add `whenNone` to Lorentz
Clarification and motivation
Oddly Lorentz
has a macro for whenSome
that does nop
on the none
branch of an ifSome
, but does not have a macro to do the opposite.
Acceptance criteria
whenNone :: (s :-> a : s) -> (Maybe a : s :-> a : s)
is implemented as part of Lorentz.Macro
.