diff --git a/bowtie/src/Bowtie/Surface/Parse.hs b/bowtie/src/Bowtie/Surface/Parse.hs
index fbdfec8..8ef9b37 100644
--- a/bowtie/src/Bowtie/Surface/Parse.hs
+++ b/bowtie/src/Bowtie/Surface/Parse.hs
@@ -33,7 +33,7 @@ parse path =
 
 -- |
 -- >>> parseTest sourceParser "type A = A\n\ntype B = B"
--- AST {astTypes = OrderedMap (fromList [(Id "A",(0,TypeDeclaration [] (fromList [(Id "A",[])]))),(Id "B",(1,TypeDeclaration [] (fromList [(Id "B",[])])))]) (fromList [(0,(Id "A",TypeDeclaration [] (fromList [(Id "A",[])]))),(1,(Id "B",TypeDeclaration [] (fromList [(Id "B",[])])))]), astTerms = OrderedMap (fromList []) (fromList [])}
+-- AST {astTypes = OrderedMap (fromList [(Id "A",(0,TypeDeclaration [] (OrderedMap (fromList [(Id "A",(0,[]))]) (fromList [(0,(Id "A",[]))])))),(Id "B",(1,TypeDeclaration [] (OrderedMap (fromList [(Id "B",(0,[]))]) (fromList [(0,(Id "B",[]))]))))]) (fromList [(0,(Id "A",TypeDeclaration [] (OrderedMap (fromList [(Id "A",(0,[]))]) (fromList [(0,(Id "A",[]))])))),(1,(Id "B",TypeDeclaration [] (OrderedMap (fromList [(Id "B",(0,[]))]) (fromList [(0,(Id "B",[]))]))))]), astTerms = OrderedMap (fromList []) (fromList [])}
 --
 -- ^^ TODO: OrderedMaps do not do well with Show
 sourceParser :: Parser AST
diff --git a/bowtie/src/Bowtie/Type/AST.hs b/bowtie/src/Bowtie/Type/AST.hs
index 839c235..bea97cf 100644
--- a/bowtie/src/Bowtie/Type/AST.hs
+++ b/bowtie/src/Bowtie/Type/AST.hs
@@ -1,6 +1,7 @@
 module Bowtie.Type.AST where
 
 import Bowtie.Lib.FreeVars
+import Bowtie.Lib.OrderedMap (OrderedMap)
 import Bowtie.Lib.Prelude
 
 import qualified Data.Set as Set
@@ -9,7 +10,7 @@ import qualified Data.Set as Set
 data TypeDeclaration
   = TypeDeclaration
       [Id]-- ^ Polymorphic over these variables
-      (HashMap Id [Type])
+      (OrderedMap Id [Type])
   deriving (Eq, Show)
 
 data Type
diff --git a/bowtie/src/Bowtie/Type/Kindcheck.hs b/bowtie/src/Bowtie/Type/Kindcheck.hs
index e533349..1afb956 100644
--- a/bowtie/src/Bowtie/Type/Kindcheck.hs
+++ b/bowtie/src/Bowtie/Type/Kindcheck.hs
@@ -19,7 +19,7 @@ kindcheck =
     -- E.g. if the first is Maybe, the second is Nothing and Just.
     constructorsFromDecls :: (Id, TypeDeclaration) -> [(Id, TypeScheme)]
     constructorsFromDecls (typeId, TypeDeclaration polyVars constructors) =
-      fmap constructorType (HashMap.toList constructors)
+      fmap constructorType (OrderedMap.toList constructors)
       where
         constructorType :: (Id, [Type]) -> (Id, TypeScheme)
         constructorType (conId, args) =
diff --git a/bowtie/src/Bowtie/Type/Parse.hs b/bowtie/src/Bowtie/Type/Parse.hs
index 0ae2b1b..99119ea 100644
--- a/bowtie/src/Bowtie/Type/Parse.hs
+++ b/bowtie/src/Bowtie/Type/Parse.hs
@@ -26,8 +26,8 @@ import Control.Applicative.Combinators.NonEmpty
 import Text.Megaparsec hiding
   (State, Token, parse, parseTest, runParser, sepBy1)
 
+import qualified Bowtie.Lib.OrderedMap as OrderedMap
 import qualified Data.Char as Char
-import qualified Data.HashMap.Strict as HashMap
 import qualified Data.List.NonEmpty as NE
 import qualified Data.Text as Text
 import qualified Text.Megaparsec as Mega
@@ -54,9 +54,9 @@ type ParserErrorBundle = ParseErrorBundle Text Void
 -- )
 --
 -- >>> parseTest declEntryParser "type Foo a b = Bar Bool a | Baz Int b"
--- (Id "Foo",TypeDeclaration [Id "a",Id "b"] (fromList [(Id "Baz",[TConstructor (Id "Int"),TVariable (Id "b")]),(Id "Bar",[TConstructor (Id "Bool"),TVariable (Id "a")])]))
+-- (Id "Foo",TypeDeclaration [Id "a",Id "b"] (OrderedMap (fromList [(Id "Baz",(1,[TConstructor (Id "Int"),TVariable (Id "b")])),(Id "Bar",(0,[TConstructor (Id "Bool"),TVariable (Id "a")]))]) (fromList [(0,(Id "Bar",[TConstructor (Id "Bool"),TVariable (Id "a")])),(1,(Id "Baz",[TConstructor (Id "Int"),TVariable (Id "b")]))])))
 --
--- TODO: switch type constructors to ordered map
+-- ^^ TODO: OrderedMaps do not do well with Show
 declEntryParser :: Parser (Id, TypeDeclaration)
 declEntryParser = do
   -- Note that we don't parse a @Break@ here. If each top level
@@ -67,7 +67,12 @@ declEntryParser = do
   typeArgs <- many (lexeme lowerIdParser)
   symbol "="
   constructors <- constructorParser `sepBy` symbol "|"
-  pure (typeId, TypeDeclaration typeArgs (HashMap.fromList constructors))
+  case OrderedMap.fromList constructors of
+    Left id ->
+      fail ("Duplicate constructors found in type declaration with id: " <> Text.unpack (unId id))
+
+    Right constructorMap ->
+      pure (typeId, TypeDeclaration typeArgs constructorMap)
 
 -- |
 -- >>> parseTest constructorParser "Bar Bool a"