Fixes #5; ternary operator without the second argument

Tested w/ dump-ast-id on:

<?php
echo (2 ? : 4);

src/Common.hs

@@ -1,4 +1,5 @@
-{-# LANGUAGE FlexibleInstances, TypeSynonymInstances #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE TypeSynonymInstances #-}
 
 module Common (
   module Text.ParserCombinators.Parsec,
@@ -9,8 +10,7 @@ import Control.Monad
 import Text.ParserCombinators.Parsec hiding (State, parse, choice)
 import Text.ParserCombinators.Parsec.Expr
 
--- parsec 2 vs 3 stuff
-
+-- | Used w/ buildExpressionParser.
 type Oper a = Operator Char () a
 
 class Parse a where
@@ -22,13 +22,12 @@ class Unparse a where
 instance (Parse a) => Parse [a] where
   parse = many parse
 
+instance (Parse a) => Parse (Maybe a) where
+  parse = Just <$> parse <|> return Nothing
+
 instance (Parse a, Parse b) => Parse (Either a b) where
   parse = Left <$> parse <|> Right <$> parse
 
-instance (Unparse a, Unparse b) => Unparse (Either a b) where
-  unparse (Left a) = unparse a
-  unparse (Right a) = unparse a
-
 instance (Unparse a, Unparse b) => Unparse (a, b) where
   unparse (a, b) = unparse a ++ unparse b
 
@@ -38,3 +37,6 @@ instance (Unparse a) => Unparse [a] where
 instance (Unparse a) => Unparse (Maybe a) where
   unparse = maybe "" unparse
 
+instance (Unparse a, Unparse b) => Unparse (Either a b) where
+  unparse (Left a) = unparse a
+  unparse (Right a) = unparse a

src/Lang/Php/Ast/ExprParse.hs

@@ -489,7 +489,7 @@ preRep, postRep :: Parser (a -> a) -> Parser (a -> a)
 preRep p = (p >>= \ f -> (f .) <$> preRep p) <|> return id
 postRep p = (p >>= \ f -> (. f) <$> postRep p) <|> return id
 
-ial :: [Parser (a -> a -> a)] -> [Oper a]
+ial, ian :: [Parser (a -> a -> a)] -> [Oper a]
 ial = map $ flip Infix AssocLeft
 ian = map $ flip Infix AssocNone
 
@@ -557,7 +557,7 @@ eptOr     = binOp BOr     tokOrP
 eptTernaryIf :: Parser ((Expr, WS) -> (Expr, WS))
 eptTernaryIf = do
   w2 <- tokQMarkP >> parse
-  (e2, w3) <- parse
+  (e2, w3) <- maybe (Nothing, []) (first Just) <$> parse
   w4 <- tokColonP >> parse
   (e3, w5) <- parse
   return $ \ (e1, w1) ->

src/Lang/Php/Ast/ExprTypes.hs

@@ -142,7 +142,7 @@ data OnceOrNot = Once | NotOnce
 data TernaryIf = TernaryIf {
   ternaryIfCond :: Expr,
   ternaryIfWS1  :: WS2,
-  ternaryIfThen :: Expr,
+  ternaryIfThen :: Maybe Expr,
   ternaryIfWS2  :: WS2,
   ternaryIfElse :: Expr}
   deriving (Data, Eq, Generic, Show, Typeable)