diff --git a/tests/cases/checker/04_custom_types.midas b/tests/cases/checker/04_custom_types.midas
index eee3eb9..6a1a6a2 100644
--- a/tests/cases/checker/04_custom_types.midas
+++ b/tests/cases/checker/04_custom_types.midas
@@ -1,6 +1,6 @@
-type Meter(float)
-type Second(float)
-type MeterPerSecond(float)
+type Meter = float
+type Second = float
+type MeterPerSecond = float
 
 extend Meter {
     op __add__(Meter) -> Meter
diff --git a/tests/cases/midas-parser/01_simple_types.midas b/tests/cases/midas-parser/01_simple_types.midas
index 9432751..a6c493d 100644
--- a/tests/cases/midas-parser/01_simple_types.midas
+++ b/tests/cases/midas-parser/01_simple_types.midas
@@ -1,15 +1,15 @@
 // Simple custom type derived from float
-type Custom(float)
+type Custom = float
 
 // Simple custom types with constraints
-type Latitude(float) where (-90 <= _ <= 90)
-type Longitude(float) where (-180 <= _ <= 180)
+type Latitude = float where (-90 <= _ <= 90)
+type Longitude = float where (-180 <= _ <= 180)
 
 // Generic custom type (a Difference of T is derived from T, e.g. a difference of floats is a float
-type Difference[T](T)
+type Difference[T] = T
 
 // Complex custom type, containing two values accessible through properties
-type GeoLocation {
+type GeoLocation = {
     lat: Latitude
     lon: Longitude
 }
@@ -24,7 +24,7 @@ extend GeoLocation {
 
 // For complex generics, you need to specify how the genericity the properties
 // are handled
-type Difference[GeoLocation] {
+type Difference[GeoLocation] = {
     lat: Difference[Latitude]
     lon: Difference[Longitude]
 }
@@ -44,11 +44,11 @@ predicate StrictlyPositive(v: float) = v > 0
 predicate Equatorial(loc: GeoLocation) = (-10 <= loc.lat <= 10)
 predicate Arctic(loc: GeoLocation) = (loc.lat >= 66)
 
-type Person {
+type Person = {
     name: str
 
     // Property with an inline constraint
-    age: int? where (0 <= _ < 150)
+    age: None | (int where (0 <= _ < 150))
 
     // Property referencing a predicate
     height: float where StrictlyPositive
diff --git a/tests/cases/midas-parser/01_simple_types.midas.ref.json b/tests/cases/midas-parser/01_simple_types.midas.ref.json
index 355b80e..5d70b51 100644
--- a/tests/cases/midas-parser/01_simple_types.midas.ref.json
+++ b/tests/cases/midas-parser/01_simple_types.midas.ref.json
@@ -31,28 +31,34 @@
       "column": 6
     },
     {
-      "type": "LEFT_PAREN",
-      "lexeme": "(",
+      "type": "WHITESPACE",
+      "lexeme": " ",
       "line": 2,
       "column": 12
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 2,
+      "column": 13
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 2,
+      "column": 14
+    },
     {
       "type": "IDENTIFIER",
       "lexeme": "float",
       "line": 2,
-      "column": 13
-    },
-    {
-      "type": "RIGHT_PAREN",
-      "lexeme": ")",
-      "line": 2,
-      "column": 18
+      "column": 15
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 2,
-      "column": 19
+      "column": 20
     },
     {
       "type": "NEWLINE",
@@ -91,118 +97,124 @@
       "column": 6
     },
     {
-      "type": "LEFT_PAREN",
-      "lexeme": "(",
+      "type": "WHITESPACE",
+      "lexeme": " ",
       "line": 5,
       "column": 14
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 5,
+      "column": 15
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 5,
+      "column": 16
+    },
     {
       "type": "IDENTIFIER",
       "lexeme": "float",
       "line": 5,
-      "column": 15
-    },
-    {
-      "type": "RIGHT_PAREN",
-      "lexeme": ")",
-      "line": 5,
-      "column": 20
+      "column": 17
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 21
+      "column": 22
     },
     {
       "type": "WHERE",
       "lexeme": "where",
       "line": 5,
-      "column": 22
+      "column": 23
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 27
+      "column": 28
     },
     {
       "type": "LEFT_PAREN",
       "lexeme": "(",
       "line": 5,
-      "column": 28
+      "column": 29
     },
     {
       "type": "MINUS",
       "lexeme": "-",
       "line": 5,
-      "column": 29
+      "column": 30
     },
     {
       "type": "NUMBER",
       "lexeme": "90",
       "line": 5,
-      "column": 30
