initial checkin

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

option/monoid.go

@@ -0,0 +1,38 @@
+package option
+
+import (
+	F "github.com/ibm/fp-go/function"
+	M "github.com/ibm/fp-go/monoid"
+	S "github.com/ibm/fp-go/semigroup"
+)
+
+func Semigroup[A any]() func(S.Semigroup[A]) S.Semigroup[Option[A]] {
+	return func(s S.Semigroup[A]) S.Semigroup[Option[A]] {
+		concat := s.Concat
+		return S.MakeSemigroup(
+			func(x, y Option[A]) Option[A] {
+				return MonadFold(x, F.Constant(y), func(left A) Option[A] {
+					return MonadFold(y, F.Constant(x), func(right A) Option[A] {
+						return Some(concat(left, right))
+					})
+				})
+			},
+		)
+	}
+}
+
+// Monoid returning the left-most non-`None` value. If both operands are `Some`s then the inner values are
+// concatenated using the provided `Semigroup`
+//
+// | x       | y       | concat(x, y)       |
+// | ------- | ------- | ------------------ |
+// | none    | none    | none               |
+// | some(a) | none    | some(a)            |
+// | none    | some(b) | some(b)            |
+// | some(a) | some(b) | some(concat(a, b)) |
+func Monoid[A any]() func(S.Semigroup[A]) M.Monoid[Option[A]] {
+	sg := Semigroup[A]()
+	return func(s S.Semigroup[A]) M.Monoid[Option[A]] {
+		return M.MakeMonoid(sg(s).Concat, None[A]())
+	}
+}