From 776fcb476f229030cb5d34f4b948603f05c01a79 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Fri, 24 Apr 2026 12:43:29 +0100
Subject: Type check type level reference

---
 .../hobgoblin/compiler/ast/typed/TypedNode.java    |  7 +++
 .../ast/typed/TypedTypeLevelExpressionNode.java    |  7 +++
 .../ast/typed/TypedTypeLevelReferenceNode.java     | 11 +++++
 .../compiler/typechecker/TypeChecker.java          | 51 ++++++++++++++++++++++
 .../compiler/typechecker/TypeCheckerContext.java   | 25 +++++++++++
 .../typechecker/UndeclaredVariableError.java       | 17 ++++++++
 .../typechecker/UndefinedVariableError.java        | 17 ++++++++
 .../compiler/typechecker/UnexpectedTypeError.java  | 25 +++++++++++
 .../hobgoblin/compiler/typechecker/Variable.java   | 11 +++++
 .../hobgoblin/compiler/types/ScalarType.java       |  8 ++++
 .../org/zwobble/hobgoblin/compiler/types/Type.java |  4 ++
 .../hobgoblin/compiler/types/TypeLevelValue.java   |  5 +++
 .../compiler/types/TypeLevelValueType.java         |  8 ++++
 .../zwobble/hobgoblin/compiler/types/TypeSet.java  | 19 ++++++++
 14 files changed, 215 insertions(+)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedNode.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelExpressionNode.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelReferenceNode.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerContext.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndeclaredVariableError.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndefinedVariableError.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UnexpectedTypeError.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/typechecker/Variable.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/types/ScalarType.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/types/Type.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValue.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValueType.java
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/types/TypeSet.java

(limited to 'src/main')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedNode.java b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedNode.java
new file mode 100644
index 0000000..6baa8d6
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedNode.java
@@ -0,0 +1,7 @@
+package org.zwobble.hobgoblin.compiler.ast.typed;
+
+import org.zwobble.hobgoblin.compiler.sources.Source;
+
+public interface TypedNode {
+    Source source();
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelExpressionNode.java b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelExpressionNode.java
new file mode 100644
index 0000000..c588f95
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelExpressionNode.java
@@ -0,0 +1,7 @@
+package org.zwobble.hobgoblin.compiler.ast.typed;
+
+import org.zwobble.hobgoblin.compiler.types.TypeLevelValue;
+
+public sealed interface TypedTypeLevelExpressionNode<T extends TypeLevelValue> extends TypedNode permits TypedTypeLevelReferenceNode {
+    T value();
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelReferenceNode.java b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelReferenceNode.java
new file mode 100644
index 0000000..bc2c50d
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/ast/typed/TypedTypeLevelReferenceNode.java
@@ -0,0 +1,11 @@
+package org.zwobble.hobgoblin.compiler.ast.typed;
+
+import org.zwobble.hobgoblin.compiler.sources.Source;
+import org.zwobble.hobgoblin.compiler.types.TypeLevelValue;
+
+public record TypedTypeLevelReferenceNode<T extends TypeLevelValue>(
+    String name,
+    T value,
+    Source source
+) implements TypedTypeLevelExpressionNode<T> {
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
new file mode 100644
index 0000000..56d4645
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeChecker.java
@@ -0,0 +1,51 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.ast.typed.TypedTypeLevelExpressionNode;
+import org.zwobble.hobgoblin.compiler.ast.typed.TypedTypeLevelReferenceNode;
+import org.zwobble.hobgoblin.compiler.ast.untyped.UntypedTypeLevelExpressionNode;
+import org.zwobble.hobgoblin.compiler.ast.untyped.UntypedTypeLevelReferenceNode;
+import org.zwobble.hobgoblin.compiler.types.TypeLevelValue;
+import org.zwobble.hobgoblin.compiler.types.TypeLevelValueType;
+import org.zwobble.hobgoblin.compiler.types.TypeSet;
+
+public class TypeChecker {
+    private TypeChecker() {
+    }
+
+    public static TypedTypeLevelExpressionNode<? extends TypeLevelValue> typeCheckTypeLevelExpression(
+        UntypedTypeLevelExpressionNode untyped,
+        TypeCheckerContext context
+    ) {
+        return switch (untyped) {
+            case UntypedTypeLevelReferenceNode untypedTypeLevelReference ->
+                typeCheckTypeLevelReference(untypedTypeLevelReference, context);
+        };
+    }
+
+    private static TypedTypeLevelExpressionNode<? extends TypeLevelValue> typeCheckTypeLevelReference(
+        UntypedTypeLevelReferenceNode untyped,
+        TypeCheckerContext context
+    ) {
+        var variable = context.lookup(untyped.name())
+            .orElseThrow(() -> new UndeclaredVariableError(untyped.name(), untyped.source()));
+
+        var type = switch (variable) {
+            case Variable.Declared declared ->
+                throw new UndefinedVariableError(untyped.name(), untyped.source());
+
+            case Variable.Defined defined ->
+                defined.type();
+        };
+
+        // TODO: handle not a type-level value
+        if (!(type instanceof TypeLevelValueType(TypeLevelValue value))) {
+            throw new UnexpectedTypeError(new TypeSet.MetaType(), type, untyped.source());
+        }
+
+        return new TypedTypeLevelReferenceNode<>(
+            untyped.name(),
+            value,
+            untyped.source()
+        );
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerContext.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerContext.java
new file mode 100644
index 0000000..e861c30
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/TypeCheckerContext.java
@@ -0,0 +1,25 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.types.Type;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Optional;
+
+public class TypeCheckerContext {
+    private final Map<String, Variable> variables = new HashMap<>();
+
+    public void declare(String name) {
+        // TODO: Check variable is not already declared.