+      "column": 31
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 32
+      "column": 33
     },
     {
       "type": "LESS_EQUAL",
       "lexeme": "<=",
       "line": 5,
-      "column": 33
+      "column": 34
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 35
+      "column": 36
     },
     {
       "type": "UNDERSCORE",
       "lexeme": "_",
       "line": 5,
-      "column": 36
+      "column": 37
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 37
+      "column": 38
     },
     {
       "type": "LESS_EQUAL",
       "lexeme": "<=",
       "line": 5,
-      "column": 38
+      "column": 39
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 5,
-      "column": 40
+      "column": 41
     },
     {
       "type": "NUMBER",
       "lexeme": "90",
       "line": 5,
-      "column": 41
+      "column": 42
     },
     {
       "type": "RIGHT_PAREN",
       "lexeme": ")",
       "line": 5,
-      "column": 43
+      "column": 44
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 5,
-      "column": 44
+      "column": 45
     },
     {
       "type": "TYPE",
@@ -223,118 +235,124 @@
       "column": 6
     },
     {
-      "type": "LEFT_PAREN",
-      "lexeme": "(",
+      "type": "WHITESPACE",
+      "lexeme": " ",
       "line": 6,
       "column": 15
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 6,
+      "column": 16
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 6,
+      "column": 17
+    },
     {
       "type": "IDENTIFIER",
       "lexeme": "float",
       "line": 6,
-      "column": 16
-    },
-    {
-      "type": "RIGHT_PAREN",
-      "lexeme": ")",
-      "line": 6,
-      "column": 21
+      "column": 18
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 22
+      "column": 23
     },
     {
       "type": "WHERE",
       "lexeme": "where",
       "line": 6,
-      "column": 23
+      "column": 24
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 28
+      "column": 29
     },
     {
       "type": "LEFT_PAREN",
       "lexeme": "(",
       "line": 6,
-      "column": 29
+      "column": 30
     },
     {
       "type": "MINUS",
       "lexeme": "-",
       "line": 6,
-      "column": 30
+      "column": 31
     },
     {
       "type": "NUMBER",
       "lexeme": "180",
       "line": 6,
-      "column": 31
+      "column": 32
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 34
+      "column": 35
     },
     {
       "type": "LESS_EQUAL",
       "lexeme": "<=",
       "line": 6,
-      "column": 35
+      "column": 36
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 37
+      "column": 38
     },
     {
       "type": "UNDERSCORE",
       "lexeme": "_",
       "line": 6,
-      "column": 38
+      "column": 39
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 39
+      "column": 40
     },
     {
       "type": "LESS_EQUAL",
       "lexeme": "<=",
       "line": 6,
-      "column": 40
+      "column": 41
     },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 6,
-      "column": 42
+      "column": 43
     },
     {
       "type": "NUMBER",
       "lexeme": "180",
       "line": 6,
-      "column": 43
+      "column": 44
     },
     {
       "type": "RIGHT_PAREN",
       "lexeme": ")",
       "line": 6,
-      "column": 46
+      "column": 47
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 6,
-      "column": 47
+      "column": 48
     },
     {
       "type": "NEWLINE",
@@ -391,28 +409,34 @@
       "column": 18
     },
     {
-      "type": "LEFT_PAREN",
-      "lexeme": "(",
+      "type": "WHITESPACE",
+      "lexeme": " ",
       "line": 9,
       "column": 19
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 9,
+      "column": 20
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 9,
+      "column": 21
+    },
     {
       "type": "IDENTIFIER",
       "lexeme": "T",
       "line": 9,
-      "column": 20
-    },
-    {
-      "type": "RIGHT_PAREN",
-      "lexeme": ")",
-      "line": 9,
-      "column": 21
+      "column": 22
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 9,
-      "column": 22
+      "column": 23
     },
     {
       "type": "NEWLINE",
@@ -456,17 +480,29 @@
       "line": 12,
       "column": 17
