X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Futils%2FUtil.lhs;fp=compiler%2Futils%2FUtil.lhs;h=db6f96a206d92315186ca3f777268bcf5edb83fe;hp=4058a97f3661227da681fc024654d51271004ce2;hb=af37b3001b086f39cbf1fe3ea2aa5c37f4f9a34f;hpb=db9c51c958c211ddd5056b5a31be0d72f40bde97

diff --git a/compiler/utils/Util.lhs b/compiler/utils/Util.lhs
index 4058a97..db6f96a 100644
--- a/compiler/utils/Util.lhs
+++ b/compiler/utils/Util.lhs
@@ -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