fn: introduce option type

this commit introduces many of the most common functions you will
want to use with the Option type. Not all of them are used
immediately in this PR.

fn/option.go

@@ -0,0 +1,149 @@
+package fn
+
+// Option[A] represents a value which may or may not be there. This is very
+// often preferable to nil-able pointers.
+type Option[A any] struct {
+	isSome bool
+	some   A
+}
+
+// Some trivially injects a value into an optional context.
+//
+// Some : A -> Option[A].
+func Some[A any](a A) Option[A] {
+	return Option[A]{
+		isSome: true,
+		some:   a,
+	}
+}
+
+// None trivially constructs an empty option
+//
+// None : Option[A].
+func None[A any]() Option[A] {
+	return Option[A]{}
+}
+
+// ElimOption is the universal Option eliminator. It can be used to safely
+// handle all possible values inside the Option by supplying two continuations.
+//
+// ElimOption : (Option[A], () -> B, A -> B) -> B.
+func ElimOption[A, B any](o Option[A], b func() B, f func(A) B) B {
+	if o.isSome {
+		return f(o.some)
+	}
+
+	return b()
+}
+
+// UnwrapOr is used to extract a value from an option, and we supply the default
+// value in the case when the Option is empty.
+//
+// UnwrapOr : (Option[A], A) -> A.
+func (o Option[A]) UnwrapOr(a A) A {
+	if o.isSome {
+		return o.some
+	}
+
+	return a
+}
+
+// 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.
+//
+// WhenSome : (Option[A], A -> ()) -> ().
+func (o Option[A]) WhenSome(f func(A)) {
+	if o.isSome {
+		f(o.some)
+	}
+}
+
+// IsSome returns true if the Option contains a value
+//
+// IsSome : Option[A] -> bool.
+func (o Option[A]) IsSome() bool {
+	return o.isSome
+}
+
+// IsNone returns true if the Option is empty
+//
+// IsNone : Option[A] -> bool.
+func (o Option[A]) IsNone() bool {
+	return !o.isSome
+}
+
+// FlattenOption joins multiple layers of Options together such that if any of
+// the layers is None, then the joined value is None. Otherwise the innermost
+// Some value is returned.
+//
+// FlattenOption : Option[Option[A]] -> Option[A].
+func FlattenOption[A any](oo Option[Option[A]]) Option[A] {
+	if oo.IsNone() {
+		return None[A]()
+	}
+	if oo.some.IsNone() {
+		return None[A]()
+	}
+
+	return oo.some
+}
+
+// ChainOption transforms a function A -> Option[B] into one that accepts an
+// Option[A] as an argument.
+//
+// ChainOption : (A -> Option[B]) -> Option[A] -> Option[B].
+func ChainOption[A, B any](f func(A) Option[B]) func(Option[A]) Option[B] {
+	return func(o Option[A]) Option[B] {
+		if o.isSome {
+			return f(o.some)
+		}
+
+		return None[B]()
+	}
+}
+
+// MapOption transforms a pure function A -> B into one that will operate
+// inside the Option context.
+//
+// MapOption : (A -> B) -> Option[A] -> Option[B].
+func MapOption[A, B any](f func(A) B) func(Option[A]) Option[B] {
+	return func(o Option[A]) Option[B] {
+		if o.isSome {
+			return Some(f(o.some))
+		}
+
+		return None[B]()
+	}
+}
+
+// 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.
+//
+// LiftA2Option : ((A, B) -> C) -> (Option[A], Option[B]) -> Option[C].
+func LiftA2Option[A, B, C any](
+	f func(A, B) C,
+) func(Option[A], Option[B]) Option[C] {
+
+	return func(o1 Option[A], o2 Option[B]) Option[C] {
+		if o1.isSome && o2.isSome {
+			return Some(f(o1.some, o2.some))
+		}
+
+		return None[C]()
+	}
+}
+
+// Alt chooses the left Option if it is full, otherwise it chooses the right
+// option. This can be useful in a long chain if you want to choose between
+// many different ways of producing the needed value.
+//
+// Alt : Option[A] -> Option[A] -> Option[A].
+func (o Option[A]) Alt(o2 Option[A]) Option[A] {
+	if o.isSome {
+		return o
+	}
+
+	return o2
+}