Trim unused imports detected by new unused-import code

[ghc-hetmet.git] / compiler / utils / Util.lhs

diff --git a/compiler/utils/Util.lhs b/compiler/utils/Util.lhs
index 4058a97..62ab5f9 100644 (file)
--- 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,
@@ -83,9 +84,8 @@ import System.IO.Unsafe ( unsafePerformIO )
 import Data.IORef       ( readIORef, writeIORef )
 import Data.List        hiding (group)
 
-import qualified Data.List as List ( elem )
 #ifdef DEBUG
-import qualified Data.List as List ( notElem )
+import qualified Data.List as List ( elem, notElem )
 import FastTypes
 #endif
 
@@ -107,6 +107,15 @@ 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 +608,10 @@ 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