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 12,
+      "column": 18
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 12,
+      "column": 19
+    },
     {
       "type": "LEFT_BRACE",
       "lexeme": "{",
       "line": 12,
-      "column": 18
+      "column": 20
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 12,
-      "column": 19
+      "column": 21
     },
     {
       "type": "WHITESPACE",
@@ -834,17 +870,29 @@
       "line": 27,
       "column": 29
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 27,
+      "column": 30
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 27,
+      "column": 31
+    },
     {
       "type": "LEFT_BRACE",
       "lexeme": "{",
       "line": 27,
-      "column": 30
+      "column": 32
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 27,
-      "column": 31
+      "column": 33
     },
     {
       "type": "WHITESPACE",
@@ -1824,17 +1872,29 @@
       "line": 47,
       "column": 12
     },
+    {
+      "type": "EQUAL",
+      "lexeme": "=",
+      "line": 47,
+      "column": 13
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 47,
+      "column": 14
+    },
     {
       "type": "LEFT_BRACE",
       "lexeme": "{",
       "line": 47,
-      "column": 13
+      "column": 15
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 47,
-      "column": 14
+      "column": 16
     },
     {
       "type": "WHITESPACE",
@@ -1922,16 +1982,10 @@
     },
     {
       "type": "IDENTIFIER",
-      "lexeme": "int",
+      "lexeme": "None",
       "line": 51,
       "column": 10
     },
-    {
-      "type": "QMARK",
-      "lexeme": "?",
-      "line": 51,
-      "column": 13
-    },
     {
       "type": "WHITESPACE",
       "lexeme": " ",
@@ -1939,8 +1993,8 @@
       "column": 14
     },
     {
-      "type": "WHERE",
-      "lexeme": "where",
+      "type": "PIPE",
+      "lexeme": "|",
       "line": 51,
       "column": 15
     },
@@ -1948,17 +2002,29 @@
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 51,
-      "column": 20
+      "column": 16
     },
     {
       "type": "LEFT_PAREN",
       "lexeme": "(",
       "line": 51,
+      "column": 17
+    },
+    {
+      "type": "IDENTIFIER",
+      "lexeme": "int",
+      "line": 51,
+      "column": 18
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 51,
       "column": 21
     },
     {
-      "type": "NUMBER",
-      "lexeme": "0",
+      "type": "WHERE",
+      "lexeme": "where",
       "line": 51,
       "column": 22
     },
@@ -1966,35 +2032,17 @@
       "type": "WHITESPACE",
       "lexeme": " ",
       "line": 51,
-      "column": 23
-    },
-    {
-      "type": "LESS_EQUAL",
-      "lexeme": "<=",
-      "line": 51,
-      "column": 24
-    },
-    {
-      "type": "WHITESPACE",
-      "lexeme": " ",
-      "line": 51,
-      "column": 26
-    },
-    {
-      "type": "UNDERSCORE",
-      "lexeme": "_",
-      "line": 51,
       "column": 27
     },
     {
-      "type": "WHITESPACE",
-      "lexeme": " ",
+      "type": "LEFT_PAREN",
+      "lexeme": "(",
       "line": 51,
       "column": 28
     },
     {
-      "type": "LESS",
-      "lexeme": "<",
+      "type": "NUMBER",
+      "lexeme": "0",
       "line": 51,
       "column": 29
     },
@@ -2004,23 +2052,65 @@
       "line": 51,
       "column": 30
     },
+    {
+      "type": "LESS_EQUAL",
+      "lexeme": "<=",
+      "line": 51,
+      "column": 31
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 51,
+      "column": 33
+    },
+    {
+      "type": "UNDERSCORE",
+      "lexeme": "_",
+      "line": 51,
+      "column": 34
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 51,
+      "column": 35
+    },
+    {
+      "type": "LESS",
+      "lexeme": "<",
+      "line": 51,
+      "column": 36
+    },
+    {
+      "type": "WHITESPACE",
+      "lexeme": " ",
+      "line": 51,
+      "column": 37
+    },
     {
       "type": "NUMBER",
       "lexeme": "150",
       "line": 51,
-      "column": 31
+      "column": 38
     },
     {
       "type": "RIGHT_PAREN",
       "lexeme": ")",
       "line": 51,
-      "column": 34
+      "column": 41
+    },
+    {
+      "type": "RIGHT_PAREN",
+      "lexeme": ")",
+      "line": 51,
+      "column": 42
     },
     {
