feat: OnError - do a side effect

README.md

@@ -130,6 +130,7 @@ To construct an IO one may use the following functions:
 - `io.Delay[A any](f func()IO[A]) IO[A]` - represents a function as a plain IO
 - `io.Fold[A any, B any](io IO[A], f func(a A)IO[B], recover func (error)IO[B]) IO[B]` - handles both happy and sad paths.
 - `io.Recover[A any](io IO[A], recover func(err error)IO[A])IO[A]` - handle only sad path and recover some errors to happy path.
+- `io.OnError[A any](io IO[A], onError func(err error) IO[fun.Unit]) IO[A]` - OnError executes a side effect when there is an error.
 
 ### Manipulation
 

io/io.go

@@ -212,6 +212,13 @@ func Recover[A any](io IO[A], recover func(err error) IO[A]) IO[A] {
 	return Fold(io, Lift[A], recover)
 }
 
+// OnError executes a side effect when there is an error.
+func OnError[A any](io IO[A], onError func(err error) IO[fun.Unit]) IO[A] {
+	return Fold(io, Lift[A], func(err error) IO[A] {
+		return AndThen(onError(err), Fail[A](err))
+	})
+}
+
 // Sequence takes a slice of IOs and returns an IO that will contain a slice of results.
 // It'll fail if any of the internal computations fail.
 func Sequence[A any](ioas []IO[A]) (res IO[[]A]) {

io/io_test.go

@@ -38,10 +38,15 @@ func TestFinally(t *testing.T) {
 	errorMessage := "on purpose failure"
 	failure := io.Fail[string](errors.New(errorMessage))
 	finalizerExecuted := false
+	onErrorExecuted := false
 	fin := io.Finally(failure, io.FromPureEffect(func() { finalizerExecuted = true }))
-	_, err := io.UnsafeRunSync(fin)
+	oe := io.OnError(fin, func(err error) io.IO[fun.Unit] {
+		return io.FromPureEffect(func() { onErrorExecuted = true })
+	})
+	_, err := io.UnsafeRunSync(oe)
 	assert.Error(t, err, errorMessage)
 	assert.True(t, finalizerExecuted)
+	assert.True(t, onErrorExecuted)
 }
 
 func Nats(count int) (ios []io.IO[int]) {