0.011%
        Average                -----          -0.086%
```

Test Plan: Validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2967

1 files changed, 4 insertions, 3 deletions

diff --git a/compiler/main/HscTypes.hs b/compiler/main/HscTypes.hs
index 8e6925fe11..51cec26006 100644
--- a/compiler/main/HscTypes.hs
+++ b/compiler/main/HscTypes.hs
@@ -194,6 +194,7 @@ import GHC.Serialized   ( Serialized )
 
 import Foreign
 import Control.Monad    ( guard, liftM, when, ap )
+import Data.Foldable    ( foldl' )
 import Data.IORef
 import Data.Time
 import Exception
@@ -1124,10 +1125,10 @@ mkIfaceHashCache :: [(Fingerprint,IfaceDecl)]
 mkIfaceHashCache pairs
   = \occ -> lookupOccEnv env occ
   where
-    env = foldr add_decl emptyOccEnv pairs
-    add_decl (v,d) env0 = foldr add env0 (ifaceDeclFingerprints v d)
+    env = foldl' add_decl emptyOccEnv pairs
+    add_decl env0 (v,d) = foldl' add env0 (ifaceDeclFingerprints v d)
       where
-        add (occ,hash) env0 = extendOccEnv env0 occ (occ,hash)
+        add env0 (occ,hash) = extendOccEnv env0 occ (occ,hash)
 
 emptyIfaceHashCache :: OccName -> Maybe (OccName, Fingerprint)
 emptyIfaceHashCache _occ = Nothing