Implement subtyping for function types

1 files changed, 96 insertions, 4 deletions

diff --git a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
index a144b78c..d6bd14be 100644
--- a/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
+++ b/src/test/java/org/shedlang/compiler/typechecker/TypeCheckerIsSubTypeTests.java
@@ -1,10 +1,7 @@
 package org.shedlang.compiler.typechecker;
 
 import org.junit.jupiter.api.Test;
-import org.shedlang.compiler.types.NamespaceName;
-import org.shedlang.compiler.types.StructType;
-import org.shedlang.compiler.types.SumTypeInfo;
-import org.shedlang.compiler.types.Types;
+import org.shedlang.compiler.types.*;
 import org.shedlang.compiler.util.P;
 
 import static org.shedlang.compiler.typechecker.TypeCheckerSubTyping.isSubType;
@@ -133,4 +130,99 @@ public class TypeCheckerIsSubTypeTests {
         assertThat(isSubType(sumType, variantType1, context), equalTo(false));
         assertThat(isSubType(sumType, variantType2, context), equalTo(false));
     }
+
+    @Test
+    public void whenFunctionsHaveDifferentPositionalParamCountsThenTheyAreNotSubtypes() {
+        var functionType1 = FunctionType.builder()
+            .addPositionalParam(Types.STRING)
+            .withReturnType(Types.UNIT)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .addPositionalParam(Types.STRING)
+            .addPositionalParam(Types.BOOL)
+            .withReturnType(Types.UNIT)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(false));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
+
+    @Test
+    public void functionPositionalParamsAreContravariant() {
+        var functionType1 = FunctionType.builder()
+            .addPositionalParam(Types.STRING)
+            .withReturnType(Types.UNIT)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .addPositionalParam(Types.TOP)
+            .withReturnType(Types.UNIT)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(true));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
+
+    @Test
+    public void whenFunctionsHaveDifferentNamedParamNamesThenTheyAreNotSubtypes() {
+        var functionType1 = FunctionType.builder()
+            .addNamedParam("x", Types.STRING)
+            .withReturnType(Types.UNIT)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .addNamedParam("y", Types.STRING)
+            .withReturnType(Types.UNIT)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(false));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
+
+    @Test
+    public void functionNamedParamsAreContravariant() {
+        var functionType1 = FunctionType.builder()
+            .addNamedParam("x", Types.STRING)
+            .withReturnType(Types.UNIT)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .addNamedParam("x", Types.TOP)
+            .withReturnType(Types.UNIT)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(true));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
+
+    @Test
+    public void functionTypeIsSubtypeOfOtherFunctionTypeWhenItHasSubeffectOfOtherFunction() {
+        var functionType1 = FunctionType.builder()
+            .withEffect(Effects.IO)
+            .withReturnType(Types.UNIT)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .withEffect(Effects.NONE)
+            .withReturnType(Types.UNIT)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(true));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
+
+    @Test
+    public void functionReturnTypeIsCovariant() {
+        var functionType1 = FunctionType.builder()
+            .withReturnType(Types.TOP)
+            .build();
+        var functionType2 = FunctionType.builder()
+            .withReturnType(Types.STRING)
+            .build();
+        var context = TypeCheckerContext.EMPTY;
+
+        assertThat(isSubType(functionType2, functionType1, context), equalTo(true));
+        assertThat(isSubType(functionType1, functionType2, context), equalTo(false));
+    }
 }