       "type": "NEWLINE",
       "lexeme": "\n",
       "line": 51,
-      "column": 35
+      "column": 43
     },
     {
       "type": "NEWLINE",
@@ -2169,259 +2259,235 @@
   ],
   "stmts": [
     {
-      "_type": "SimpleTypeStmt",
+      "_type": "TypeStmt",
       "name": "Custom",
-      "template": null,
-      "base": {
-        "_type": "TypeExpr",
-        "name": "float",
-        "template": null,
-        "optional": false
-      },
-      "constraint": null
+      "params": [],
+      "type": {
+        "_type": "NamedType",
+        "name": "float"
+      }
     },
     {
-      "_type": "SimpleTypeStmt",
+      "_type": "TypeStmt",
       "name": "Latitude",
-      "template": null,
-      "base": {
-        "_type": "TypeExpr",
-        "name": "float",
-        "template": null,
-        "optional": false
-      },
-      "constraint": {
-        "_type": "GroupingExpr",
-        "expr": {
-          "_type": "BinaryExpr",
-          "left": {
+      "params": [],
+      "type": {
+        "_type": "ConstraintType",
+        "type": {
+          "_type": "NamedType",
+          "name": "float"
+        },
+        "constraint": {
+          "_type": "GroupingExpr",
+          "expr": {
             "_type": "BinaryExpr",
             "left": {
-              "_type": "UnaryExpr",
-              "operator": "-",
+              "_type": "BinaryExpr",
+              "left": {
+                "_type": "UnaryExpr",
+                "operator": "-",
+                "right": {
+                  "_type": "LiteralExpr",
+                  "value": 90.0
+                }
+              },
+              "operator": "<=",
               "right": {
-                "_type": "LiteralExpr",
-                "value": 90.0
+                "_type": "WildcardExpr"
               }
             },
             "operator": "<=",
             "right": {
-              "_type": "WildcardExpr"
+              "_type": "LiteralExpr",
+              "value": 90.0
             }
-          },
-          "operator": "<=",
-          "right": {
-            "_type": "LiteralExpr",
-            "value": 90.0
           }
         }
       }
     },
     {
-      "_type": "SimpleTypeStmt",
+      "_type": "TypeStmt",
       "name": "Longitude",
-      "template": null,
-      "base": {
-        "_type": "TypeExpr",
-        "name": "float",
-        "template": null,
-        "optional": false
-      },
-      "constraint": {
-        "_type": "GroupingExpr",
-        "expr": {
-          "_type": "BinaryExpr",
-          "left": {
+      "params": [],
+      "type": {
+        "_type": "ConstraintType",
+        "type": {
+          "_type": "NamedType",
+          "name": "float"
+        },
+        "constraint": {
+          "_type": "GroupingExpr",
+          "expr": {
             "_type": "BinaryExpr",
             "left": {
-              "_type": "UnaryExpr",
-              "operator": "-",
+              "_type": "BinaryExpr",
+              "left": {
+                "_type": "UnaryExpr",
+                "operator": "-",
+                "right": {
+                  "_type": "LiteralExpr",
+                  "value": 180.0
+                }
+              },
+              "operator": "<=",
               "right": {
-                "_type": "LiteralExpr",
-                "value": 180.0
+                "_type": "WildcardExpr"
               }
             },
             "operator": "<=",
             "right": {
-              "_type": "WildcardExpr"
+              "_type": "LiteralExpr",
+              "value": 180.0
             }
-          },
-          "operator": "<=",
-          "right": {
-            "_type": "LiteralExpr",
-            "value": 180.0
           }
         }
       }
     },
     {
-      "_type": "SimpleTypeStmt",
+      "_type": "TypeStmt",
       "name": "Difference",
-      "template": {
-        "_type": "TemplateExpr",
-        "type": {
-          "_type": "TypeExpr",
+      "params": [
+        {
           "name": "T",
-          "template": null,
-          "optional": false
+          "bound": null
         }
-      },
-      "base": {
-        "_type": "TypeExpr",
-        "name": "T",
-        "template": null,
