2 files changed, 68 insertions, 0 deletions
diff --git a/src/main/java/org/shedlang/compiler/types/TypeSubstitution.java b/src/main/java/org/shedlang/compiler/types/TypeSubstitution.java
new file mode 100644
index 00000000..bd679292
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/types/TypeSubstitution.java
@@ -0,0 +1,49 @@
+package org.shedlang.compiler.types;
+
+public class TypeSubstitution {
+    private TypeSubstitution() {}
+
+    public static Type substitute(Type type, TypeSubstitutionMap map) {
+        return switch (type) {
+            case BoolType boolType -> type;
+
+            case BottomType bottomType -> type;
+
+            case ConstructedType constructedType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case FunctionType functionType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case IntType intType -> type;
+
+            case NamespaceType namespaceType -> type;
+
+            case StringType stringType -> type;
+
+            case StructuredType structuredType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case SumType sumType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case TopType topType -> type;
+
+            case TupleType tupleType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case TypeLevelValueType typeLevelValueType -> {
+                throw new UnsupportedOperationException("TODO");
+            }
+
+            case TypeParam typeParam -> map.substitute(typeParam);
+
+            case UnitType unitType -> type;
+        };
+    }
+}
diff --git a/src/main/java/org/shedlang/compiler/types/TypeSubstitutionMap.java b/src/main/java/org/shedlang/compiler/types/TypeSubstitutionMap.java
new file mode 100644
index 00000000..2709fc02
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/types/TypeSubstitutionMap.java
@@ -0,0 +1,19 @@
+package org.shedlang.compiler.types;
+
+import java.util.Map;
+
+public class TypeSubstitutionMap {
+    public static TypeSubstitutionMap of(Map.Entry<TypeParam, Type>... map) {
+        return new TypeSubstitutionMap(Map.ofEntries(map));
+    }
+
+    private final Map<TypeParam, Type> map;
+
+    public TypeSubstitutionMap(Map<TypeParam, Type> map) {
+        this.map = map;
+    }
+
+    public Type substitute(TypeParam typeParam) {
+        return this.map.getOrDefault(typeParam, typeParam);
+    }
+}