[IronPython] Naming and resolution of generic types (complete!)

[Dino Viehland]

I apologize for the extreme delay on this reply...  This mail got stuck in our mailing list somewhere and didn't come out until today.

The way we've chosen to solve this is to go w/ the single type that allows you to disambiguate after the fact.  If you have System.IComparable (another good example is System.Nullable which has both generic & non-generic versions) we build one 'type' that allows you to disambiguate using __getitem__.

This is the same way you need to get a generic instantiation (e.g. if there were no IComparable conflicts you'd need to do IComparable[int] to get the specific instantiation) so it fits in nicely.  Otherwise the type is the non-generic version.

It would seem our str/repr of this type is pretty bad right now (showing you only the generic version) - we should make that more helpful (e.g. something like <type IComparable, IComparable[T]> maybe ?).

I guess the other remaining issue is how to get access to the generic type that isn't bound to a specific instantiation - currently this isn't much of a problem w/ IronPython as there's not much you can actually do w/ the open generic type, but we should probably solve it long term.

I've opened 2 bugs - the first one (str/repr of the type) should be easy to fix, but some thought probably needs to go into the open generic type issue.  We could make some __ __ method to do this, allow indexing by None, or something else - neither of those sounds particularly wonderful.


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-----Original Message-----
From: users-bounces at lists.ironpython.com [mailto:users-bounces at lists.ironpython.com] On Behalf Of Brian Lloyd
Sent: Friday, March 31, 2006 12:43 PM
To: pythondotnet at python.org
Cc: users at lists.ironpython.com
Subject: [IronPython] Naming and resolution of generic types (complete!)


Hi all - I'm cross-posting this to the IP list as the subject seems to
be an open issue there too.

I'm working on generics support for Python for .NET, and there a couple
of thorny issues that could use some discussion regarding naming and
resolution of generic types.

In C# you can explicitly name a generic type, a la Dictionary<,>. That
syntax won't work in Python, of course. Looking at IP, it seems to allow
the apparent Python name to be the unmangled IL name so you can
naturally use the name.

The problem is that the mangled name is illegal as a python name, and
the unmangled name isn't guaranteed to be unique - you can potentially
have any number of generic types as well as a non-generic type with the
same base name in the same namespace:

namespace Spam {

    public class SpamCan {}
    public class SpamCan<T> {}
    public class SpamCan<T, V> {}
    ...
}

I imagine that maybe IP has enough information available at compile-time
to do the right thing for some common usage (binding and instantiating
the types), but the overloaded name can still be ambiguous. A real-life
example of this is System.IComparable - in IP, it doesn't seem possible
to get to the non-generic version of IComparable anymore (or at least it
was not obvious to me how to do it):

>>> import System
>>> System.IComparable
<type 'IComparable`1'>

It seems like we need to figure out some acceptable way of spelling
generic type names explicitly in Python (an equivalent to IComparable<>
in C#) that works within the existing syntax.

There don't appear to be a lot of great options :( It has to be a valid
Python name to work in an import statement, so that rules out strange
operator shenanigans akin to the [] hack used for generic type binding.

One would be to mimic the existing IL mangling in some way, a la:

>From System import IComparable   # the non-generic type
>From System import IComparable_1 # the 1-param generic

# or
from System import IComparable_T
from System.Collections.Generic import Dictionary_TT

These are all pretty gross, and still don't totally prevent hiding of
legit non-generic classes with those names (though it makes it less
likely that names will be hidden than the current approach).

I suppose another approach would be to continue to have only one type
end up with the simple name, but provide a way to disambiguate it after
the fact:

>>> import System
>>> System.IComparable
<type 'IComparable`1'>

>>> # abandon all hope, ye who enter here...
>>> NonGenericIComparable = System.IComparable<<0
>>> OneParamGenericIComparable = System.IComparable<<1
>>> TwoParamVersionIfThereWasOne = System.IComparable<<2

That feels to me like it violates Python Zen in several ways, though.

Thoughts?


-Brian

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