+        this.variables.put(name, new Variable.Declared());
+    }
+
+    public void define(String name, Type type) {
+        // TODO: Check variable is not already defined.
+        this.variables.put(name, new Variable.Defined(type));
+    }
+
+    public Optional<Variable> lookup(String name) {
+        return Optional.ofNullable(this.variables.get(name));
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndeclaredVariableError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndeclaredVariableError.java
new file mode 100644
index 0000000..c6130fa
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndeclaredVariableError.java
@@ -0,0 +1,17 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+
+public class UndeclaredVariableError extends SourceError {
+    private final String variableName;
+
+    public UndeclaredVariableError(String variableName, Source source) {
+        super("Undeclared variable: " + variableName, source);
+        this.variableName = variableName;
+    }
+
+    public String variableName() {
+        return variableName;
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndefinedVariableError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndefinedVariableError.java
new file mode 100644
index 0000000..95be5a6
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UndefinedVariableError.java
@@ -0,0 +1,17 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+
+public class UndefinedVariableError extends SourceError {
+    private final String variableName;
+
+    public UndefinedVariableError(String variableName, Source source) {
+        super("Variable has been declared but not defined: " + variableName, source);
+        this.variableName = variableName;
+    }
+
+    public String variableName() {
+        return variableName;
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UnexpectedTypeError.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UnexpectedTypeError.java
new file mode 100644
index 0000000..c2ca4b6
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/UnexpectedTypeError.java
@@ -0,0 +1,25 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.errors.SourceError;
+import org.zwobble.hobgoblin.compiler.sources.Source;
+import org.zwobble.hobgoblin.compiler.types.Type;
+import org.zwobble.hobgoblin.compiler.types.TypeSet;
+
+public class UnexpectedTypeError extends SourceError {
+    private final TypeSet expected;
+    private final Type actual;
+
+    public UnexpectedTypeError(TypeSet expected, Type actual, Source source) {
+        super("Expected " + expected.describe() + " but was " + actual.describe(), source);
+        this.expected = expected;
+        this.actual = actual;
+    }
+
+    public TypeSet expected() {
+        return expected;
+    }
+
+    public Type actual() {
+        return actual;
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/Variable.java b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/Variable.java
new file mode 100644
index 0000000..1d3a4b6
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/typechecker/Variable.java
@@ -0,0 +1,11 @@
+package org.zwobble.hobgoblin.compiler.typechecker;
+
+import org.zwobble.hobgoblin.compiler.types.Type;
+
+public sealed interface Variable {
+    record Declared() implements Variable {
+    }
+
+    record Defined(Type type) implements Variable {
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/types/ScalarType.java b/src/main/java/org/zwobble/hobgoblin/compiler/types/ScalarType.java
new file mode 100644
index 0000000..22032a1
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/types/ScalarType.java
@@ -0,0 +1,8 @@
+package org.zwobble.hobgoblin.compiler.types;
+
+public record ScalarType(String name) implements Type {
+    @Override
+    public String describe() {
+        return name;
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/types/Type.java b/src/main/java/org/zwobble/hobgoblin/compiler/types/Type.java
new file mode 100644
index 0000000..2692261
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/types/Type.java
@@ -0,0 +1,4 @@
+package org.zwobble.hobgoblin.compiler.types;
+
+public sealed interface Type extends TypeLevelValue permits ScalarType, TypeLevelValueType {
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValue.java b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValue.java
new file mode 100644
index 0000000..9740cf9
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValue.java
@@ -0,0 +1,5 @@
+package org.zwobble.hobgoblin.compiler.types;
+
+public sealed interface TypeLevelValue permits Type {
+    String describe();
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValueType.java b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValueType.java
new file mode 100644
index 0000000..b885c25
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeLevelValueType.java
@@ -0,0 +1,8 @@
+package org.zwobble.hobgoblin.compiler.types;
+
+public record TypeLevelValueType(TypeLevelValue value) implements Type {
+    @Override
+    public String describe() {
+        return "TypeLevelValue[" + value.describe() + "]";
+    }
+}
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeSet.java b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeSet.java
new file mode 100644
index 0000000..f587897
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/types/TypeSet.java
@@ -0,0 +1,19 @@
+package org.zwobble.hobgoblin.compiler.types;
+
+public sealed interface TypeSet {
+    String describe();
+
+    record SingleType(Type type) implements TypeSet {
+        @Override
+        public String describe() {
+            return type.describe();
+        }
+    }
+
+    record MetaType() implements TypeSet {
+        @Override
+        public String describe() {
+            return "metatype";
+        }
+    }
+}
-- 
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