diff --git a/CompState.hs b/CompState.hs
index 02fcac9..1dd6ec8 100644
--- a/CompState.hs
+++ b/CompState.hs
@@ -44,6 +44,11 @@ data CompFrontend = FrontendOccam | FrontendRain
 
 -- | State necessary for compilation.
 data CompState = CompState {
+    -- This structure needs to be printable with pshow.
+    -- There are explicit rules for the Maps and Sets used here
+    -- in PrettyShow.hs; if you add any new ones here then remember
+    -- to add matching rules there.
+
     -- Set by Main (from command-line options)
     csMode :: CompMode,
     csBackend :: CompBackend,
diff --git a/Pass.hs b/Pass.hs
index c35079d..64aa5f2 100644
--- a/Pass.hs
+++ b/Pass.hs
@@ -89,7 +89,7 @@ debugAST p
           veryDebug $ "}}}"
           veryDebug $ "{{{ State"
           ps <- get
-          veryDebug $ show ps
+          veryDebug $ pshow ps
           veryDebug $ "}}}"
 
 -- | Number lines in a piece of text.
diff --git a/PrettyShow.hs b/PrettyShow.hs
index e8abfc2..87697c1 100644
--- a/PrettyShow.hs
+++ b/PrettyShow.hs
@@ -24,6 +24,7 @@ module PrettyShow (pshow) where
 
 import Data.Generics
 import qualified Data.Map as Map
+import qualified Data.Set as Set
 import Text.PrettyPrint.HughesPJ
 
 import qualified AST as A
@@ -66,6 +67,9 @@ doMap :: (Data a, Data b) => Map.Map a b -> Doc
 doMap map = braces $ sep $ punctuate (text ",") [doAny k <+> text ":" <+> doAny v
                                                  | (k, v) <- Map.toAscList map]
 
+doSet :: Data a => Set.Set a -> Doc
+doSet set = brackets $ sep $ punctuate (text ",") (map doAny $ Set.toList set)
+
 foldPatternList :: Pattern -> [Pattern]
 foldPatternList (Match con patList)
   = if (showConstr con == "(:)")
@@ -123,6 +127,8 @@ doAny = doGeneral `ext1Q` doList `extQ` doString `extQ` doMeta `extQ` doPattern
           `extQ` (doMap :: Map.Map String A.NameDef -> Doc)
           `extQ` (doMap :: Map.Map String [A.Type] -> Doc)
           `extQ` (doMap :: Map.Map String [A.Actual] -> Doc)
+          `extQ` (doSet :: Set.Set String -> Doc)
+          `extQ` (doSet :: Set.Set A.Name -> Doc)
 
 -- | Convert an arbitrary data structure to a string in a reasonably pretty way.
 -- This is currently rather slow.