X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=compiler%2Futils%2FUtil.lhs;h=af811104762c0fb7fc99252ee6755dc2a9945514;hb=893a3c0ad6c39caf71ac28af900733513e1f153e;hp=4058a97f3661227da681fc024654d51271004ce2;hpb=7251944b3932ebcef656efdf201edfd8121aea76;p=ghc-hetmet.git

diff --git a/compiler/utils/Util.lhs b/compiler/utils/Util.lhs
index 4058a97..af81110 100644
--- a/compiler/utils/Util.lhs
+++ b/compiler/utils/Util.lhs
@@ -32,6 +32,7 @@ module Util (
 
         -- * List operations controlled by another list
         takeList, dropList, splitAtList, split,
+        dropTail,
 
         -- * For loop
         nTimes,
@@ -107,6 +108,15 @@ infixr 9 `thenCmp`
 %*                                                                      *
 %************************************************************************
 
+These booleans are global constants, set by CPP flags.  They allow us to
+recompile a single module (this one) to change whether or not debug output
+appears. They sometimes let us avoid even running CPP elsewhere.
+
+It's important that the flags are literal constants (True/False). Then,
+with -0, tests of the flags in other modules will simplify to the correct
+branch of the conditional, thereby dropping debug code altogether when
+the flags are off.
+
 \begin{code}
 ghciSupported :: Bool
 #ifdef GHCI
@@ -599,6 +609,10 @@ splitAtList (_:xs) (y:ys) = (y:ys', ys'')
     where
       (ys', ys'') = splitAtList xs ys
 
+-- drop from the end of a list
+dropTail :: Int -> [a] -> [a]
+dropTail n = reverse . drop n . reverse
+
 snocView :: [a] -> Maybe ([a],a)
         -- Split off the last element
 snocView [] = Nothing