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Do GraalVM JavaScript's JS-to-Java object conversions contain type information?

To mimic on GraalVM JavaScript some of Node's functions under util.types, I turned to Java functions that I can access through Java.type or the Java package global variables. GraalVM's has internal functions like JSProxy.isJSProxy and JSSet.isJSSet that do this. I made them accessible by marking them as a exported, recompiled, and tested them (I accessed them with, e.g., Java.type("com.oracle.truffle.js.builtins.runtime.JSProxy").isJSProxy).
As I found out, GraalVM JavaScript's internal functions are useless to me. For example, JSProxy.isJSProxy only returns true if it is passed an instance of JSProxyObject. Oracle's documentation says that
JavaScript objects are exposed to Java code as instances of com.oracle.truffle.api.interop.java.TruffleMap. This class implements Java’s Map interface.
TruffleMap doesn't appear in the source code of GraalVM JavaScript and only appears once in the source code of GraalVM, so it's likely deprecated, but that's besides the point because the representation method is probably the same.
Whatever the external Java-side representation of JS objects is right now, does it contain information about what the object's type on the JavaScript side is? Can I determine if it's a JS proxy? Can I determine if it's a Set object? (And so on for native JavaScript object types.)

To be honest your question is quite confusing to me, but are you just looking for the instanceof operator?
import org.graalvm.polyglot.*;
import org.graalvm.polyglot.proxy.*;
public class Test {
public static class MyClass {
public int id = 42;
}
public static void main(String[] args) {
try (Context context = Context.newBuilder().allowAllAccess(true).build()) {
context.getBindings("js").putMember("javaObj", new MyClass());
boolean valid = context
.eval("js", "javaObj instanceof Java.type('Test$MyClass')")
.asBoolean();
assert valid;
}
}
}

In python, "export" a custom-tailored object from a module

In JavaScript, specifically in node.js setting, one can spell module.exports = 13; in module.js, then x = import ("module.js"); elsewhere and have 13 assigned to x directly.
This saves some code when a module exports a single function, and I notice a lot of widely used packages (such as through2) make use of it.
Is there a way to do the same in Python? With some black magic, maybe?
I do have heard of a thing called loader that's, I guess, supposed to do some manipulations with a module before making it available. In particular, I think SaltStack makes use of something like that in salt.loader, but the code is too hard for me to follow. I imagine we could write a function similar to this:
def loader(module):
m = __import__(module)
return m["__exports__"]
— Then define __exports__ somewhere in a module we want to import and enjoy functionality very similar to JavaScript's module.exports mechanics. But unfortunately TypeError: 'module' object has no attribute '__getitem__' prevents us from doing that.

Python has importing built in to the language at a more basic level than Javascript does, almost all use cases are covered by a simple import statement.
For your example, all it really boils down to is:
from module import exports as x
So, there's not need to look for code savings by changing module.
The other part of the question is how, as a module author, would you restrict people to seeing only a single symbol.
Generally this is not required except to help users know what are public functions vs implementation details. Python has a few common idioms for this:
Any names that start with a leading underscore, such as _helper, are considered private. They can be accessed as normal, but the implication is you should not.
If a module level variable __all__ = [...] exists, only the strings it contains are considered public. The names must seperatedly be declared in the module.
As well as being documentation, both of these do affect one aspect of the module import:
from module import *
Using a star import is generally discouraged, but only public names will be brought in to the local namespace.

After some thinking I understood that, while we can't say m["__exports__"] due to module object's class not having __getitem__ method, we can still access some of the module's elements with "dot" notation: m.__exports__ works.
Another way: screen all module level names off with an underscore and assign the object to be exported to a variable named after the module, then from module import *.
loader.py:
def loader(module):
m = __import__(module)
return m.__exports__
exports.py:
def _f():
return 13
_a = 31
exports = {"p6": _f, "p8": _a}
__exports__ = exports
Python 2.7:
>>> import loader
>>> e = loader.load ("exports")
>>> e
{'p8': 31, 'p6': <function _f at 0x7fb79d494cf8>}
>>> from exports import *
>>> exports
{'p8': 31, 'p6': <function _f at 0x7fb79d494cf8>}
Python 3:
>>> import loader
>>> e = loader.load ("exports")
>>> e
{'p6': <function _f at 0x7f88ae229ae8>, 'p8': 31}
>>> from exports import *
>>> exports
{'p6': <function _f at 0x7f88ae229ae8>, 'p8': 31}
In the first way proposed, I unfortunately cannot use __all__ in loader.load to filter only listed names from a module being loaded since __getitem__ is not defined for module object.
In the second way proposed I don't get so much control (in that a malicious module can export arbitrary names and manipulate my namespace) and flexibility (in that I cannot assign the module's exported object to arbitrary name anywhere in my code).
So, there is still a bit left to be desired here.

