experimenting with kinds and higher order types, not done yet

common/api/src/main/java/com/dfsek/terra/api/util/generic/Applicative.java

@@ -0,0 +1,10 @@
+package com.dfsek.terra.api.util.generic;
+
+import java.util.function.Function;
+
+
+public interface Applicative<T, A extends K<A, T>> extends Functor<T> {
+    <U, A1 extends K<A1, U>> Applicative<U, A1> pure(U t);
+
+    <U, A1 extends K<A1, U>> Applicative<U, A1> apply(K<A, Function<T, U>> amap);
+}

common/api/src/main/java/com/dfsek/terra/api/util/generic/Functor.java

@@ -3,6 +3,6 @@ package com.dfsek.terra.api.util.generic;
 import java.util.function.Function;
 
 
-public interface Functor<T, F extends Functor<?, F>> {
+public interface Functor<T> {
-    <U> Functor<U, F> map(Function<T, U> map);
+    <U> Functor<U> map(Function<T, U> map);
 }

common/api/src/main/java/com/dfsek/terra/api/util/generic/K.java

@@ -0,0 +1,11 @@
+package com.dfsek.terra.api.util.generic;
+
+/**
+ * Kind
+ */
+public interface K<T, U> {
+    @SuppressWarnings("unchecked")
+    default T self() {
+        return (T) this;
+    }
+}

common/api/src/main/java/com/dfsek/terra/api/util/generic/Maybe.java

@@ -1,10 +1,27 @@
 package com.dfsek.terra.api.util.generic;
 
 import java.util.Optional;
+import java.util.function.Function;
 
 
-public interface Maybe<T> {
+public interface Maybe<T> extends Monad<T, Maybe<?>> {
+    @Override
+    <T2> Maybe<T2> bind(Function<T, Monad<T2, Maybe<?>>> map);
+
+    @Override
+    <T2> Maybe<T2> identity();
+
+    @Override
+    <U> Maybe<U> map(Function<T, U> map);
+
+    @Override
+    Monad<T, Maybe<?>> multiply(Maybe<?> t);
+
     default Optional<T> toOptional() {
 
     }
+
+    record Just<T>(T value) implements Maybe<T> {
+
+    }
 }

common/api/src/main/java/com/dfsek/terra/api/util/generic/Monad.java

@@ -6,12 +6,24 @@ import java.util.function.Function;
 /**
  * A monad is a monoid in the category of endofunctors.
  */
-public interface Monad<T, M extends Monad<?, M>> extends Functor<T, M>, Monoid<T, M> {
+public interface Monad<T, M extends K<M, T>> extends Applicative<T, M>, Monoid<T, M>{
-    <T2> Monad<T2, M> bind(Function<T, Monad<T2, M>> map);
+    <T2, M2 extends K<M2, T2>> Monad<T2, M2> bind(Function<T, Monad<T2, M2>> map);
-    <T2> Monad<T2, M> pure(T2 t);
+    @Override
+    <T2, M2 extends K<M2, T2>> Monad<T2, M2> identity();
 
     @Override
-    default <U> Monad<U, M> map(Function<T, U> map) {
+    <U, M2 extends K<M2, U>> Monad<U, M2> pure(U t);
+
+    @Override
+    Monad<T, M> multiply(M t);
+
+    @Override
+    default <U, M2 extends K<M2, U>> Monad<U, M2> map(Function<T, U> map) {
         return bind(m -> pure(map.apply(m)));
     }
+
+    @Override
+    default <U, A1 extends K<A1, U>> Monad<U, A1> apply(K<M, Function<T, U>> amap) {
+        amap.self()
+    }
 }

common/api/src/main/java/com/dfsek/terra/api/util/generic/Monoid.java

@@ -1,7 +1,7 @@
 package com.dfsek.terra.api.util.generic;
 
-public interface Monoid<T, M extends Monoid<?, M>> extends Semigroup<T, M>{
+public interface Monoid<T, M extends K<M, T>> extends Semigroup<T, M>{
-    <T1> Monoid<T1, M> pure(T1 t);
+    <T1, M1 extends K<M1, T1>> Monoid<T1, M1> identity();
 
     @Override
     Monoid<T, M> multiply(M t);

common/api/src/main/java/com/dfsek/terra/api/util/generic/Semigroup.java

@@ -1,5 +1,5 @@
 package com.dfsek.terra.api.util.generic;
 
-public interface Semigroup<T, S extends Semigroup<?, S>> {
+public interface Semigroup<T, S extends K<S, T>> {
     Semigroup<T, S> multiply(S t);
 }