Check when expression is instance of sum type

3 files changed, 46 insertions, 0 deletions

diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 58396e80..3b4df564 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -1139,6 +1139,13 @@ public class TypeChecker {
         var typedExpressionResult = typeCheckExpression(untyped.expression(), context);
         var typedExpression = typedExpressionResult.value();
         context = typedExpressionResult.context();
+
+        if (!(typedExpression.type() instanceof SumType)) {
+            throw new WhenExpressionIsNotInstanceOfSumTypeError(
+                typedExpression.type(),
+                typedExpression.source()
+            );
+        }
         
         var typedCasesResult = context.mapWithContext(
             untyped.cases(),
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java
new file mode 100644
index 00000000..1583c8ae
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java
@@ -0,0 +1,18 @@
+package org.shedlang.compiler.typechecker.errors;
+
+import org.shedlang.compiler.errors.SourceError;
+import org.shedlang.compiler.sources.Source;
+import org.shedlang.compiler.types.Type;
+
+public class WhenExpressionIsNotInstanceOfSumTypeError extends SourceError {
+    private final Type actualType;
+
+    public WhenExpressionIsNotInstanceOfSumTypeError(Type actualType, Source source) {
+        super("when expression must be an instance of a sum type, but was " + actualType.describe(), source);
+        this.actualType = actualType;
+    }
+
+    public Type actualType() {
+        return actualType;
+    }
+}
diff --git a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
index aec35c33..031b20c6 100644
--- a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
+++ b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerWhenTests.java
@@ -2,12 +2,14 @@ package org.shedlang.compiler.typechecker;
 
 import org.junit.jupiter.api.Test;
 import org.shedlang.compiler.ast.untyped.Untyped;
+import org.shedlang.compiler.typechecker.errors.WhenExpressionIsNotInstanceOfSumTypeError;
 import org.shedlang.compiler.types.*;
 import org.shedlang.compiler.util.P;
 
 import java.util.List;
 import java.util.Map;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.shedlang.compiler.ast.typed.TypedNodeMatchers.*;
 import static org.zwobble.precisely.AssertThat.assertThat;
 import static org.zwobble.precisely.Matchers.equalTo;
@@ -20,6 +22,25 @@ public class TypeCheckerWhenTests {
     private static final StructType VARIANT_TYPE_2 = Types.struct(NAMESPACE_NAME, "Variant2");
 
     @Test
+    public void whenExpressionIsNotInstanceOfSumTypeThenErrorIsThrown() {
+        var untyped = Untyped.when(
+            Untyped.reference("x"),
+            List.of()
+        );
+        var context = TypeCheckerContext.EMPTY
+            .enterGlobal(P.map(
+                Map.entry("x", Types.STRING)
+            ));
+
+        var error = assertThrows(
+            WhenExpressionIsNotInstanceOfSumTypeError.class,
+            () -> TypeChecker.typeCheckExpression(untyped, context)
+        );
+
+        assertThat(error.actualType(), equalTo(Types.STRING));
+    }
+
+    @Test
     public void whenAllCasesAreHandledThenExpressionTypeChecks() {
         var untyped = Untyped.when(
             Untyped.reference("x"),