diff --git a/src/main/java/io/github/joselion/maybe/EffectHandler.java b/src/main/java/io/github/joselion/maybe/EffectHandler.java
index f980e7a..593d9fa 100644
--- a/src/main/java/io/github/joselion/maybe/EffectHandler.java
+++ b/src/main/java/io/github/joselion/maybe/EffectHandler.java
@@ -82,7 +82,7 @@ public EffectHandler<E> doOnSuccess(final Runnable effect) {
    * @param effect a consumer function that recieves the caught error
    * @return the same handler to continue chainning operations
    */
-  public <X extends Throwable> EffectHandler<E> doOnError(final Class<X> ofType, final Consumer<X> effect) {
+  public <X extends Throwable> EffectHandler<E> doOnError(final Class<X> ofType, final Consumer<? super X> effect) {
     this.error
       .filter(ofType::isInstance)
       .map(ofType::cast)
@@ -98,7 +98,7 @@ public <X extends Throwable> EffectHandler<E> doOnError(final Class<X> ofType, f
    * @param effect a consumer function that recieves the caught error
    * @return the same handler to continue chainning operations
    */
-  public EffectHandler<E> doOnError(final Consumer<E> effect) {
+  public EffectHandler<E> doOnError(final Consumer<? super E> effect) {
     this.error.ifPresent(effect);
 
     return this;
@@ -116,7 +116,7 @@ public EffectHandler<E> doOnError(final Consumer<E> effect) {
    * @return an empty handler if an error instance of the provided type was
    *         caught. The same handler instance otherwise
    */
-  public <X extends E> EffectHandler<E> catchError(final Class<X> ofType, final Consumer<X> handler) {
+  public <X extends Throwable> EffectHandler<E> catchError(final Class<X> ofType, final Consumer<? super X> handler) {
     return this.error
       .filter(ofType::isInstance)
       .map(ofType::cast)
@@ -137,7 +137,7 @@ public <X extends E> EffectHandler<E> catchError(final Class<X> ofType, final Co
    * @return an empty handler if the error is present. The same handler
    *         instance otherwise
    */
-  public EffectHandler<E> catchError(final Consumer<E> handler) {
+  public EffectHandler<E> catchError(final Consumer<? super E> handler) {
     return this.error
       .map(caught -> {
         handler.accept(caught);
@@ -157,11 +157,11 @@ public EffectHandler<E> catchError(final Consumer<E> handler) {
    *         invoked callback
    */
   public <X extends Throwable> EffectHandler<X> runEffect(
-    final ThrowingRunnable<X> onSuccess,
-    final ThrowingConsumer<E, X> onError
+    final ThrowingRunnable<? extends X> onSuccess,
+    final ThrowingConsumer<? super E, ? extends X> onError
   ) {
     return this.error
-      .map(Maybe.partialEffect(onError))
+      .map(Maybe.<E, X>partialEffect(onError))
       .orElseGet(() -> Maybe.fromEffect(onSuccess));
   }
 
@@ -175,7 +175,7 @@ public <X extends Throwable> EffectHandler<X> runEffect(
    * @return a new {@link EffectHandler} that is either empty or with the
    *         thrown error
    */
-  public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingRunnable<X> effect) {
+  public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingRunnable<? extends X> effect) {
     return this.runEffect(effect, err -> { });
   }
 
@@ -185,7 +185,7 @@ public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingRunnable<X
    *
    * @param effect a consumer function that receives the caught error
    */
-  public void orElse(final Consumer<E> effect) {
+  public void orElse(final Consumer<? super E> effect) {
     this.error.ifPresent(effect);
   }
 
