diff --git a/common/Errors.hs b/common/Errors.hs
index 3a0e735..008fafb 100644
--- a/common/Errors.hs
+++ b/common/Errors.hs
@@ -23,6 +23,7 @@ module Errors (checkJust, Die(..),
 
 import Control.Monad.Error
 import Control.Monad.Trans
+import Data.Generics
 import Data.List
 import System.IO
 import System.IO.Error
@@ -49,7 +50,7 @@ data WarningType
   | WarnUnknownPreprocessorDirective
   | WarnUninitialisedVariable
   | WarnUnusedVariable
-  deriving (Eq, Show, Ord, Read, Enum, Bounded)
+  deriving (Eq, Show, Ord, Read, Enum, Bounded, Typeable, Data)
 -- I intend the above warnings to be part of a command-line mechanism to enable
 -- or suppress them according to various flags.  So that you might write:
 -- -WnoWarnParserOddity
diff --git a/data/CompState.hs b/data/CompState.hs
index 243002e..3257e89 100644
--- a/data/CompState.hs
+++ b/data/CompState.hs
@@ -31,7 +31,7 @@ import Data.Set (Set)
 import qualified Data.Set as Set
 
 import qualified AST as A
-import Errors (Die, dieP, ErrorReport, Warn, WarningType, warnP)
+import Errors (Die, dieP, ErrorReport, Warn, WarningType(..), warnP)
 import Metadata
 import OrdAST ()
 import UnifyType
@@ -101,6 +101,7 @@ data CompState = CompState {
     csVerboseLevel :: Int,
     csOutputFile :: String,
     csKeepTemporaries :: Bool,
+    csEnabledWarnings :: Set WarningType,
 
     -- Set by preprocessor
     csCurrentFile :: String,
@@ -138,6 +139,12 @@ emptyState = CompState {
     csVerboseLevel = 0,
     csOutputFile = "-",
     csKeepTemporaries = False,
+    csEnabledWarnings = Set.fromList
+      [ WarnInternal
+      , WarnParserOddity
+      , WarnUnknownPreprocessorDirective
+      , WarnUnusedVariable],
+-- TODO enable WarnUninitialisedVariable by default
 
     csCurrentFile = "none",
     csUsedFiles = Set.empty,