> On Oct 29, 2017, at 1:34 PM, Chris Lattner via swift-evolution
> <[email protected]> wrote:
>
>>
>> On Oct 29, 2017, at 8:23 AM, Chris Lattner via swift-evolution
>> <[email protected]> wrote:
>>
>>
>>> On Oct 29, 2017, at 4:04 AM, Lukas Stabe <[email protected]> wrote:
>>>
>>>> On 28. Oct 2017, at 23:10, Chris Lattner via swift-evolution
>>>> <[email protected]> wrote:
>>>>
>>>> … which is to say, exactly identical to the Python version except that new
>>>> variables need to be declared with let/var. This can be done by blessing
>>>> Python.Object (which is identical to “PyObject*” at the machine level)
>>>> with some special dynamic name lookup behavior: Dot syntax turns into a
>>>> call to PyObject_GetAttrString, subscripts turn into PyObject_GetItem,
>>>> calls turn into PyObject_Call, etc. ARC would be implemented with INCREF
>>>> etc.
>>>
>>> That sounds like a very interesting prospect. Do you think it would make
>>> sense to make the language features that facilitate this (dynamic dispatch
>>> of method calls, property accesses, subscript and ARC) available to Swift
>>> classes annotated in some way, so that interop like this can be implemented
>>> as a library without special treatment by the Swift compiler? This could
>>> also enable more dynamic DSL like features.
>>
>> I haven’t explored enough of the design space to be sure, but I’d want to
>> make sure that a library version of this could be done without giving up
>> ergonomics of the result. If you were interested in being able to interop
>> with other languages that are dynamically typed and reference counted, then
>> something like this could be possible in principle:
>
> Thinking about the Perl case makes it clear to me that this should not be
> built into the compiler as a monolithic thing. Perl supports several
> different types (SV/AV/HV) which represent different concepts (scalars,
> arrays, hashes) so baking it all together into one thing would be the wrong
> way to map it. In fact, the magic we need is pretty small, and seems
> generally useful for other things. Consider a design like this:
>
>
> // not magic, things like Int, String and many other conform to this.
> protocol Pythonable {
> init?(_ : PythonObject)
> func toPython() -> PythonObject
> }
It’s not magic unless you expect the compiler or runtime to help with
conversion between Int/String/etc. and PythonObject, as with
_ObjectiveCBridgeable.
>
> // Not magic.
> struct PythonObject : /*protocols below*/ {
> var state : UnsafePointer<PyObject>
>
> subscript(_ : Pythonable…) -> PythonObject {
> ...
> }
> }
>
> // Magic, must be on the struct definition.
> // Could alternatively allow custom copy/move/… ctors like C++.
> protocol CustomValueWitnessTable {
> static func init(..)
> static func copy(..)
> static func move(..)
> static func destroy(..)
> }
Swift’s implementation model supports this. As a surface-level construct it’s
going to be mired in UnsafeMutablePointers, and it’s not at all clear to me
that we want this level of control there. Presumably, binding to Python is
going to require some compiler effort—defining how it is that Python objects
are initialized/copied/moved/destroyed seems like a reasonable part of that
effort.
> // Magic, allows anyobject-like member lookup on a type when lookup otherwise
> fails.
> protocol DynamicMemberLookupable {
> associatedtype MemberLookupResultType
> func dynamicMemberLookup(_ : String) -> MemberLookupResultType
> }
AnyObject lookup looks for an actual declaration on any type anywhere. One
could extend that mechanism to, say, return all Python methods and assume that
you can call any Python method with any PythonObject instance. AnyObject lookup
is fairly unprincipled as a language feature, because there’s no natural scope
in which to perform name lookup, and involves hacks at many levels that don’t
always work (e.g., AnyObject lookup… sometimes… fails across multiple source
files for hard-to-explain reasons). You’re taking on that brokenness if you
expand AnyObject lookup to another ecosystem.
Although it doesn’t really seem like AnyObject lookup is the thing you’re
asking for here. It seems more like you want dynamicMemberLookup(_:) to capture
“self” and the method name, and then be a callable thing as below...
>
> // Magic, allows “overloaded/sugared postfix ()”.
> protocol CustomCallable {
> func call( …)
> }
>
> The only tricky thing about this is the call part of things. At least in the
> case of python, we want something like this:
>
> foo.bar(1, 2, a: x, b: y)
>
> to turn into:
> foo.dynamicMemberLookup(“bar”).call(1, 2, kwargs: [“a”:x, “b”:y])
>
> We don’t want this to be a memberlookup of a value that has “bar” as a
> basename and “a:” and “b:” as parameter labels.
Well, I think the MemberLookupResult is going to get the name “bar”, argument
labels “_:_:a:b:”, and arguments “1”, “2”, “x”, “y”, because that’s the Swift
model of argument labels. It can then reshuffle them however it needs to for
the underlying interaction with the Python interpreter.
There are definite design trade-offs here. With AnyObject lookup, it’s a
known-broken feature but because it depends on synthesized Swift method
declarations, it’ll behave mostly the same way as other Swift method
declarations—static overloading, known (albeit weak) type signatures, etc. But,
it might require more of Python’s model to be grafted onto those method
declarations. With dynamic member lookup, you’re throwing away all type safety
(even for a motivated Python developer who might be willing to annotate APIs
with types) and creating a general language mechanism for doing that.
- Doug
>
> -Chris
>
>>
>> protocol DynamicDispatchable { // Protocol is “magic" known by the compiler.
>> func retain()
>> func release()
>> func memberLookup(_ : String) -> Self
>> func subscript<T>(_ : T) -> Self
>> func call(_ args: [Self]) -> Self
>> }
>>
>> module Python {
>> struct Object : DynamicDispatchable {
>> var state : UnsafePointer<PyObject>
>>
>> func retain() {
>> INCREF(self)
>> }
>>
>> func memberLookup(_ : String) -> Object {
>> PyObject_GetAttrString(…)
>> }
>> etc
>> }
>>
>> module Perl5 {
>> struct Object : DynamicDispatchable {
>> var state : UnsafePointer<SV>
>>
>> func retain() {
>> SvREFCNT_inc(self)
>> }
>> ….
>>
>>
>>
>> Are there other uses for such a thing?
>>
>> -Chris
>>
>>
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