@@ -208,7 +208,7 @@ public void orThrow() throws E {
    * @param mapper a function that maps the new exception to throw
    * @throws X a mapped exception
    */
-  public <X extends Throwable> void orThrow(final Function<E, X> mapper) throws X {
+  public <X extends Throwable> void orThrow(final Function<? super E, ? extends X> mapper) throws X {
     if (this.error.isPresent()) {
       throw mapper.apply(this.error.get());
     }
diff --git a/src/main/java/io/github/joselion/maybe/Maybe.java b/src/main/java/io/github/joselion/maybe/Maybe.java
index cf675dc..4f022cf 100644
--- a/src/main/java/io/github/joselion/maybe/Maybe.java
+++ b/src/main/java/io/github/joselion/maybe/Maybe.java
@@ -97,7 +97,9 @@ public static <T> Maybe<T> fromOptional(final Optional<T> value) {
    * @return a {@link ResolveHandler} with either the value resolved or the thrown
    *         exception to be handled
    */
-  public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(final ThrowingSupplier<T, E> resolver) {
+  public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(
+    final ThrowingSupplier<? extends T, ? extends E> resolver
+  ) {
     try {
       return ResolveHandler.ofSuccess(resolver.get());
     } catch (Throwable e) { // NOSONAR
@@ -116,7 +118,7 @@ public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(final T
    * @return an {@link EffectHandler} with either the thrown exception to be
    *         handled or nothing
    */
-  public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRunnable<E> effect) {
+  public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRunnable<? extends E> effect) {
     try {
       effect.run();
       return EffectHandler.empty();
@@ -153,7 +155,7 @@ public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRu
    *         that receives an {@code S} value, and produces a {@code ResolveHandler<T, E>}
    */
   public static <S, T, E extends Throwable> Function<S, ResolveHandler<T, E>> partialResolver(
-    final ThrowingFunction<S, T, E> resolver
+    final ThrowingFunction<? super S, ? extends T, ? extends E> resolver
   ) {
     return value -> Maybe.fromResolver(() -> resolver.apply(value));
   }
@@ -183,7 +185,7 @@ public static <S, T, E extends Throwable> Function<S, ResolveHandler<T, E>> part
    *         that receives an {@code S} value, and produces an {@code EffectHandler<E>}
    */
   public static <S, E extends Throwable> Function<S, EffectHandler<E>> partialEffect(
-    final ThrowingConsumer<S, E> effect
+    final ThrowingConsumer<? super S, ? extends E> effect
   ) {
     return value -> Maybe.fromEffect(() -> effect.accept(value));
   }
@@ -217,7 +219,7 @@ public static <R extends AutoCloseable, E extends Throwable> ResourceHolder<R, E
    *         or run an effect using the prepared resource
    */
   public static <R extends Closeable, E extends Throwable> ResourceHolder<R, E> solveResource(
-    final ThrowingSupplier<R, E> supplier
+    final ThrowingSupplier<? extends R, ? extends E> supplier
   ) {
     return Maybe
       .fromResolver(supplier)
@@ -234,10 +236,10 @@ public static <R extends Closeable, E extends Throwable> ResourceHolder<R, E> so
    * @return a {@code Maybe} with the mapped value if present,
    *         {@link #nothing()} otherwise
    */
-  public <U> Maybe<U> map(final Function<T, U> mapper) {
+  public <U> Maybe<U> map(final Function<? super T, ? extends U> mapper) {
     return Maybe
       .fromOptional(this.value)
-      .resolve(mapper::apply)
+      .<U, Throwable>resolve(mapper::apply)
       .toMaybe();
   }
 
@@ -254,10 +256,11 @@ public <U> Maybe<U> map(final Function<T, U> mapper) {
    * @return a {@code Maybe} with the mapped value if present,
    *         {@link #nothing()} otherwise
    */
-  public <U> Maybe<U> flatMap(final Function<T, Maybe<U>> mapper) {
+  public <U> Maybe<U> flatMap(final Function<? super T, Maybe<? extends U>> mapper) {
     return Maybe
       .fromOptional(this.value)
       .resolve(mapper::apply)
+      .map(Commons::<Maybe<U>>cast)
       .orElseGet(Maybe::nothing);
   }
 
@@ -273,10 +276,12 @@ public <U> Maybe<U> flatMap(final Function<T, Maybe<U>> mapper) {
    * @return a {@link ResolveHandler} with either the resolved value, or the
    *         thrown exception to be handled
    */
-  public <U, E extends Throwable> ResolveHandler<U, E> resolve(final ThrowingFunction<T, U, E> resolver) {
+  public <U, E extends Throwable> ResolveHandler<U, E> resolve(
+    final ThrowingFunction<? super T, ? extends U, ? extends E> resolver
+  ) {
     try {
       return this.value
-        .map(Maybe.partialResolver(resolver))
+        .map(Maybe.<T, U, E>partialResolver(resolver))
         .orElseThrow();
     } catch (final NoSuchElementException e) {
       final var error = Commons.<E>cast(e);
@@ -293,10 +298,10 @@ public <U, E extends Throwable> ResolveHandler<U, E> resolve(final ThrowingFunct
    * @return an {@link EffectHandler} with either the thrown exception to be
    *         handled or nothing
    */
-  public <E extends Throwable> EffectHandler<E> runEffect(final ThrowingConsumer<T, E> effect) {
+  public <E extends Throwable> EffectHandler<E> runEffect(final ThrowingConsumer<? super T, ? extends E> effect) {
     try {
       return this.value
-        .map(Maybe.partialEffect(effect))
+        .map(Maybe.<T, E>partialEffect(effect))
         .orElseThrow();
     } catch (final NoSuchElementException e) {
       final var error = Commons.<E>cast(e);
diff --git a/src/main/java/io/github/joselion/maybe/ResolveHandler.java b/src/main/java/io/github/joselion/maybe/ResolveHandler.java
index ceebbe5..c742486 100644
--- a/src/main/java/io/github/joselion/maybe/ResolveHandler.java
+++ b/src/main/java/io/github/joselion/maybe/ResolveHandler.java
@@ -80,7 +80,7 @@ Optional<E> error() {
    * @param effect a function that receives the resolved value
    * @return the same handler to continue chainning operations
    */
-  public ResolveHandler<T, E> doOnSuccess(final Consumer<T> effect) {
+  public ResolveHandler<T, E> doOnSuccess(final Consumer<? super T> effect) {
     this.value.doOnRight(effect);
 
