polish the PGF API and make Expr and Type abstract types. Tree is a type synonym of Expr

src/PGF/CId.hs

@@ -1,16 +1,19 @@
 module PGF.CId (CId(..), 
-                mkCId, readCId, prCId,
-                wildCId, 
-                pCId, pIdent) where
+                mkCId, wildCId,
+                readCId, showCId,
+                
+                -- utils
+                pCId, pIdent, ppCId) where
 
 import Control.Monad
 import qualified Data.ByteString.Char8 as BS
 import Data.Char
 import qualified Text.ParserCombinators.ReadP as RP
+import qualified Text.PrettyPrint as PP
 
 
 -- | An abstract data type that represents
--- function identifier in PGF.
+-- identifiers for functions and categories in PGF.
 newtype CId = CId BS.ByteString deriving (Eq,Ord)
 
 wildCId :: CId
@@ -20,17 +23,18 @@ wildCId = CId (BS.singleton '_')
 mkCId :: String -> CId
 mkCId s = CId (BS.pack s)
 
+-- | Reads an identifier from 'String'. The function returns 'Nothing' if the string is not valid identifier.
 readCId :: String -> Maybe CId
 readCId s = case [x | (x,cs) <- RP.readP_to_S pCId s, all isSpace cs] of
               [x] -> Just x
               _   -> Nothing
 
 -- | Renders the identifier as 'String'
-prCId :: CId -> String
-prCId (CId x) = BS.unpack x
+showCId :: CId -> String
+showCId (CId x) = BS.unpack x
 
 instance Show CId where
-    showsPrec _ = showString . prCId
+    showsPrec _ = showString . showCId
 
 instance Read CId where
     readsPrec _ = RP.readP_to_S pCId
@@ -45,4 +49,7 @@ pIdent :: RP.ReadP String
 pIdent = liftM2 (:) (RP.satisfy isIdentFirst) (RP.munch isIdentRest)
   where
     isIdentFirst c = c == '_' || isLetter c
-    isIdentRest c = c == '_' || c == '\'' || isAlphaNum c
+    isIdentRest c = c == '_' || c == '\'' || isAlphaNum c
+
+ppCId :: CId -> PP.Doc
+ppCId = PP.text . showCId