-        "optional": false
-      },
-      "constraint": null
+      ],
+      "type": {
+        "_type": "NamedType",
+        "name": "T"
+      }
     },
     {
-      "_type": "ComplexTypeStmt",
+      "_type": "TypeStmt",
       "name": "GeoLocation",
-      "template": null,
-      "properties": [
-        {
-          "_type": "PropertyStmt",
-          "name": "lat",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "Latitude",
-            "template": null,
-            "optional": false
+      "params": [],
+      "type": {
+        "_type": "ComplexType",
+        "properties": [
+          {
+            "_type": "PropertyStmt",
+            "name": "lat",
+            "type": {
+              "_type": "NamedType",
+              "name": "Latitude"
+            }
           },
-          "constraint": null
-        },
-        {
-          "_type": "PropertyStmt",
-          "name": "lon",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "Longitude",
-            "template": null,
-            "optional": false
-          },
-          "constraint": null
-        }
-      ]
+          {
+            "_type": "PropertyStmt",
+            "name": "lon",
+            "type": {
+              "_type": "NamedType",
+              "name": "Longitude"
+            }
+          }
+        ]
+      }
     },
     {
       "_type": "ExtendStmt",
       "type": {
-        "_type": "TypeExpr",
-        "name": "GeoLocation",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "GeoLocation"
       },
       "operations": [
         {
           "_type": "OpStmt",
           "name": "__sub__",
           "operand": {
-            "_type": "TypeExpr",
-            "name": "GeoLocation",
-            "template": null,
-            "optional": false
+            "_type": "NamedType",
+            "name": "GeoLocation"
           },
           "result": {
-            "_type": "TypeExpr",
-            "name": "Difference",
-            "template": {
-              "_type": "TemplateExpr",
-              "type": {
-                "_type": "TypeExpr",
-                "name": "GeoLocation",
-                "template": null,
-                "optional": false
-              }
+            "_type": "GenericType",
+            "type": {
+              "_type": "NamedType",
+              "name": "Difference"
             },
-            "optional": false
+            "params": [
+              {
+                "_type": "NamedType",
+                "name": "GeoLocation"
+              }
+            ]
           }
         }
       ]
     },
     {
-      "_type": "ComplexTypeStmt",
+      "_type": "TypeStmt",
       "name": "Difference",
-      "template": {
-        "_type": "TemplateExpr",
-        "type": {
-          "_type": "TypeExpr",
+      "params": [
+        {
           "name": "GeoLocation",
-          "template": null,
-          "optional": false
+          "bound": null
         }
-      },
-      "properties": [
-        {
-          "_type": "PropertyStmt",
-          "name": "lat",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "Difference",
-            "template": {
-              "_type": "TemplateExpr",
+      ],
+      "type": {
+        "_type": "ComplexType",
+        "properties": [
+          {
+            "_type": "PropertyStmt",
+            "name": "lat",
+            "type": {
+              "_type": "GenericType",
               "type": {
-                "_type": "TypeExpr",
-                "name": "Latitude",
-                "template": null,
-                "optional": false
-              }
-            },
-            "optional": false
+                "_type": "NamedType",
+                "name": "Difference"
+              },
+              "params": [
+                {
+                  "_type": "NamedType",
+                  "name": "Latitude"
+                }
+              ]
+            }
           },
-          "constraint": null
-        },
-        {
-          "_type": "PropertyStmt",
-          "name": "lon",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "Difference",
-            "template": {
-              "_type": "TemplateExpr",
+          {
+            "_type": "PropertyStmt",