     return this;
@@ -96,7 +96,7 @@ public ResolveHandler<T, E> doOnSuccess(final Consumer<T> effect) {
    * @param effect a consumer function that receives the caught error
    * @return the same handler to continue chainning operations
    */
-  public <X extends Throwable> ResolveHandler<T, E> doOnError(final Class<X> ofType, final Consumer<X> effect) {
+  public <X extends Throwable> ResolveHandler<T, E> doOnError(final Class<X> ofType, final Consumer<? super X> effect) {
     this.value
       .leftToOptional()
       .filter(ofType::isInstance)
@@ -113,7 +113,7 @@ public <X extends Throwable> ResolveHandler<T, E> doOnError(final Class<X> ofTyp
    * @param effect a consumer function that receives the caught error
    * @return the same handler to continue chainning operations
    */
-  public ResolveHandler<T, E> doOnError(final Consumer<E> effect) {
+  public ResolveHandler<T, E> doOnError(final Consumer<? super E> effect) {
     this.value.doOnLeft(effect);
 
     return this;
@@ -131,7 +131,10 @@ public ResolveHandler<T, E> doOnError(final Consumer<E> effect) {
    * @return a handler containing a new value if an error instance of the
    *         provided type was caught. The same handler instance otherwise
    */
-  public <X extends E> ResolveHandler<T, E> catchError(final Class<X> ofType, final Function<X, T> handler) {
+  public <X extends Throwable> ResolveHandler<T, E> catchError(
+    final Class<X> ofType,
+    final Function<? super X, ? extends T> handler
+  ) {
     return this.value
       .leftToOptional()
       .filter(ofType::isInstance)
@@ -150,7 +153,7 @@ public <X extends E> ResolveHandler<T, E> catchError(final Class<X> ofType, fina
    * @return a handler containing a new value if an error was caught. The same
    *         handler instance otherwise
    */
-  public ResolveHandler<T, E> catchError(final Function<E, T> handler) {
+  public ResolveHandler<T, E> catchError(final Function<? super E, ? extends T> handler) {
     return this.value
       .mapLeft(handler)
       .mapLeft(ResolveHandler::<T, E>ofSuccess)
@@ -175,8 +178,8 @@ public ResolveHandler<T, E> catchError(final Function<E, T> handler) {
    * @return a new handler with either the resolved value or the error
    */
   public <S, X extends Throwable> ResolveHandler<S, X> resolve(
-    final ThrowingFunction<T, S, X> onSuccess,
-    final ThrowingFunction<E, S, X> onError
+    final ThrowingFunction<? super T, ? extends S, ? extends X> onSuccess,
+    final ThrowingFunction<? super E, ? extends S, ? extends X> onError
   ) {
     return this.value.unwrap(
       Maybe.partialResolver(onError),
@@ -194,7 +197,9 @@ public <S, X extends Throwable> ResolveHandler<S, X> resolve(
    *                 resolves another
    * @return a new handler with either the resolved value or an error
    */
-  public <S, X extends Throwable> ResolveHandler<S, X> resolve(final ThrowingFunction<T, S, X> resolver) {
+  public <S, X extends Throwable> ResolveHandler<S, X> resolve(
+    final ThrowingFunction<? super T, ? extends S, ? extends X> resolver
+  ) {
     return this.value
       .mapLeft(Commons::<X>cast)
       .unwrap(
@@ -214,8 +219,8 @@ public <S, X extends Throwable> ResolveHandler<S, X> resolve(final ThrowingFunct
    *         invoked callback
    */
   public <X extends Throwable> EffectHandler<X> runEffect(
-    final ThrowingConsumer<T, X> onSuccess,
-    final ThrowingConsumer<E, X> onError
+    final ThrowingConsumer<? super T, ? extends X> onSuccess,
+    final ThrowingConsumer<? super E, ? extends X> onError
   ) {
     return this.value.unwrap(
       Maybe.partialEffect(onError),
@@ -233,7 +238,7 @@ public <X extends Throwable> EffectHandler<X> runEffect(
    * @return a new {@link EffectHandler} representing the result of the success
    *         callback or containg the error
    */
-  public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingConsumer<T, X> effect) {
+  public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingConsumer<? super T, ? extends X> effect) {
     return this.value
       .mapLeft(Commons::<X>cast)
       .unwrap(
@@ -251,7 +256,7 @@ public <X extends Throwable> EffectHandler<X> runEffect(final ThrowingConsumer<T
    * @param mapper a function that receives the resolved value and produces another
    * @return a new handler with either the mapped value, or the previous error
    */
-  public <U> ResolveHandler<U, E> map(final Function<T, U> mapper) {
+  public <U> ResolveHandler<U, E> map(final Function<? super T, ? extends U> mapper) {
     return this.value
       .mapRight(mapper)
       .unwrap(
@@ -302,7 +307,7 @@ public T orElse(final T fallback) {
    *               another value
    * @return the resolved value if present. Another value otherwise
    */
-  public T orElse(final Function<E, T> mapper) {
+  public T orElse(final Function<? super E, ? extends T> mapper) {
     return this.value.unwrap(mapper, Function.identity());
   }
 
