From: naur@post11.tele.dk <unknown>
Date: Sat, 7 Feb 2009 18:14:27 +0000 (+0000)
Subject: Rules to make genericLength strict for Int/Integer lengths, see #2962
X-Git-Tag: 2009-06-25~56
X-Git-Url: http://git.megacz.com/?a=commitdiff_plain;h=349544274787b6d587e85e75e6c754a618b813aa;p=ghc-base.git

Rules to make genericLength strict for Int/Integer lengths, see #2962
---

diff --git a/Data/List.hs b/Data/List.hs
index 16cbeb8..bd4e14c 100644
--- a/Data/List.hs
+++ b/Data/List.hs
@@ -568,6 +568,17 @@ genericLength           :: (Num i) => [b] -> i
 genericLength []        =  0
 genericLength (_:l)     =  1 + genericLength l
 
+{-# RULES
+  "genericLengthInt"     genericLength = (strictGenericLength :: [a] -> Int);
+  "genericLengthInteger" genericLength = (strictGenericLength :: [a] -> Integer);
+ #-}
+
+strictGenericLength     :: (Num i) => [b] -> i
+strictGenericLength l   =  gl l 0
+                        where
+                           gl [] a     = a
+                           gl (_:xs) a = let a' = a + 1 in a' `seq` gl xs a'
+
 -- | The 'genericTake' function is an overloaded version of 'take', which
 -- accepts any 'Integral' value as the number of elements to take.
 genericTake             :: (Integral i) => i -> [a] -> [a]