Fixed optional on nil value (#23)

README.md

@@ -235,6 +235,7 @@ Collectors:
 - Filtering
 - Reducing
 - ToSlice
+- ToMap*
 
 Check the [godoc](https://pkg.go.dev/github.com/seborama/fuego/v11) for full details.
 
@@ -282,14 +283,20 @@ Focus on _**what**_ needs doing in your streams (and delegate the details of the
 
 ## [Golang, Receivers and Functions](#golang-receivers-and-functions)
 
-Some tests (e.g. TestCollector_Filtering) are using receiver Getter methods in guise of `Function[T, R any]`. Here is the explanation how this is possible.
+Some tests (e.g. `TestCollector_Filtering`) are using receiver Getter methods in guise of `Function[T, R any]`. Here is the explanation how this is possible.
 
 `Function[T, R any]` is defined as `func(T) R`.
 
 A method Getter is typically defined as `func (T) Property() R {...}`.
 
 While `Property()` does not take any argument, Go allows `T.Property` be called as `T.Property(t)`, where `t` is the receiver. This is a `func(T) R` and hence a `Function[T, R any]`.
 
+Example - `TestCollector_Filtering`:
+
+`employee.Department(employees[0])` is the same as `employees[0].Department()`, and of course, they both return a `string`.
+
+The first syntax has one advantage for our purpose though: it is a `Function[T, R any]`.
+
 [(toc)](#table-of-content)
 
 ## [Known limitations](#known-limitations)

collector.go

@@ -162,6 +162,8 @@ func Reducing[T any](f2 BiFunction[T, T, T]) Collector[T, Optional[T], T] {
 	}
 
 	finisher := func(e Optional[T]) T {
+		// alternative:
+		// return e.OrElse(*new(T))
 		return e.Get()
 	}
 

optional.go

@@ -1,10 +1,10 @@
 package fuego
 
+import (
+	"reflect"
+)
+
 // Optional is a container object which may or may not contain a value (NO: nil is considered a non-value).
-// IMPORTANT NOTE:
-// Currently, Go 1.18 does not permit nil generic types.
-// See: https://github.com/golang/go/issues/22729
-//
 // See IsPresent().
 //
 // Additional methods that depend on the presence or absence of a contained value are provided,
@@ -105,16 +105,28 @@ func (o Optional[T]) Map(f Function[T, Any]) Optional[Any] {
 }
 
 // OptionalOf returns an Optional describing the given (NO: non-nil) value.
-// IMPORTANT NOTE:
 // Currently, Go 1.18 does not permit nil generic types.
 // See: https://github.com/golang/go/issues/22729
 func OptionalOf[T any](val T) Optional[T] {
 	return Optional[T]{
 		value:   val,
-		present: true,
+		present: !isNil(val),
 	}
 }
 
+func isNil(v any) bool {
+	// hat tip to TeaEntityLab/fpGo:
+	// https://github.com/TeaEntityLab/fpGo/blob/5a35fcbc23e384be5f9b33069e3e3ecc9c661bf4/fp.go#L1218
+	val := reflect.ValueOf(v)
+	kind := reflect.ValueOf(v).Kind()
+
+	if kind == reflect.Ptr {
+		return val.IsNil()
+	}
+
+	return !val.IsValid()
+}
+
 // OptionalEmpty returns an empty Optional instance. No value is present for this Optional.
 func OptionalEmpty[T any]() Optional[T] {
 	return Optional[T]{

optional_test.go

@@ -16,11 +16,9 @@ func TestOptional_OptionalOf_ZeroValue_IsPresent_True(t *testing.T) {
 	assert.True(t, o.IsPresent())
 }
 
-func TestOptional_OptionalOf_Nil_Panics(t *testing.T) {
-	// IMPORTANT NOTE:
-	// Currently, Go 1.18 does not permit nil generic types.
-	// See: https://github.com/golang/go/issues/22729
-	// assert.PanicsWithValue(t, PanicNilNotPermitted, func() { OptionalOf[*string](nil) })
+func TestOptional_OptionalOf_Nil_IsPresent_False(t *testing.T) {
+	o := OptionalOf[*string](nil)
+	assert.False(t, o.IsPresent())
 }
 
 func TestOptional_OptionalEmpty_IsPresent_False(t *testing.T) {