@@ -317,7 +322,7 @@ public T orElse(final Function<E, T> mapper) {
    *                 opration failed to resolve
    * @return the resolved value if present. Another value otherwise
    */
-  public T orElseGet(final Supplier<T> supplier) {
+  public T orElseGet(final Supplier<? extends T> supplier) {
     return this.value
       .rightToOptional()
       .orElseGet(supplier);
@@ -360,7 +365,7 @@ public T orThrow() throws E {
    * @return the resolved/handled value if present
    * @throws X a mapped exception
    */
-  public <X extends Throwable> T orThrow(final Function<E, X> mapper) throws X {
+  public <X extends Throwable> T orThrow(final Function<? super E, ? extends X> mapper) throws X {
     return this.value
       .rightToOptional()
       .orElseThrow(() -> mapper.apply(this.value.leftOrNull()));
@@ -444,7 +449,7 @@ public <R extends AutoCloseable> ResourceHolder<R, E> mapToResource(final Functi
    *         present or the error otherwise.
    */
   public <R extends AutoCloseable, X extends Throwable> ResourceHolder<R, X> solveResource(
-    final ThrowingFunction<T, R, X> solver
+    final ThrowingFunction<? super T, ? extends R, ? extends X> solver
   ) {
     return this.value
       .mapLeft(Commons::<X>cast)
diff --git a/src/main/java/io/github/joselion/maybe/ResourceHolder.java b/src/main/java/io/github/joselion/maybe/ResourceHolder.java
index fd4d756..f93b198 100644
--- a/src/main/java/io/github/joselion/maybe/ResourceHolder.java
+++ b/src/main/java/io/github/joselion/maybe/ResourceHolder.java
@@ -90,7 +90,9 @@ Optional<E> error() {
    * @return a {@link ResolveHandler} with either the value resolved or the thrown
    *         exception to be handled
    */
-  public <T, X extends Throwable> ResolveHandler<T, X> resolveClosing(final ThrowingFunction<R, T, X> resolver) {
+  public <T, X extends Throwable> ResolveHandler<T, X> resolveClosing(
+    final ThrowingFunction<? super R, ? extends T, ? extends X> resolver
+  ) {
     return this.value
       .mapLeft(Commons::<X>cast)
       .unwrap(
@@ -122,7 +124,9 @@ public <T, X extends Throwable> ResolveHandler<T, X> resolveClosing(final Throwi
    * @return an {@link EffectHandler} with either the thrown exception to be
    *         handled or nothing
    */
-  public <X extends Throwable> EffectHandler<X> runEffectClosing(final ThrowingConsumer<R, X> effect) {
+  public <X extends Throwable> EffectHandler<X> runEffectClosing(
+    final ThrowingConsumer<? super R, ? extends X> effect
+  ) {
     return this.value
       .mapLeft(Commons::<X>cast)
       .unwrap(
diff --git a/src/main/java/io/github/joselion/maybe/util/Either.java b/src/main/java/io/github/joselion/maybe/util/Either.java
index 5fb6af2..1e197ef 100644
--- a/src/main/java/io/github/joselion/maybe/util/Either.java
+++ b/src/main/java/io/github/joselion/maybe/util/Either.java
@@ -7,6 +7,8 @@
 
 import org.eclipse.jdt.annotation.Nullable;
 
+import io.github.joselion.maybe.helpers.Commons;
+
 /**
  * Either is a monadic wrapper that contains one of two possible values which
  * are represented as {@code Left} or {@code Right}. the values can be of
@@ -61,7 +63,7 @@ static <L, R> Either<L, R> ofRight(final R value) {
    * @param onRight a function to handle the right value if present
    * @return either the left or the right handled value
    */
-  <T> T unwrap(Function<L, T> onLeft, Function<R, T> onRight);
+  <T> T unwrap(Function<? super L, ? extends T> onLeft, Function<? super R, ? extends T> onRight);
 
   /**
    * Returns true if the {@code Left} value is present, false otherwise.
@@ -69,7 +71,7 @@ static <L, R> Either<L, R> ofRight(final R value) {
    * @return true if left is present, false otherwise
    */
   default boolean isLeft() {
-    return unwrap(left -> true, right -> false);
+    return this.unwrap(left -> true, right -> false);
   }
 
   /**
@@ -78,7 +80,7 @@ default boolean isLeft() {
    * @return true if right is present, false otherwise
    */
   default boolean isRight() {
-    return unwrap(left -> false, rigth -> true);
+    return this.unwrap(left -> false, rigth -> true);
   }
 
