The type for option values. Either None or a value Some v.

none is None.

some v is Some v.

value o ~default is v if o is Some v and default otherwise.

bind o f is f v if o is Some v and None if o is None.

join oo is Some v if oo is Some (Some v) and None otherwise.

map f o is None if o is None and Some (f v) if o is Some v.

product o0 o1 is Some (v0, v1) if o0 is Some v0 and o1 is Some v1 and None otherwise.

• since 5.5

fold ~none ~some o is none if o is None and some v if o is Some v.

iter f o is f v if o is Some v and () otherwise.

blend f o1 o2 is o1 if o2 is None, o2 if o1 is None, and Some (f v1 v2) if o1 is Some v1 and o2 is Some v2.

• since 5.5

for_all p behaves like List.for_all p on a list of zero or one element:

• for_all p None is true,
• for_all p (Some v) is p v.

• since 5.5

exists p behaves like List.exists p on a list of zero or one element:

• exists p None is false,
• exists p (Some v) is p v.

• since 5.5

is_none o is true if and only if o is None.

is_some o is true if and only if o is Some o.

equal eq o0 o1 is true if and only if o0 and o1 are both None or if they are Some v0 and Some v1 and eq v0 v1 is true.

compare cmp o0 o1 is a total order on options using cmp to compare values wrapped by Some _. None is smaller than Some _ values.

to_result ~none o is Ok v if o is Some v and Error none otherwise.

to_list o is [] if o is None and [v] if o is Some v.

to_seq o is o as a sequence. None is the empty sequence and Some v is the singleton sequence containing v.