[project @ 1998-12-02 13:17:09 by simonm]

[ghc-hetmet.git] / ghc / lib / std / PrelList.lhs

diff --git a/ghc/lib/std/PrelList.lhs b/ghc/lib/std/PrelList.lhs
index 4f76427..e3c83ba 100644 (file)
--- a/ghc/lib/std/PrelList.lhs
+++ b/ghc/lib/std/PrelList.lhs
@@ -46,19 +46,35 @@ head                    :: [a] -> a
 head (x:_)              =  x
 head []                 =  errorEmptyList "head"
 
+tail                    :: [a] -> [a]
+tail (_:xs)             =  xs
+tail []                 =  errorEmptyList "tail"
+
 last                    :: [a] -> a
+#ifdef USE_REPORT_PRELUDE
 last [x]                =  x
 last (_:xs)             =  last xs
 last []                 =  errorEmptyList "last"
-
-tail                    :: [a] -> [a]
-tail (_:xs)             =  xs
-tail []                 =  errorEmptyList "tail"
+#else
+-- eliminate repeated cases
+last []                =  errorEmptyList "last"
+last (x:xs)            =  last' x xs
+  where last' x []     = x
+       last' _ (x:xs) = last' x xs
+#endif
 
 init                    :: [a] -> [a]
+#ifdef USE_REPORT_PRELUDE
 init [x]                =  []
 init (x:xs)             =  x : init xs
 init []                 =  errorEmptyList "init"
+#else
+-- eliminate repeated cases
+init []                 =  errorEmptyList "init"
+init (x:xs)             =  init' x xs
+  where init' x []     = []
+       init' x (y:xs) = x : init' y xs
+#endif
 
 null                    :: [a] -> Bool
 null []                 =  True
@@ -408,7 +424,6 @@ unlines                     =  concatMap (++ "\n")
 -- here's a more efficient version
 unlines [] = []
 unlines (l:ls) = l ++ '\n' : unlines ls
-
 #endif
 
 unwords                        :: [String] -> String