1 files changed, 17 insertions, 2 deletions
diff --git a/src/main/java/org/shedlang/compiler/config/ConfigChecker.java b/src/main/java/org/shedlang/compiler/config/ConfigChecker.java
index 46ec748f..fe607f83 100644
--- a/src/main/java/org/shedlang/compiler/config/ConfigChecker.java
+++ b/src/main/java/org/shedlang/compiler/config/ConfigChecker.java
@@ -2,6 +2,11 @@ package org.shedlang.compiler.config;
 
 import org.shedlang.compiler.config.errors.ConfigError;
 import org.shedlang.compiler.typechecker.TypeInfos;
+import org.shedlang.compiler.types.Effects;
+import org.shedlang.compiler.types.FunctionType;
+import org.shedlang.compiler.types.Types;
+
+import java.util.List;
 
 public class ConfigChecker {
     private ConfigChecker() {
@@ -17,8 +22,18 @@ public class ConfigChecker {
         var namespaceTypeInfo = typeInfos.namespaceTypeInfo(binConfig.mainNamespace())
             .orElseThrow(() -> new ConfigError("Namespace " + binConfig.mainNamespace() + " not found"));
 
-        if (namespaceTypeInfo.fieldType("main").isEmpty()) {
-            throw new ConfigError("Namespace " + binConfig.mainNamespace() + " has no field main");
+        var mainType = namespaceTypeInfo.fieldType("main")
+            .orElseThrow(() -> new ConfigError("Namespace " + binConfig.mainNamespace() + " has no field main"));
+
+        var expectedMainType = new FunctionType(
+            List.of(),
+            List.of(),
+            Effects.IO,
+            Types.UNIT
+        );
+
+        if (!mainType.equals(expectedMainType)) {
+            throw new ConfigError("main function should have type " + expectedMainType.describe() + ", but " + binConfig.mainNamespace() + ".main has type " + mainType.describe());
         }
     }
 }