diff --git a/ats-format.cabal b/ats-format.cabal
--- a/ats-format.cabal
+++ b/ats-format.cabal
@@ -1,5 +1,5 @@
 name:                ats-format
-version:             0.1.0.17
+version:             0.1.0.18
 synopsis:            A source-code formatter for ATS
 description:         An opinionated source-code formatter for ATS (http://www.ats-lang.org/).
 homepage:            https://hub.darcs.net/vmchale/ats-format#readme
diff --git a/src/Language/ATS/Parser.y b/src/Language/ATS/Parser.y
--- a/src/Language/ATS/Parser.y
+++ b/src/Language/ATS/Parser.y
@@ -448,16 +448,23 @@
         | Records comma identifier eq Type { ($3, $5) : $1 }
         | Records comma identifierSpace eq Type { ($3, $5) : $1 }
 
+IdentifiersIn : identifier { [$1] }
+              | IdentifiersIn comma identifier { $3 : $1 }
+
 -- | Parse a constructor for a sum type
-SumLeaf : vbar identifier { ($2, Nothing) }
-        | vbar identifierSpace of Type { ($2, Just $4) }
+SumLeaf : vbar identifier { ($2, [], Nothing) }
+        | vbar identifierSpace of Type { ($2, [], Just $4) }
+        | vbar IdentifierOr openParen IdentifiersIn closeParen { ($2, $4, Nothing) }
+        | vbar IdentifierOr openParen IdentifiersIn closeParen of Type { ($2, $4, Just $7) }
 
 -- | Parse all constructors of a sum type
 Leaves : SumLeaf { [$1] }
        | Leaves SumLeaf { $2 : $1 }
-       | identifierSpace of Type { [($1, Just $3)] }
-       | identifier { [($1, Nothing)] }
+       | identifierSpace of Type { [($1, [], Just $3)] }
+       | identifier { [($1, [], Nothing)] }
        | dollar {% Left $ Expected $1 "|" "$" }
+       | identifier openParen IdentifiersIn closeParen  { [($1, $3, Nothing)] }
+       | identifier openParen IdentifiersIn closeParen of Type { [($1, $3, Just $6)] }
 
 Universals : { [] }
            | doubleBraces { [] }
diff --git a/src/Language/ATS/PrettyPrint.hs b/src/Language/ATS/PrettyPrint.hs
--- a/src/Language/ATS/PrettyPrint.hs
+++ b/src/Language/ATS/PrettyPrint.hs
@@ -356,12 +356,16 @@
 prettyDL (DataPropLeaf us e Nothing:xs) = prettyDL xs $$ indent 2 ("|" <+> foldMap pretty us <+> pretty e)
 prettyDL (DataPropLeaf us e (Just e'):xs) = prettyDL xs $$ indent 2 ("|" <+> foldMap pretty us <+> pretty e <+> "of" <+> pretty e')
 
-prettyLeaf :: [(String, Maybe Type)] -> Doc
+prettyLeaf :: [(String, [String], Maybe Type)] -> Doc
 prettyLeaf []                = mempty
-prettyLeaf [(s, Nothing)]    = indent 2 ("|" <+> string s)
-prettyLeaf [(s, Just e)]     = indent 2 ("|" <+> string s <+> "of" <+> pretty e)
-prettyLeaf ((s, Nothing):xs) = prettyLeaf xs $$ indent 2 ("|" <+> string s)
-prettyLeaf ((s, Just e):xs)  = prettyLeaf xs $$ indent 2 ("|" <+> string s <+> "of" <+> pretty e)
+prettyLeaf [(s, [], Nothing)]    = indent 2 ("|" <+> string s)
+prettyLeaf [(s, [], Just e)]     = indent 2 ("|" <+> string s <+> "of" <+> pretty e)
+prettyLeaf ((s, [], Nothing):xs) = prettyLeaf xs $$ indent 2 ("|" <+> string s)
+prettyLeaf ((s, [], Just e):xs)  = prettyLeaf xs $$ indent 2 ("|" <+> string s <+> "of" <+> pretty e)
+prettyLeaf [(s, as, Nothing)]    = indent 2 ("|" <+> string s <> prettyArgs as)
+prettyLeaf [(s, as, Just e)]     = indent 2 ("|" <+> string s <> prettyArgs as <+> "of" <+> pretty e)
+prettyLeaf ((s, as, Nothing):xs) = prettyLeaf xs $$ indent 2 ("|" <+> string s <> prettyArgs as)
+prettyLeaf ((s, as, Just e):xs)  = prettyLeaf xs $$ indent 2 ("|" <+> string s <> prettyArgs as <+> "of" <+> pretty e)
 
 prettyHelper :: Doc -> [Doc] -> [Doc]
 prettyHelper _ [x]    = [x]
diff --git a/src/Language/ATS/Types.hs b/src/Language/ATS/Types.hs
--- a/src/Language/ATS/Types.hs
+++ b/src/Language/ATS/Types.hs
@@ -69,8 +69,8 @@
                  | RecordViewType String [Arg] [Universal] [(String, Type)]
                  | TypeDef AlexPosn String [Arg] Type
                  | ViewTypeDef AlexPosn String [Arg] Type
-                 | SumType String [Arg] [(String, Maybe Type)] -- TODO [Arg] for here
-                 | SumViewType String [Arg] [(String, Maybe Type)]
+                 | SumType String [Arg] [(String, [String], Maybe Type)]
+                 | SumViewType String [Arg] [(String, [String], Maybe Type)]
                  | AbsType AlexPosn String [Arg] (Maybe Type)
                  | AbsViewType AlexPosn String [Arg] (Maybe Type)
                  | AbsView AlexPosn String [Arg] (Maybe Type)