+            "name": "lon",
+            "type": {
+              "_type": "GenericType",
               "type": {
-                "_type": "TypeExpr",
-                "name": "Longitude",
-                "template": null,
-                "optional": false
-              }
-            },
-            "optional": false
-          },
-          "constraint": null
-        }
-      ]
+                "_type": "NamedType",
+                "name": "Difference"
+              },
+              "params": [
+                {
+                  "_type": "NamedType",
+                  "name": "Longitude"
+                }
+              ]
+            }
+          }
+        ]
+      }
     },
     {
       "_type": "ExtendStmt",
       "type": {
-        "_type": "TypeExpr",
-        "name": "Latitude",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "Latitude"
       },
       "operations": [
         {
           "_type": "OpStmt",
           "name": "__sub__",
           "operand": {
-            "_type": "TypeExpr",
-            "name": "Latitude",
-            "template": null,
-            "optional": false
+            "_type": "NamedType",
+            "name": "Latitude"
           },
           "result": {
-            "_type": "TypeExpr",
-            "name": "Difference",
-            "template": {
-              "_type": "TemplateExpr",
-              "type": {
-                "_type": "TypeExpr",
-                "name": "Latitude",
-                "template": null,
-                "optional": false
-              }
+            "_type": "GenericType",
+            "type": {
+              "_type": "NamedType",
+              "name": "Difference"
             },
-            "optional": false
+            "params": [
+              {
+                "_type": "NamedType",
+                "name": "Latitude"
+              }
+            ]
           }
         }
       ]
@@ -2429,34 +2495,29 @@
     {
       "_type": "ExtendStmt",
       "type": {
-        "_type": "TypeExpr",
-        "name": "Longitude",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "Longitude"
       },
       "operations": [
         {
           "_type": "OpStmt",
           "name": "__sub__",
           "operand": {
-            "_type": "TypeExpr",
-            "name": "Longitude",
-            "template": null,
-            "optional": false
+            "_type": "NamedType",
+            "name": "Longitude"
           },
           "result": {
-            "_type": "TypeExpr",
-            "name": "Difference",
-            "template": {
-              "_type": "TemplateExpr",
-              "type": {
-                "_type": "TypeExpr",
-                "name": "Longitude",
-                "template": null,
-                "optional": false
-              }
+            "_type": "GenericType",
+            "type": {
+              "_type": "NamedType",
+              "name": "Difference"
             },
-            "optional": false
+            "params": [
+              {
+                "_type": "NamedType",
+                "name": "Longitude"
+              }
+            ]
           }
         }
       ]
@@ -2466,10 +2527,8 @@
       "name": "Positive",
       "subject": "v",
       "type": {
-        "_type": "TypeExpr",
-        "name": "float",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "float"
       },
       "condition": {
         "_type": "BinaryExpr",
@@ -2489,10 +2548,8 @@
       "name": "StrictlyPositive",
       "subject": "v",
       "type": {
-        "_type": "TypeExpr",
-        "name": "float",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "float"
       },
       "condition": {
         "_type": "BinaryExpr",
@@ -2512,10 +2569,8 @@
       "name": "Equatorial",
       "subject": "loc",
       "type": {
-        "_type": "TypeExpr",
-        "name": "GeoLocation",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "GeoLocation"
       },
       "condition": {
         "_type": "GroupingExpr",
@@ -2554,10 +2609,8 @@
       "name": "Arctic",
       "subject": "loc",
       "type": {
-        "_type": "TypeExpr",
-        "name": "GeoLocation",
-        "template": null,
-        "optional": false