   /**
@@ -87,8 +89,8 @@ default boolean isRight() {
    * @param effect a consumer function that receives the left value
    * @return the same {@code Either} instance
    */
-  default Either<L, R> doOnLeft(final Consumer<L> effect) {
-    return unwrap(
+  default Either<L, R> doOnLeft(final Consumer<? super L> effect) {
+    return this.unwrap(
       left -> {
         effect.accept(left);
         return Either.ofLeft(left);
@@ -103,8 +105,8 @@ default Either<L, R> doOnLeft(final Consumer<L> effect) {
    * @param effect effect a consumer function that receives the right value
    * @return the same {@code Either} instance
    */
-  default Either<L, R> doOnRight(final Consumer<R> effect) {
-    return unwrap(
+  default Either<L, R> doOnRight(final Consumer<? super R> effect) {
+    return this.unwrap(
       Either::ofLeft,
       right -> {
         effect.accept(right);
@@ -120,8 +122,8 @@ default Either<L, R> doOnRight(final Consumer<R> effect) {
    * @param mapper a function that receives the left value and returns another
    * @return an {@code Either} instance with the mapped left value
    */
-  default <T> Either<T, R> mapLeft(final Function<L, T> mapper) {
-    return unwrap(
+  default <T> Either<T, R> mapLeft(final Function<? super L, ? extends T> mapper) {
+    return this.unwrap(
       left -> Either.ofLeft(mapper.apply(left)),
       Either::ofRight
     );
@@ -134,8 +136,8 @@ default <T> Either<T, R> mapLeft(final Function<L, T> mapper) {
    * @param mapper a function that receives the right value and returns another
    * @return an {@code Either} instance with the mapped right value
    */
-  default <T> Either<L, T> mapRight(final Function<R, T> mapper) {
-    return unwrap(
+  default <T> Either<L, T> mapRight(final Function<? super R, ? extends T> mapper) {
+    return this.unwrap(
       Either::ofLeft,
       right -> Either.ofRight(mapper.apply(right))
     );
@@ -153,8 +155,11 @@ default <T> Either<L, T> mapRight(final Function<R, T> mapper) {
    * @param rigthMapper a function that receives the right value and returns another
    * @return an {@code Either} instance with the mapped left or right value
    */
-  default <T, S> Either<T, S> map(final Function<L, T> leftMapper, final Function<R, S> rigthMapper) {
-    return unwrap(
+  default <T, S> Either<T, S> map(
+    final Function<? super L, ? extends T> leftMapper,
+    final Function<? super R, ? extends S> rigthMapper
+  ) {
+    return this.unwrap(
       left -> Either.ofLeft(leftMapper.apply(left)),
       right -> Either.ofRight(rigthMapper.apply(right))
     );
@@ -171,8 +176,10 @@ default <T, S> Either<T, S> map(final Function<L, T> leftMapper, final Function<
    * @param mapper a function that receives the left value and returns an {@code Either}
    * @return an {@code Either} instance with the mapped left value
    */
-  default <T> Either<T, R> flatMapLeft(final Function<L, Either<T, R>> mapper) {
-    return unwrap(mapper, Either::ofRight);
+  default <T> Either<T, R> flatMapLeft(final Function<? super L, ? extends Either<? extends T, ? extends R>> mapper) {
+    return this
+      .mapLeft(mapper)
+      .unwrap(Commons::cast, Either::ofRight);
   }
 
   /**
@@ -186,8 +193,10 @@ default <T> Either<T, R> flatMapLeft(final Function<L, Either<T, R>> mapper) {
    * @param mapper a function that receives the right value and returns an {@code Either}
    * @return an {@code Either} instance with the mapped right value
    */
-  default <T> Either<L, T> flatMapRight(final Function<R, Either<L, T>> mapper) {
-    return unwrap(Either::ofLeft, mapper);
+  default <T> Either<L, T> flatMapRight(final Function<? super R, ? extends Either<? extends L, ? extends T>> mapper) {
+    return this
+      .mapRight(mapper)
+      .unwrap(Either::ofLeft, Commons::cast);
   }
 
   /**
@@ -204,10 +213,13 @@ default <T> Either<L, T> flatMapRight(final Function<R, Either<L, T>> mapper) {
    * @return an {@code Either} instance with the mapped left or right value
    */
   default <T, S> Either<T, S> flatMap(
-    final Function<L, Either<T, S>> leftMapper,
-    final Function<R, Either<T, S>> rigthMapper
+    final Function<? super L, ? extends Either<? extends T, ? extends S>> leftMapper,
+    final Function<? super R, ? extends Either<? extends T, ? extends S>> rigthMapper
   ) {
-    return unwrap(leftMapper, rigthMapper);
+    return this
+      .mapLeft(leftMapper)
+      .mapRight(rigthMapper)
+      .unwrap(Commons::cast, Commons::cast);
   }
 
   /**
@@ -218,7 +230,7 @@ default <T, S> Either<T, S> flatMap(
    * @return the left value or a fallback
    */
   default L leftOrElse(final L fallback) {
-    return unwrap(Function.identity(), rigth -> fallback);
+    return this.unwrap(Function.identity(), rigth -> fallback);
   }
 
   /**
@@ -229,7 +241,7 @@ default L leftOrElse(final L fallback) {
    * @return the right value or a fallback
    */
   default R rightOrElse(final R fallback) {
-    return unwrap(left -> fallback, Function.identity());
+    return this.unwrap(left -> fallback, Function.identity());
   }
 
