[Python-ideas] Maybe/Option builtin

[Devin Jeanpierre]

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.

Python has no way to represent "I have a value, and that value is None".

e.g. what is the difference between x.get('a') and x.get('b') for x ==
{'a': None} ?

Otherwise, agree 100% . And anyway, retrofitting this into Python
can't really work.

-- Devin

> 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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