3 files changed, 41 insertions, 0 deletions
diff --git a/src/main/java/org/shedlang/compiler/ast/typed/FreeVariable.java b/src/main/java/org/shedlang/compiler/ast/typed/FreeVariable.java
index f1791a9a..dc755b63 100644
--- a/src/main/java/org/shedlang/compiler/ast/typed/FreeVariable.java
+++ b/src/main/java/org/shedlang/compiler/ast/typed/FreeVariable.java
@@ -5,4 +5,7 @@ import org.shedlang.compiler.sources.Source;
 import java.util.Set;
 
 public record FreeVariable(int index, String name, Set<Source> referenceSources) {
+    public Source firstReferenceSource() {
+        return referenceSources.stream().findFirst().orElseThrow();
+    }
 }
diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 1a27345e..05e89031 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -622,6 +622,27 @@ public class TypeChecker {
 
         context = context.exit();
 
+        for (var freeVariable : freeVariables) {
+            var variable = context.compileTimeLookup(freeVariable.name())
+                .orElseThrow(() -> new UnknownVariableError(
+                    freeVariable.name(),
+                    freeVariable.firstReferenceSource()
+                ));
+
+            switch (variable.variableType()) {
+                case CONSTANT -> {}
+                case IMPORT -> {}
+                case PARAM -> {}
+                case VAL -> {}
+
+                case VAR ->
+                    throw new CannotCaptureVarError(
+                        freeVariable.name(),
+                        freeVariable.firstReferenceSource()
+                    );
+            };
+        }
+
         var typed = new TypedFunctionBodyNode(
             freeVariables,
             typedBody,
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/CannotCaptureVarError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/CannotCaptureVarError.java
new file mode 100644
index 00000000..93d93fb3
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/CannotCaptureVarError.java
@@ -0,0 +1,17 @@
+package org.shedlang.compiler.typechecker.errors;
+
+import org.shedlang.compiler.errors.SourceError;
+import org.shedlang.compiler.sources.Source;
+
+public class CannotCaptureVarError extends SourceError {
+    private final String name;
+
+    public CannotCaptureVarError(String name, Source source) {
+        super("var " + name + " cannot be captured", source);
+        this.name = name;
+    }
+
+    public String name() {
+        return name;
+    }
+}