[Python-ideas] Optional static typing -- late to the party

[David Mertz]

On Wed, Aug 20, 2014 at 11:14 AM, Bob Ippolito <bob at redivi.com> wrote:
> There's the misunderstanding: PyContracts style contracts are not "easy
enough" in Haskell.

Well, for example, I found something like this for Haskell:

newtype Digit = Digit { digitVal :: Int }
  deriving (Eq, Ord, Show)
mkDigit :: Int -> Maybe Digit
mkDigit n
  | n >= 0 && n < 10 = Just (Digit n)
  | otherwise = Nothing


OK, sure it's not just one line.  But now we have a predicate-restricted
data type right in the type system.  If we introduce a static type system
in Python, I'd like it to be able to do that.  PyContracts--or something
close to it--lets us do that in a DSL.  But I would be equally happy to use
some Python code that could be tucked away but reused.  E.g., completely
hypothetical syntax:

class Digit(mypy.Int):

    def __predicate__(self):

        return 0 <= self < 10

def times_modulo(incr: Digit, num: Int) -> Digit:

    result = 0

    for i in range(num):

        result += incr

        result %= 10

    return result


I could just stick Digit and all my other custom types in 'mytypes.py', and
the actual annotations would have a richer type system, plus remain
readable (if I chose decent names for the types).

> mypy's types are not like a C type system. There are a few missing
features that are being worked on, but it is much better than some here
perceive it to be.
>
>
> On Wednesday, August 20, 2014, David Mertz <mertz at gnosis.cx> wrote:
>>
>> Hi Bob,
>>
>> I enjoyed watching your talk (on video, not live), and I certainly see
the appeal of a Haskell-like type system.  However, what we seem to be
discussing here with mypy annotations looks a lot closer to a C type system
than to a Haskell type system.  All those things that PyContracts get us
are easy enough in Haskell--why aim so low if we are thinking of a change
in Python?
>>
>>
>> On Wed, Aug 20, 2014 at 10:42 AM, Bob Ippolito <bob at redivi.com> wrote:
>>>
>>> On Wed, Aug 20, 2014 at 10:23 AM, David Mertz <mertz at gnosis.cx> wrote:
>>>>
>>>> I have to say that I find the capabilities in PyContracts--which are
absent currently in mypy--to be very compelling.  If we are going to add
encouragement to use type annotations, adding richer types seems extremely
worthwhile.
>>>
>>>
>>> It's easy to check any expressible condition at runtime, but this is
not the case ahead of time as with a static analysis tool or linter. It'd
be great to have this sort of capability in mypy, but what you're asking
for is at the fringes of current research and will not be practical to add
to Python any time soon.
>>>
>>> -bob
>>>
>>
>>
>>
>> --
>> Keeping medicines from the bloodstreams of the sick; food
>> from the bellies of the hungry; books from the hands of the
>> uneducated; technology from the underdeveloped; and putting
>> advocates of freedom in prisons.  Intellectual property is
>> to the 21st century what the slave trade was to the 16th.




--
Keeping medicines from the bloodstreams of the sick; food
from the bellies of the hungry; books from the hands of the
uneducated; technology from the underdeveloped; and putting
advocates of freedom in prisons.  Intellectual property is
to the 21st century what the slave trade was to the 16th.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20140820/d3976c35/attachment-0001.html>