   /**
@@ -239,7 +251,7 @@ default R rightOrElse(final R fallback) {
    * @return the left value or null
    */
   default @Nullable L leftOrNull() {
-    return unwrap(Function.identity(), rigth -> null);
+    return this.unwrap(Function.identity(), rigth -> null);
   }
 
   /**
@@ -249,7 +261,7 @@ default R rightOrElse(final R fallback) {
    * @return the right value or null
    */
   default @Nullable R rightOrNull() {
-    return unwrap(left -> null, Function.identity());
+    return this.unwrap(left -> null, Function.identity());
   }
 
   /**
@@ -297,7 +309,7 @@ L value() {
     }
 
     @Override
-    public <T> T unwrap(final Function<L, T> onLeft, final Function<R, T> onRight) {
+    public <T> T unwrap(final Function<? super L, ? extends T> onLeft, final Function<? super R, ? extends T> onRight) {
       return onLeft.apply(this.value);
     }
 
@@ -351,7 +363,7 @@ R value() {
     }
 
     @Override
-    public <T> T unwrap(final Function<L, T> onLeft, final Function<R, T> onRight) {
+    public <T> T unwrap(final Function<? super L, ? extends T> onLeft, final Function<? super R, ? extends T> onRight) {
       return onRight.apply(this.value);
     }
 
diff --git a/src/main/java17/io/github/joselion/maybe/Maybe.java b/src/main/java17/io/github/joselion/maybe/Maybe.java
index 8004d27..17e9d87 100644
--- a/src/main/java17/io/github/joselion/maybe/Maybe.java
+++ b/src/main/java17/io/github/joselion/maybe/Maybe.java
@@ -97,7 +97,9 @@ public static <T> Maybe<T> fromOptional(final Optional<T> value) {
    * @return a {@link ResolveHandler} with either the value resolved or the thrown
    *         exception to be handled
    */
-  public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(final ThrowingSupplier<T, E> resolver) {
+  public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(
+    final ThrowingSupplier<? extends T, ? extends E> resolver
+  ) {
     try {
       return ResolveHandler.ofSuccess(resolver.get());
     } catch (Throwable e) { // NOSONAR
@@ -116,7 +118,7 @@ public static <T, E extends Throwable> ResolveHandler<T, E> fromResolver(final T
    * @return an {@link EffectHandler} with either the thrown exception to be
    *         handled or nothing
    */
-  public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRunnable<E> effect) {
+  public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRunnable<? extends E> effect) {
     try {
       effect.run();
       return EffectHandler.empty();
@@ -153,7 +155,7 @@ public static <E extends Throwable> EffectHandler<E> fromEffect(final ThrowingRu
    *         that receives an {@code S} value, and produces a {@code ResolveHandler<T, E>}
    */
   public static <S, T, E extends Throwable> Function<S, ResolveHandler<T, E>> partialResolver(
-    final ThrowingFunction<S, T, E> resolver
+    final ThrowingFunction<? super S, ? extends T, ? extends E> resolver
   ) {
     return value -> Maybe.fromResolver(() -> resolver.apply(value));
   }
@@ -183,7 +185,7 @@ public static <S, T, E extends Throwable> Function<S, ResolveHandler<T, E>> part
    *         that receives an {@code S} value, and produces an {@code EffectHandler<E>}
    */
   public static <S, E extends Throwable> Function<S, EffectHandler<E>> partialEffect(
-    final ThrowingConsumer<S, E> effect
+    final ThrowingConsumer<? super S, ? extends E> effect
   ) {
     return value -> Maybe.fromEffect(() -> effect.accept(value));
   }
@@ -217,7 +219,7 @@ public static <R extends AutoCloseable, E extends Throwable> ResourceHolder<R, E
    *         or run an effect using the prepared resource
    */
   public static <R extends Closeable, E extends Throwable> ResourceHolder<R, E> solveResource(
-    final ThrowingSupplier<R, E> supplier
+    final ThrowingSupplier<? extends R, ? extends E> supplier
   ) {
     return Maybe
       .fromResolver(supplier)
@@ -234,10 +236,10 @@ public static <R extends Closeable, E extends Throwable> ResourceHolder<R, E> so
    * @return a {@code Maybe} with the mapped value if present,
    *         {@link #nothing()} otherwise
    */
-  public <U> Maybe<U> map(final Function<T, U> mapper) {
+  public <U> Maybe<U> map(final Function<? super T, ? extends U> mapper) {
     return Maybe
       .fromOptional(this.value)
-      .resolve(mapper::apply)
+      .<U, Throwable>resolve(mapper::apply)
       .toMaybe();
   }
 
@@ -254,10 +256,11 @@ public <U> Maybe<U> map(final Function<T, U> mapper) {
    * @return a {@code Maybe} with the mapped value if present,
    *         {@link #nothing()} otherwise
    */
-  public <U> Maybe<U> flatMap(final Function<T, Maybe<U>> mapper) {
+  public <U> Maybe<U> flatMap(final Function<? super T, Maybe<? extends U>> mapper) {
     return Maybe
       .fromOptional(this.value)
       .resolve(mapper::apply)
+      .map(Commons::<Maybe<U>>cast)
       .orElseGet(Maybe::nothing);
   }
 
