Check type of main function

2 files changed, 42 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());
         }
     }
 }
diff --git a/src/test/java/org/shedlang/compiler/config/ConfigCheckerTests.java b/src/test/java/org/shedlang/compiler/config/ConfigCheckerTests.java
index b6134ed2..404e57e6 100644
--- a/src/test/java/org/shedlang/compiler/config/ConfigCheckerTests.java
+++ b/src/test/java/org/shedlang/compiler/config/ConfigCheckerTests.java
@@ -66,4 +66,29 @@ public class ConfigCheckerTests {
 
         assertThat(error.getMessage(), equalTo("Namespace Main has no field main"));
     }
+
+    @Test
+    public void whenMainFunctionHasWrongTypeThenBinConfigIsInvalid() throws IOException {
+        var binConfig = new BinConfig("x", NamespaceName.of("Main"));
+        var typeInfos = TypeInfos.EMPTY
+            .addNamespaceTypeInfo(
+                NamespaceName.of("Main"),
+                NamespaceTypeInfo.fromFields(P.orderedMap(
+                    Map.entry(
+                        "main",
+                        new FunctionType(List.of(), List.of(), Effects.IO, Types.STRING)
+                    )
+                ))
+            );
+
+        var error = assertThrows(
+            ConfigError.class,
+            () -> ConfigChecker.configCheckBin(binConfig, typeInfos)
+        );
+
+        assertThat(
+            error.getMessage(),
+            equalTo("main function should have type fn () ! Io -> Unit, but Main.main has type fn () ! Io -> String")
+        );
+    }
 }