xtofl/fmappy

fmappy's Issues

Add a way to register/use the Optional functor

Some functions need None as an argument, some need to be shielded from it.

def is_none(arg):
  return arg is None

def triple(arg):
  return arg + arg + arg

assert fmap(is_none, (1, 2, 3, None, 4)) == (False, False, False, True, False)
with pytest.raises():
  fmap(triple, (1, 2, 3, None))

We need a nice way for the client code to indicate this.

• Simplest solution would be to treat Optional as a non-registered case: fmap_optional(triple, (1, 2, None)) == (3, 6, None).
• A registration and explicit lookup could work: fmap[Optional](triple, (1, 2, None)) == (3, 6, None)
• ...?