[IronPython] LINQ from IronPython

[Dody Gunawinata]

I am not well averse with generator expression, but I think it will have
problem supporting the 9 keywords in LINQ Query Comprehension Syntax so
sticking to extension methods would make more sense.

C# LINQ Query comprehension is purely syntactic (
http://blogs.msdn.com/ericlippert/archive/2009/12/07/query-transformations-are-syntactic.aspx
),

On Tue, Dec 22, 2009 at 6:59 AM, Jeff Hardy <jdhardy at gmail.com> wrote:

> This is one of those things that I've been pondering for a while, so I
> thougt I'd throw it out there and see what everyone else thinks (I
> fully expect to hear that I'm crazy...). I'd like to be able to query
> an existing data model (Entity Framework in this case, but that's not
> important) from IronPython without pulling all of the data to the
> client. By my read there are two necessary things and one nice-to-have
> to enable this.
>
> This is just a bunch of random thoughts about how to get LINQ support
> into IronPython; I know nothing about the details of the IronPython
> compiler, so I'm making no assumptions about the difficulty of any of
> this.
>
> Extension Methods
> --
> The biggest hurdle (but probably the most fruitful) is enabling
> extension methods. Unfortunately, I don't know enough about how
> IronPython looks up methods on .NET classes to know how easy this is,
> so I'll just ramble about possible syntaxes and assume the lookups
> will work.
>
> The first option is identical to C#:
>   from System import Linq    # or...
>   import System.Linq
>
> Either form pulls in all extension methods in all public, static types
> in that namespace.
>
> The second option is similar to C#, but requires knowing the extension
> class as well:
>   from System.Linq import Queryable
>
> This form would only pull in extension methods defined in the Queryable
> class.
>
> I prefer the second option over the first ('explicit is better than
> implicit'), but both have problems in not being completely clear what
> is happening.
>
> Another option is to be completely explicit:
>   clr.Extend(myObj, System.Linq.Queryable)
>
> It's nice and explicit about what is happening, but could get tedious
> quickly.
>
> Expression Trees (C# lambdas)
> --
> Trying to do
>   Queryable.Where(entities.Customers, lambda c: c.Id % 2 == 0))
> gives (not surprisingly)
>   The type arguments for method 'Where' cannot be inferred from the usage.
>
> Putting in the generic argument manually
>   Queryable.Where[Customer](entities.Customers, lambda c: c.Id % 2 == 0))
> I get (again, not surprisingly)
>   `expected Expression[Func[Course, int, bool]], got function`
>
> So, at the very least lambdas would have to be convertible to
> Expression<> objects. I know the DLR uses expression trees internally,
> but I don't know if they're available at the proper time for that
> conversion to be possible. The type inference doesn't seem anymore
> difficult than what's already done, but who knows...
>
> Generator Expressions as LINQ exprssions
> --
> With extension methods and expression trees, the Q in LINQ is handled;
> now the LIN ('Language INtegrated') needs to be filled in.
>
> Most of the LINQ queries I do are of the form:
>   from customer in entities.Customers
>   where customer.Name == name
>   select customer
>
> This just so happens to look a lot like
>   customer for customer in entities.Customers if customer.Name == name
>
> i.e. a generator expression.
>
> Generator expressions and LINQ queries have a lot in common - chief
> amongst them is that they're both lazy. Would it be possible, if
> `entities.Customers ` implements IQueryable, for a generator
> expression to be converted into a LINQ query? Would it even be
> desirable?
>
> The alternative is
>   entities.Customers.Where(lambda c: c.Name == name)
>
> which is perfectly fine, but something more would be nice.
> --
>
> One thing all of this is that it doesn't change the syntax of Python
> at all, just the semantics. I'm not sure if that's good or bad,
> though.
>
> Thoughts?
>
> - Jeff
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