[Python-ideas] Maybe/Option builtin

[Haoyi Li]

I've been using [x] and [] for my Option type. It works great, even can be
chained monadically using for comprehensions =)


On Wed, May 21, 2014 at 4:50 PM, Steven D'Aprano <steve at pearwood.info>wrote:

> On Wed, May 21, 2014 at 10:49:36AM -0400, mulhern wrote:
> > I feel that a Maybe/Option type, analogous to the types found in Haskell
> or
> > OCaml would actually be useful in Python. The value analogous to the None
> > constructor should be Python's None.
> >
> > Obviously, it wouldn't give the type-checking benefits that it gives in
> > statically checked languages, but every use of a Maybe object as if it
> were
> > the contained object would give an error, alerting the user to the fact
> > that None is a possibility and allowing them to address the problem
> sooner
> > rather than later.
>
> Since this is a Python list, you shouldn't take it for granted that we
> will be familiar with Haskell or Ocaml, nor expect us to go off and
> study those languages well enough to understand what a Maybe object is
> used for or how it will fit into Python's execution model.
>
> I'm no expect on either of those two languages, but it seems to me that
> that the only point of Maybe is to allow the static type checker to
> distinguish between (for example) "this function returns a string" and
> "this function returns either a string or None". Since Python doesn't do
> static, compile-time type checks, I'm having difficulty in seeing what
> would be the point of Maybe in Python.
>
> As I said, I'm not an expert in Haskell, but I don't think this proposal
> is helpful, and certainly not helpful enough to make it a built-in or
> standard part of the language. If I have missed some benefit of Maybe,
> please explain how it would apply in Python terms.
>
>
>
> > I feel that it would be kind of tricky to implement it as a class.
> > Something like:
> >
> > class Maybe(object):
> >     def __init__(self, value=None):
> >         self.value = value
> >     def value(self):
> >         return self.value
> >
> > is a start but I'm not able to see how to make
> >
> > if Maybe():
> >     print("nothing") # never prints
>
> In Python 2, define __nonzero__. In Python 3, define __bool__.
>
> https://docs.python.org/2/reference/datamodel.html#object.__nonzero__
> https://docs.python.org/3/reference/datamodel.html#object.__bool__
>
>
> > but
> >
> > if Maybe({}):
> >     print("yes a value") #always prints
> >
> > which is definitely the desired behaviour.
>
> Not to me it isn't. This goes against the standard Python convention
> that empty containers are falsey. Since Maybe({}) wraps an empty dict,
> it should be considered a false value.
>
>
>
>
> --
> Steven
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