fn: add utility funcs/method for option type

fn/option.go

@@ -48,6 +48,22 @@ func (o Option[A]) UnwrapOr(a A) A {
 	return a
 }
 
+// UnwrapOrFunc is used to extract a value from an option, and we supply a
+// thunk to be evaluated in the case when the Option is empty.
+func (o Option[A]) UnwrapOrFunc(f func() A) A {
+	return ElimOption(o, f, func(a A) A { return a })
+}
+
+// UnwrapOrFuncErr is used to extract a value from an option, and we supply a
+// thunk to be evaluated in the case when the Option is empty.
+func (o Option[A]) UnwrapOrFuncErr(f func() (A, error)) (A, error) {
+	if o.isSome {
+		return o.some, nil
+	}
+
+	return f()
+}
+
 // WhenSome is used to conditionally perform a side-effecting function that
 // accepts a value of the type that parameterizes the option. If this function
 // performs no side effects, WhenSome is useless.
@@ -117,6 +133,19 @@ func MapOption[A, B any](f func(A) B) func(Option[A]) Option[B] {
 	}
 }
 
+// MapOptionZ transforms a pure function A -> B into one that will operate
+// inside the Option context. Unlike MapOption, this function will return the
+// default/zero argument of the return type if the Option is empty.
+func MapOptionZ[A, B any](o Option[A], f func(A) B) B {
+	var zero B
+
+	if o.IsNone() {
+		return zero
+	}
+
+	return f(o.some)
+}
+
 // LiftA2Option transforms a pure function (A, B) -> C into one that will
 // operate in an Option context. For the returned function, if either of its
 // arguments are None, then the result will be None.