@@ -273,10 +276,12 @@ public <U> Maybe<U> flatMap(final Function<T, Maybe<U>> mapper) {
    * @return a {@link ResolveHandler} with either the resolved value, or the
    *         thrown exception to be handled
    */
-  public <U, E extends Throwable> ResolveHandler<U, E> resolve(final ThrowingFunction<T, U, E> resolver) {
+  public <U, E extends Throwable> ResolveHandler<U, E> resolve(
+    final ThrowingFunction<? super T, ? extends U, ? extends E> resolver
+  ) {
     try {
       return this.value
-        .map(Maybe.partialResolver(resolver))
+        .map(Maybe.<T, U, E>partialResolver(resolver))
         .orElseThrow();
     } catch (final NoSuchElementException e) {
       final var error = Commons.<E>cast(e);
@@ -293,10 +298,10 @@ public <U, E extends Throwable> ResolveHandler<U, E> resolve(final ThrowingFunct
    * @return an {@link EffectHandler} with either the thrown exception to be
    *         handled or nothing
    */
-  public <E extends Throwable> EffectHandler<E> runEffect(final ThrowingConsumer<T, E> effect) {
+  public <E extends Throwable> EffectHandler<E> runEffect(final ThrowingConsumer<? super T, ? extends E> effect) {
     try {
       return this.value
-        .map(Maybe.partialEffect(effect))
+        .map(Maybe.<T, E>partialEffect(effect))
         .orElseThrow();
     } catch (final NoSuchElementException e) {
       final var error = Commons.<E>cast(e);
diff --git a/src/main/java17/io/github/joselion/maybe/util/Either.java b/src/main/java17/io/github/joselion/maybe/util/Either.java
index c4d61ab..13498fe 100644
--- a/src/main/java17/io/github/joselion/maybe/util/Either.java
+++ b/src/main/java17/io/github/joselion/maybe/util/Either.java
@@ -7,6 +7,8 @@
 
 import org.eclipse.jdt.annotation.Nullable;
 
+import io.github.joselion.maybe.helpers.Commons;
+
 /**
  * Either is a monadic wrapper that contains one of two possible values which
  * are represented as {@code Left} or {@code Right}. the values can be of
@@ -61,7 +63,7 @@ static <L, R> Either<L, R> ofRight(final R value) {
    * @param onRight a function to handle the right value if present
    * @return either the left or the right handled value
    */
-  <T> T unwrap(Function<L, T> onLeft, Function<R, T> onRight);
+  <T> T unwrap(Function<? super L, ? extends T> onLeft, Function<? super R, ? extends T> onRight);
 
   /**
    * Returns true if the {@code Left} value is present, false otherwise.
@@ -69,7 +71,7 @@ static <L, R> Either<L, R> ofRight(final R value) {
    * @return true if left is present, false otherwise
    */
   default boolean isLeft() {
-    return unwrap(left -> true, right -> false);
+    return this.unwrap(left -> true, right -> false);
   }
 
   /**
@@ -78,7 +80,7 @@ default boolean isLeft() {
    * @return true if right is present, false otherwise
    */
   default boolean isRight() {
-    return unwrap(left -> false, rigth -> true);
+    return this.unwrap(left -> false, rigth -> true);
   }
 
   /**
@@ -87,8 +89,8 @@ default boolean isRight() {
    * @param effect a consumer function that receives the left value
    * @return the same {@code Either} instance
    */
-  default Either<L, R> doOnLeft(final Consumer<L> effect) {
-    return unwrap(
+  default Either<L, R> doOnLeft(final Consumer<? super L> effect) {
+    return this.unwrap(
       left -> {
         effect.accept(left);
         return Either.ofLeft(left);
@@ -103,8 +105,8 @@ default Either<L, R> doOnLeft(final Consumer<L> effect) {
    * @param effect effect a consumer function that receives the right value
    * @return the same {@code Either} instance
    */
-  default Either<L, R> doOnRight(final Consumer<R> effect) {
-    return unwrap(
+  default Either<L, R> doOnRight(final Consumer<? super R> effect) {
+    return this.unwrap(
       Either::ofLeft,
       right -> {
         effect.accept(right);
@@ -120,8 +122,8 @@ default Either<L, R> doOnRight(final Consumer<R> effect) {
    * @param mapper a function that receives the left value and returns another
    * @return an {@code Either} instance with the mapped left value
    */
-  default <T> Either<T, R> mapLeft(final Function<L, T> mapper) {
-    return unwrap(
+  default <T> Either<T, R> mapLeft(final Function<? super L, ? extends T> mapper) {
+    return this.unwrap(
       left -> Either.ofLeft(mapper.apply(left)),
       Either::ofRight
     );
@@ -134,8 +136,8 @@ default <T> Either<T, R> mapLeft(final Function<L, T> mapper) {
    * @param mapper a function that receives the right value and returns another
    * @return an {@code Either} instance with the mapped right value
    */
-  default <T> Either<L, T> mapRight(final Function<R, T> mapper) {
-    return unwrap(
+  default <T> Either<L, T> mapRight(final Function<? super R, ? extends T> mapper) {
+    return this.unwrap(
       Either::ofLeft,
       right -> Either.ofRight(mapper.apply(right))
     );
@@ -153,8 +155,11 @@ default <T> Either<L, T> mapRight(final Function<R, T> mapper) {
    * @param rigthMapper a function that receives the right value and returns another
    * @return an {@code Either} instance with the mapped left or right value
    */
-  default <T, S> Either<T, S> map(final Function<L, T> leftMapper, final Function<R, S> rigthMapper) {
-    return unwrap(
+  default <T, S> Either<T, S> map(
+    final Function<? super L, ? extends T> leftMapper,
+    final Function<? super R, ? extends S> rigthMapper
+  ) {
+    return this.unwrap(
       left -> Either.ofLeft(leftMapper.apply(left)),
       right -> Either.ofRight(rigthMapper.apply(right))
     );
@@ -171,8 +176,10 @@ default <T, S> Either<T, S> map(final Function<L, T> leftMapper, final Function<
    * @param mapper a function that receives the left value and returns an {@code Either}
    * @return an {@code Either} instance with the mapped left value
    */
-  default <T> Either<T, R> flatMapLeft(final Function<L, Either<T, R>> mapper) {
-    return unwrap(mapper, Either::ofRight);
+  default <T> Either<T, R> flatMapLeft(final Function<? super L, ? extends Either<? extends T, ? extends R>> mapper) {
+    return this
+      .mapLeft(mapper)
+      .unwrap(Commons::cast, Either::ofRight);
   }
 
