diff --git a/src/PGF.hs b/src/PGF.hs
index 19b3d2f8a..113cc08b8 100644
--- a/src/PGF.hs
+++ b/src/PGF.hs
@@ -69,8 +69,6 @@ import PGF.TypeCheck
 import PGF.Paraphrase
 import PGF.Macros
 import PGF.Data
-import PGF.Expr
-import PGF.Type
 import PGF.Raw.Convert
 import PGF.Raw.Parse
 import PGF.Raw.Print (printTree)
diff --git a/src/PGF/Data.hs b/src/PGF/Data.hs
index 644e33750..8ee95c579 100644
--- a/src/PGF/Data.hs
+++ b/src/PGF/Data.hs
@@ -1,6 +1,8 @@
-module PGF.Data where
+module PGF.Data (module PGF.Data, module PGF.Expr, module PGF.Type) where
 
 import PGF.CId
+import PGF.Expr hiding (Value, Env)
+import PGF.Type
 import GF.Text.UTF8
 
 import qualified Data.Map as Map
@@ -40,42 +42,6 @@ data Concr = Concr {
   parser  :: Maybe ParserInfo       -- parser
   }
 
-data Type =
-   DTyp [Hypo] CId [Expr]
-  deriving (Eq,Ord,Show)
-
-data Literal = 
-   LStr String                      -- ^ string constant
- | LInt Integer                     -- ^ integer constant
- | LFlt Double                      -- ^ floating point constant
- deriving (Eq,Ord,Show)
-
--- | The tree is an evaluated expression in the abstract syntax
--- of the grammar. The type is especially restricted to not
--- allow unapplied lambda abstractions. The tree is used directly 
--- from the linearizer and is produced directly from the parser.
-data Tree = 
-   Abs [CId] Tree                   -- ^ lambda abstraction. The list of variables is non-empty
- | Var CId                          -- ^ variable
- | Fun CId [Tree]                   -- ^ function application
- | Lit Literal                      -- ^ literal
- | Meta Int                         -- ^ meta variable
-  deriving (Show, Eq, Ord)
-
--- | An expression represents a potentially unevaluated expression
--- in the abstract syntax of the grammar. It can be evaluated with
--- the 'expr2tree' function and then linearized or it can be used
--- directly in the dependent types.
-data Expr =
-   EAbs CId Expr                    -- ^ lambda abstraction
- | EApp Expr Expr                   -- ^ application
- | ELit Literal                     -- ^ literal
- | EMeta  Int                       -- ^ meta variable
- | EVar   CId                       -- ^ variable or function reference
- | EEq [Equation]                   -- ^ lambda function defined as a set of equations with pattern matching
- | EPi CId Expr Expr                -- ^ dependent function type
-  deriving (Eq,Ord,Show)
-
 data Term =
    R [Term]
  | P Term Term
@@ -98,18 +64,6 @@ data Alternative =
    Alt [String] [String]
   deriving (Eq,Ord,Show)
 
-data Hypo =
-   Hyp CId Type
-  deriving (Eq,Ord,Show)
-
--- | The equation is used to define lambda function as a sequence
--- of equations with pattern matching. The list of 'Expr' represents
--- the patterns and the second 'Expr' is the function body for this
--- equation.
-data Equation =
-   Equ [Expr] Expr
-  deriving (Eq,Ord,Show)
-
 
 type FCat      = Int
 type FIndex    = Int
diff --git a/src/PGF/Expr.hs b/src/PGF/Expr.hs
index 454989728..0dde19310 100644
--- a/src/PGF/Expr.hs
+++ b/src/PGF/Expr.hs
@@ -1,4 +1,7 @@
-module PGF.Expr(readTree, showTree, pTree, ppTree,
+module PGF.Expr(Tree(..), Literal(..),                
+                readTree, showTree, pTree, ppTree,
+
+                Expr(..), Equation(..),
                 readExpr, showExpr, pExpr, ppExpr,
 
                 tree2expr, expr2tree,
@@ -11,7 +14,6 @@ module PGF.Expr(readTree, showTree, pTree, ppTree,
                ) where
 
 import PGF.CId
-import PGF.Data
 
 import Data.Char
 import Data.Maybe
@@ -20,6 +22,45 @@ import qualified Text.PrettyPrint as PP
 import qualified Text.ParserCombinators.ReadP as RP
 import qualified Data.Map as Map
 
