derive4j/derive4j

  public Optional<Map<TypeVariable, TypeMirror>> unify(TypeMirror from, TypeMirror to) {
    return unify0(from, to, new HashMap<>()).map(solution -> {
      // sanity check just in case there is something wrong:
      if (!Types.isSameType(resolve(from, tv -> get(tv, solution)), to)) {
        throw new RuntimeException("Error in unify algo");
      }
      return Collections.unmodifiableMap(solution);
    });
  }

  private Optional<HashMap<TypeVariable, TypeMirror>> unify0(TypeMirror from, TypeMirror to,
      HashMap<TypeVariable, TypeMirror> unified) {
    if (Types.isSameType(from, to)) {
      return Optional.of(unified);
    }
    if (asTypeElement(from).map(fromTE -> !asTypeElement(to).filter(fromTE::equals).isPresent()).orElse(false)) {
      return Optional.empty();
    }
    return asTypeVariable.visit(from)
        .map(tv -> Optional.ofNullable(unified.get(tv))
            .map(u -> Types.isSameType(u, to)
                ? Optional.of(unified)
                : Optional.<HashMap<TypeVariable, TypeMirror>>empty())
            .orElseGet(() -> {
              unified.put(tv, to);
              return Optional.of(unified);
            }))
        .orElseGet(() -> asDeclaredType(from).flatMap(fromDT -> asDeclaredType(to).flatMap(toDT -> traverseOptional(
            zip(fromDT.getTypeArguments(), toDT.getTypeArguments()),
            p2 -> p2.match((a1, a2) -> unify0(a1, a2, unified))).map(__ -> unified))));
  }