From 6a317ee95b04e3fdb4b4be6f710221aeb20d4ee9 Mon Sep 17 00:00:00 2001
From: Ben Gamari <ben@smart-cactus.org>
Date: Mon, 4 Jul 2016 20:19:41 -0400
Subject: Try fusing away unpackFS

---
 compiler/utils/Encoding.hs   | 17 ++++++++---------
 compiler/utils/FastString.hs |  3 ++-
 2 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/compiler/utils/Encoding.hs b/compiler/utils/Encoding.hs
index 6028397cfd..8da88318d9 100644
--- a/compiler/utils/Encoding.hs
+++ b/compiler/utils/Encoding.hs
@@ -35,7 +35,7 @@ import Numeric
 import ExtsCompat46
 import qualified Data.ByteString as BS
 import qualified Data.ByteString.Unsafe   as BS
-import System.IO.Unsafe ( unsafeInterleaveIO )
+import System.IO.Unsafe ( unsafePerformIO )
 
 -- -----------------------------------------------------------------------------
 -- UTF-8
@@ -114,19 +114,18 @@ utf8CharStart p = go p
                         then go (p `plusPtr` (-1))
                         else return p
 
-utf8DecodeStringLazy :: BS.ByteString -> IO [Char]
+utf8DecodeStringLazy :: BS.ByteString -> [Char]
 utf8DecodeStringLazy !bs
-  = unpack 0
+  = build (unpack 0)
   where
-    unpack !offset
-        | BS.null bs' = return []
+    unpack !offset cons nil
+        | BS.null bs' = nil
         | otherwise  =
-          BS.unsafeUseAsCString bs' $ \ptr ->
+          unsafePerformIO $ BS.unsafeUseAsCString bs' $ \ptr ->
             case utf8DecodeChar (castPtr ptr) of
-              (c, nBytes) -> do
-                chs <- unsafeInterleaveIO $ unpack (offset + nBytes)
-                return (c : chs)
+              (c, nBytes) -> return $ c `cons` unpack (offset + nBytes) cons nil
         where !bs' = BS.drop offset bs
+{-# INLINEABLE utf8DecodeStringLazy #-}
 
 utf8DecodeString :: Ptr Word8 -> Int -> IO [Char]
 utf8DecodeString ptr len
diff --git a/compiler/utils/FastString.hs b/compiler/utils/FastString.hs
index 21cbfebc3a..32330f2e18 100644
--- a/compiler/utils/FastString.hs
+++ b/compiler/utils/FastString.hs
@@ -482,7 +482,8 @@ nullFS f = BS.null (fs_bs f)
 
 -- | Unpacks and decodes the FastString
 unpackFS :: FastString -> String
-unpackFS (FastString _ _ bs _) = inlinePerformIO $ utf8DecodeStringLazy bs
+unpackFS (FastString _ _ bs _) = utf8DecodeStringLazy bs
+{-# INLINEABLE unpackFS #-}
 
 -- | Gives the UTF-8 encoded bytes corresponding to a 'FastString'
 bytesFS :: FastString -> [Word8]
-- 
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