+data Literal = 
+   LStr String                      -- ^ string constant
+ | LInt Integer                     -- ^ integer constant
+ | LFlt Double                      -- ^ floating point constant
+ deriving (Eq,Ord,Show)
+
+-- | The tree is an evaluated expression in the abstract syntax
+-- of the grammar. The type is especially restricted to not
+-- allow unapplied lambda abstractions. The tree is used directly 
+-- from the linearizer and is produced directly from the parser.
+data Tree = 
+   Abs [CId] Tree                   -- ^ lambda abstraction. The list of variables is non-empty
+ | Var CId                          -- ^ variable
+ | Fun CId [Tree]                   -- ^ function application
+ | Lit Literal                      -- ^ literal
+ | Meta Int                         -- ^ meta variable
+  deriving (Show, Eq, Ord)
+
+-- | An expression represents a potentially unevaluated expression
+-- in the abstract syntax of the grammar. It can be evaluated with
+-- the 'expr2tree' function and then linearized or it can be used
+-- directly in the dependent types.
+data Expr =
+   EAbs CId Expr                    -- ^ lambda abstraction
+ | EApp Expr Expr                   -- ^ application
+ | ELit Literal                     -- ^ literal
+ | EMeta  Int                       -- ^ meta variable
+ | EVar   CId                       -- ^ variable or function reference
+ | EEq [Equation]                   -- ^ lambda function defined as a set of equations with pattern matching
+ | EPi CId Expr Expr                -- ^ dependent function type
+  deriving (Eq,Ord,Show)
+
+-- | The equation is used to define lambda function as a sequence
+-- of equations with pattern matching. The list of 'Expr' represents
+-- the patterns and the second 'Expr' is the function body for this
+-- equation.
+data Equation =
+   Equ [Expr] Expr
+  deriving (Eq,Ord,Show)
 
 -- | parses 'String' as an expression
 readTree :: String -> Maybe Tree
diff --git a/src/PGF/Type.hs b/src/PGF/Type.hs
index cfe0bbe72..9ec5b3022 100644
--- a/src/PGF/Type.hs
+++ b/src/PGF/Type.hs
@@ -1,7 +1,8 @@
-module PGF.Type ( readType, showType, pType, ppType ) where
+module PGF.Type ( Type(..), Hypo(..),
+                  readType, showType,
+                  pType, ppType ) where
 
 import PGF.CId
-import PGF.Data
 import PGF.Expr
 import Data.Char
 import qualified Text.PrettyPrint as PP
@@ -9,16 +10,32 @@ import qualified Text.ParserCombinators.ReadP as RP
 import Control.Monad
 import Debug.Trace
 
--- | parses 'String' as an expression
+-- | To read a type from a 'String', use 'read' or 'readType'.
+data Type =
+   DTyp [Hypo] CId [Expr]
+  deriving (Eq,Ord)
+
+data Hypo =
+   Hyp CId Type
+  deriving (Eq,Ord,Show)
+
+-- | Reads a 'Type' from a 'String'.
 readType :: String -> Maybe Type
 readType s = case [x | (x,cs) <- RP.readP_to_S pType s, all isSpace cs] of
                [x] -> Just x
                _   -> Nothing
 
+instance Show Type where
+    showsPrec i x = showString (PP.render (ppType i x))
+
+instance Read Type where
+    readsPrec _ = RP.readP_to_S pType
+
 -- | renders type as 'String'
 showType :: Type -> String
 showType = PP.render . ppType 0
 
+pType :: RP.ReadP Type
 pType = do
   RP.skipSpaces
   hyps <- RP.sepBy (pHypo >>= \h -> RP.string "->" >> return h) RP.skipSpaces
@@ -45,7 +62,7 @@ pType = do
       args <- RP.sepBy pFactor RP.skipSpaces
       return (mkCId cat, args)
 
-
+ppType :: Int -> Type -> PP.Doc
 ppType d (DTyp ctxt cat args)
   | null ctxt = ppRes cat args
   | otherwise = ppParens (d > 0) (foldr ppCtxt (ppRes cat args) ctxt)
@@ -56,5 +73,6 @@ ppType d (DTyp ctxt cat args)
 
     ppRes cat es = PP.text (prCId cat) PP.<+> PP.hsep (map (ppExpr 2) es)
 
+ppParens :: Bool -> PP.Doc -> PP.Doc
 ppParens True  = PP.parens
 ppParens False = id
diff --git a/src/server/MainFastCGI.hs b/src/server/MainFastCGI.hs
index 38748fcc4..69f5fb419 100644
--- a/src/server/MainFastCGI.hs
+++ b/src/server/MainFastCGI.hs
@@ -54,6 +54,7 @@ pgfMain pgf command =
     getCat = 
        do mcat  <- getInput "cat"
           case mcat of
+            Nothing -> return Nothing
             Just "" -> return Nothing
             Just cat | cat `notElem` PGF.categories pgf ->
                          throwCGIError 400 "Unknown category" ["Unknown category: " ++ cat]