   /**
@@ -186,8 +193,10 @@ default <T> Either<T, R> flatMapLeft(final Function<L, Either<T, R>> mapper) {
    * @param mapper a function that receives the right value and returns an {@code Either}
    * @return an {@code Either} instance with the mapped right value
    */
-  default <T> Either<L, T> flatMapRight(final Function<R, Either<L, T>> mapper) {
-    return unwrap(Either::ofLeft, mapper);
+  default <T> Either<L, T> flatMapRight(final Function<? super R, ? extends Either<? extends L, ? extends T>> mapper) {
+    return this
+      .mapRight(mapper)
+      .unwrap(Either::ofLeft, Commons::cast);
   }
 
   /**
@@ -204,10 +213,13 @@ default <T> Either<L, T> flatMapRight(final Function<R, Either<L, T>> mapper) {
    * @return an {@code Either} instance with the mapped left or right value
    */
   default <T, S> Either<T, S> flatMap(
-    final Function<L, Either<T, S>> leftMapper,
-    final Function<R, Either<T, S>> rigthMapper
+    final Function<? super L, ? extends Either<? extends T, ? extends S>> leftMapper,
+    final Function<? super R, ? extends Either<? extends T, ? extends S>> rigthMapper
   ) {
-    return unwrap(leftMapper, rigthMapper);
+    return this
+      .mapLeft(leftMapper)
+      .mapRight(rigthMapper)
+      .unwrap(Commons::cast, Commons::cast);
   }
 
   /**
@@ -218,7 +230,7 @@ default <T, S> Either<T, S> flatMap(
    * @return the left value or a fallback
    */
   default L leftOrElse(final L fallback) {
-    return unwrap(Function.identity(), rigth -> fallback);
+    return this.unwrap(Function.identity(), rigth -> fallback);
   }
 
   /**
@@ -229,7 +241,7 @@ default L leftOrElse(final L fallback) {
    * @return the right value or a fallback
    */
   default R rightOrElse(final R fallback) {
-    return unwrap(left -> fallback, Function.identity());
+    return this.unwrap(left -> fallback, Function.identity());
   }
 
   /**
@@ -239,7 +251,7 @@ default R rightOrElse(final R fallback) {
    * @return the left value or null
    */
   default @Nullable L leftOrNull() {
-    return unwrap(Function.identity(), rigth -> null);
+    return this.unwrap(Function.identity(), rigth -> null);
   }
 
   /**
@@ -249,7 +261,7 @@ default R rightOrElse(final R fallback) {
    * @return the right value or null
    */
   default @Nullable R rightOrNull() {
-    return unwrap(left -> null, Function.identity());
+    return this.unwrap(left -> null, Function.identity());
   }
 
   /**
@@ -291,7 +303,7 @@ record Left<L, R>(L value) implements Either<L, R> {
     }
 
     @Override
-    public <T> T unwrap(final Function<L, T> onLeft, final Function<R, T> onRight) {
+    public <T> T unwrap(final Function<? super L, ? extends T> onLeft, final Function<? super R, ? extends T> onRight) {
       return onLeft.apply(this.value);
     }
 
@@ -338,7 +350,7 @@ record Right<L, R>(R value) implements Either<L, R> {
     }
 
     @Override
-    public <T> T unwrap(final Function<L, T> onLeft, final Function<R, T> onRight) {
+    public <T> T unwrap(final Function<? super L, ? extends T> onLeft, final Function<? super R, ? extends T> onRight) {
       return onRight.apply(this.value);
     }