Add NullAway static null-safety checker via Error Prone

pom.xml

@@ -19,6 +19,11 @@
   </properties>
 
   <dependencies>
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <version>1.0.0</version>
+    </dependency>
     <dependency>
       <groupId>org.junit.jupiter</groupId>
       <artifactId>junit-jupiter</artifactId>
@@ -47,6 +52,38 @@
         <plugin>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.14.1</version>
+          <configuration>
+            <fork>true</fork>
+            <meminitial>128m</meminitial>
+            <maxmem>2048m</maxmem>
+            <compilerArgs>
+              <arg>-XDcompilePolicy=simple</arg>
+              <arg>--should-stop=ifError=FLOW</arg>
+              <arg>-Xplugin:ErrorProne -Xep:NullAway:ERROR -XepOpt:NullAway:AnnotatedPackages=com.garciat.typeclasses</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.file=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.main=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.parser=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.processing=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED</arg>
+              <arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED</arg>
+              <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED</arg>
+              <arg>-J--add-opens=jdk.compiler/com.sun.tools.javac.comp=ALL-UNNAMED</arg>
+            </compilerArgs>
+            <annotationProcessorPaths>
+              <path>
+                <groupId>com.google.errorprone</groupId>
+                <artifactId>error_prone_core</artifactId>
+                <version>2.45.0</version>
+              </path>
+              <path>
+                <groupId>com.uber.nullaway</groupId>
+                <artifactId>nullaway</artifactId>
+                <version>0.12.14</version>
+              </path>
+            </annotationProcessorPaths>
+          </configuration>
         </plugin>
         <plugin>
           <artifactId>maven-surefire-plugin</artifactId>
@@ -86,6 +123,9 @@
         <groupId>com.spotify.fmt</groupId>
         <artifactId>fmt-maven-plugin</artifactId>
       </plugin>
+      <plugin>
+        <artifactId>maven-compiler-plugin</artifactId>
+      </plugin>
     </plugins>
   </build>
 </project>

src/main/java/com/garciat/typeclasses/classes/PrintAll.java

@@ -1,11 +1,14 @@
 package com.garciat.typeclasses.classes;
 
+import static com.garciat.typeclasses.types.Unit.unit;
+
 import com.garciat.typeclasses.api.TypeClass;
 import com.garciat.typeclasses.api.TypeClass.Witness;
 import com.garciat.typeclasses.impl.utils.Lists;
 import com.garciat.typeclasses.types.F1;
 import com.garciat.typeclasses.types.F2;
 import com.garciat.typeclasses.types.F3;
+import com.garciat.typeclasses.types.Unit;
 import java.util.List;
 import java.util.function.Function;
 
@@ -21,12 +24,12 @@ static <T> T of(PrintAll<T> printAll) {
   }
 
   @Witness
-  static PrintAll<Void> base() {
+  static PrintAll<Unit> base() {
     return strings -> {
       for (String s : strings) {
         System.out.println(s);
       }
-      return null;
+      return unit();
     };
   }
 

src/main/java/com/garciat/typeclasses/types/FwdList.java

@@ -1,6 +1,7 @@
 package com.garciat.typeclasses.types;
 
 import static com.garciat.typeclasses.api.TypeClass.Witness.Overlap.OVERLAPPING;
+import static com.garciat.typeclasses.types.Unit.unit;
 
 import com.garciat.typeclasses.api.hkt.Kind;
 import com.garciat.typeclasses.api.hkt.Kind.KArr;
@@ -47,12 +48,12 @@ default <B> B foldl(B identity, BiFunction<B, A, B> f) {
   }
 
   default void forEach(Consumer<A> action) {
-    this.<Void>match(
-        () -> null,
+    this.<Unit>match(
+        () -> unit(),
         (head, tail) -> {
           action.accept(head);
           tail.forEach(action);
-          return null;
+          return unit();
         });
   }
 

src/main/java/com/garciat/typeclasses/types/Unit.java

@@ -0,0 +1,7 @@
+package com.garciat.typeclasses.types;
+
+public record Unit() {
+  public static Unit unit() {
+    return new Unit();
+  }
+}

src/test/java/com/garciat/typeclasses/ExamplesTest.java

@@ -18,6 +18,7 @@
 import com.garciat.typeclasses.types.JavaList;
 import com.garciat.typeclasses.types.Maybe;
 import com.garciat.typeclasses.types.Sum;
+import com.garciat.typeclasses.types.Unit;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -70,7 +71,7 @@ void example() {
     F3<Integer, Integer, Integer, Integer> sum = SumAllInt.of(witness(new Ty<>() {}));
     System.out.println(sum.apply(1, 2, 3));
 
-    F3<String, JavaList<String>, Integer, Void> printer = PrintAll.of(witness(new Ty<>() {}));
+    F3<String, JavaList<String>, Integer, Unit> printer = PrintAll.of(witness(new Ty<>() {}));
     printer.apply("Items:", JavaList.of("apple", "banana", "cherry"), 0);
 
     Foldable<FwdList.Tag> foldableFwdList = witness(new Ty<>() {});

src/test/java/com/garciat/typeclasses/TypeClassesTest.java

@@ -216,10 +216,12 @@ void witnessMapWithDependencies() {
   // Test helper classes
   // ============================================
 
+  @SuppressWarnings("NullAway")
   static class NoWitnessType {
     String value;
   }
 
+  @SuppressWarnings("NullAway")
   static class CustomType {
     String value;
 

src/test/java/com/garciat/typeclasses/impl/ParsedTypeTest.java

@@ -77,6 +77,7 @@ class TestClass<T> {}
 
   @Test
   void parseTypeVariableInParameterizedType() throws Exception {
+    @SuppressWarnings("NullAway")
     class TestClass<T> {
       List<T> field;
     }
@@ -88,6 +89,7 @@ class TestClass<T> {
 
   @Test
   void parseWildcardTypeThrows() throws Exception {
+    @SuppressWarnings("NullAway")
     class TestClass {
       List<?> field;
     }