+        "_type": "NamedType",
+        "name": "GeoLocation"
       },
       "condition": {
         "_type": "GroupingExpr",
@@ -2580,79 +2633,87 @@
       }
     },
     {
-      "_type": "ComplexTypeStmt",
+      "_type": "TypeStmt",
       "name": "Person",
-      "template": null,
-      "properties": [
-        {
-          "_type": "PropertyStmt",
-          "name": "name",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "str",
-            "template": null,
-            "optional": false
+      "params": [],
+      "type": {
+        "_type": "ComplexType",
+        "properties": [
+          {
+            "_type": "PropertyStmt",
+            "name": "name",
+            "type": {
+              "_type": "NamedType",
+              "name": "str"
+            }
           },
-          "constraint": null
-        },
-        {
-          "_type": "PropertyStmt",
-          "name": "age",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "int",
-            "template": null,
-            "optional": true
-          },
-          "constraint": {
-            "_type": "GroupingExpr",
-            "expr": {
-              "_type": "BinaryExpr",
-              "left": {
-                "_type": "BinaryExpr",
-                "left": {
-                  "_type": "LiteralExpr",
-                  "value": 0.0
+          {
+            "_type": "PropertyStmt",
+            "name": "age",
+            "type": {
+              "_type": "UnionType",
+              "types": [
+                {
+                  "_type": "NamedType",
+                  "name": "None"
                 },
-                "operator": "<=",
-                "right": {
-                  "_type": "WildcardExpr"
+                {
+                  "_type": "ConstraintType",
+                  "type": {
+                    "_type": "NamedType",
+                    "name": "int"
+                  },
+                  "constraint": {
+                    "_type": "GroupingExpr",
+                    "expr": {
+                      "_type": "BinaryExpr",
+                      "left": {
+                        "_type": "BinaryExpr",
+                        "left": {
+                          "_type": "LiteralExpr",
+                          "value": 0.0
+                        },
+                        "operator": "<=",
+                        "right": {
+                          "_type": "WildcardExpr"
+                        }
+                      },
+                      "operator": "<",
+                      "right": {
+                        "_type": "LiteralExpr",
+                        "value": 150.0
+                      }
+                    }
+                  }
                 }
+              ]
+            }
+          },
+          {
+            "_type": "PropertyStmt",
+            "name": "height",
+            "type": {
+              "_type": "ConstraintType",
+              "type": {
+                "_type": "NamedType",
+                "name": "float"
               },
-              "operator": "<",
-              "right": {
-                "_type": "LiteralExpr",
-                "value": 150.0
+              "constraint": {
+                "_type": "VariableExpr",
+                "name": "StrictlyPositive"
               }
             }
-          }
-        },
-        {
-          "_type": "PropertyStmt",
-          "name": "height",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "float",
-            "template": null,
-            "optional": false
           },
-          "constraint": {
-            "_type": "VariableExpr",
-            "name": "StrictlyPositive"
+          {
+            "_type": "PropertyStmt",
+            "name": "home",
+            "type": {
+              "_type": "NamedType",
+              "name": "GeoLocation"
+            }
           }
-        },
-        {
-          "_type": "PropertyStmt",
-          "name": "home",
-          "type": {
-            "_type": "TypeExpr",
-            "name": "GeoLocation",
-            "template": null,
-            "optional": false
-          },
-          "constraint": null
-        }
-      ]
+        ]
+      }
     }
   ],
   "errors": []