jacinda-3.3.0.2: lib/maybe.jac

fn isJust(x) :=
  option #f ([:#t) x;

fn isNothing(x) :=
  option #t ([:#f) x;

{. Haskell's (<|>)
fn alternative(a, b) :=
  option a Some b;

fn lift(f,x,y) :=
  option None (λz.f z"y) x;