diff --git a/src/Tessera/Env.hs b/src/Tessera/Env.hs
new file mode 100644
index 0000000..c63a311
--- /dev/null
+++ b/src/Tessera/Env.hs
@@ -0,0 +1,76 @@
+module Tessera.Env where
+
+import Data.List (foldl')
+import qualified Data.Map.Strict as Map
+
+import Tessera.Syntax
+
+data TypeDef = TypeDef
+  { typeDefinitionParams :: [String]
+  , typeDefinitionConstructors :: [Constructor]
+  } deriving (Eq, Show)
+
+data Env = Env
+  { envTypes :: Map.Map String TypeDef
+  , envConstructorTypes :: Map.Map String (String, [Type])
+  } deriving (Eq, Show)
+
+builtinTypes :: Map.Map String TypeDef
+builtinTypes = Map.fromList
+  [ ("Int", TypeDef [] [])
+  , ("Bool", TypeDef [] [Constructor "True" [], Constructor "False" []])
+  ]
+
+builtinConstructors :: Map.Map String (String, [Type])
+builtinConstructors = Map.fromList
+  [ ("True", ("Bool", []))
+  , ("False", ("Bool", []))
+  ]
+
+buildEnv :: Program -> Either Diagnostic Env
+buildEnv program = do
+  types <- foldl' addType (Right builtinTypes) [d | d <- program, isData d]
+  let constructors = foldl' addConstructors builtinConstructors [d | d <- program, isData d]
+  return (Env types constructors)
+  where
+    isData (DataDeclaration _ _ _) = True
+    isData _ = False
+
+    addType acc (DataDeclaration name params constructors)
+      | name `elem` ["Int", "Bool"] = Left (Diagnostic (Span 0 0) ("cannot redeclare builtin type " ++ name))
+      | otherwise = do
+          table <- acc
+          case Map.lookup name table of
+            Just _ -> Left (Diagnostic (Span 0 0) ("duplicate type declaration " ++ name))
+            Nothing -> Right (Map.insert name (TypeDef params constructors) table)
+    addType acc _ = acc
+
+    addConstructors table (DataDeclaration name _ constructors) =
+      foldl' (\m c -> Map.insert (constructorName c) (name, constructorArgs c) m) table constructors
+    addConstructors table _ = table
+
+lookupConstructor :: Env -> String -> Maybe (String, [Type])
+lookupConstructor env name = Map.lookup name (envConstructorTypes env)
+
+lookupType :: Env -> String -> Maybe TypeDef
+lookupType env name = Map.lookup name (envTypes env)
+
+constructorArity :: Env -> String -> Maybe Int
+constructorArity env name = length . snd <$> lookupConstructor env name
+
+substitute :: Map.Map String Type -> Type -> Type
+substitute mapping ty = case ty of
+  TVar name -> Map.findWithDefault (TVar name) name mapping
+  TName name arguments -> TName name (map (substitute mapping) arguments)
+  TTuple elements -> TTuple (map (substitute mapping) elements)
+
+instantiate :: [String] -> [Type] -> [Type] -> [Type]
+instantiate params arguments = map (substitute mapping)
+  where mapping = Map.fromList (zip params arguments)
+
+typeEquals :: Type -> Type -> Bool
+typeEquals left right = case (left, right) of
+  (TName a as, TName b bs) -> a == b && length as == length bs && and (zipWith typeEquals as bs)
+  (TVar a, TVar b) -> a == b
+  (TTuple as, TTuple bs) -> length as == length bs && and (zipWith typeEquals as bs)
+  _ -> False