Is it possible to import variables in JavaScript (node.js)?

I have variables in app.js:
var G = {};
module.exports = G;
var DATA = G.DATA = 'DATA';
var F1 = G.F1　= function(val)
{
return val;
};
In this manner, I can export variables under the object G, and at the same time, can access the variable directly writing DATA without G. prefix.
So far so good.
Now, I want to run a test for app.js in test.js:
var G = require('./app.js');
console.log(G.DATA); // -> DATA
This works, but I also want to access the variable directly writing DATA without G. prefix like console.log(DATA); // -> DATA
Surely, I could do like
var DATA = G.DATA; for every variables(property) export&required module G object, but obviously it's a tedious process to add every variable to the test file manually to correspond the G objects.
Is there any way to do this automatically?
So far, I'm pessmistic since
JS function encloses var in the own scope, so in theory there's no way to have a helper function to var for every object property.
Thanks.
PS. I would like to avoid any eval or VM of node solution. I have tried them in past, and too much problems.

I could assign a local variables for every property export&required module G object, but obviously it's a tedious process to add every variable to the test file manually to correspond the G objects.
No, that's how it is supposed to work. You - and only you - are in charge of what local variables exist in your module scope. No changes in the "export variables" of an included module should break your code.
Accessing properties on the imported module (with a self-chosen name) is the way to go. This is quite equivalent to Python's import app or import app as g.
If you want some particular properties as local variables, you will usually choose them manually, as in Python's from app import DATA, F1. In JS, you will need a multiple var statement like the one you've shown in your question. However, there is a syntax feature called destructuring assignment which will make this more fluid. You can use this in JavaScript 1.7+ (Gecko), CoffeeScript, or EcmaScript 6:
var {DATA, F1} = require("app.js");
Is there any way to do this automatically?
Yes and No. You should not do this, but you can - just like Python's frowned-upon from app import *. To cite what they say, which is equally true for JavaScript:
[It] introduces an unknown set of names into the interpreter, possibly
hiding some things you have already defined.
Note that in general the practice of importing * from a module or
package is frowned upon, since it often causes poorly readable code.
However, it is okay to use it to save typing in interactive sessions.
In JavaScript, you can[1] use the with-statement:
with (require("app.js")) {
…
}
[1]: Not in ES5.1 strict mode, though - which is recommended for optimisation

Function name convention for "convert foo to bar"

I have a very common pattern of "given a Foo, return a Bar," for example, given a user_id, return a User.
Is there a conventional naming pattern for these sorts of functions? Following Joel on Software, I've personally used a lot of bar_from_foo(), but I rarely see other people do this and it quickly becomes verbose, e.g.
widgets = user_widgets_from_user(user_from_param_map(params))
Is there a conventional way to name, or namespace (e.g. User.from_map()) in any of the popular languages out there? I am particularly interested in Python but any language you can think of would br useful.

I would take advantage of Clojure's naming flexibility and call it:
(defn foo->bar [] ...)
To me that makes the intent quite clear and it's pretty short.

In Python, and various other OO languages, this should be a constructor on Bar that takes a single Foo as its only argument. Since Python doesn't do method overloading (and attempts to emulate it are usually fragile), if Bar.__init__ already takes a different signature, then the convention is exactly your last one. Importantly, this is usually defined as a class method rather than a static method (for the benefit of subclasses):
class Bar:
#classmethod
def from_foo(cls, f):
'''Create a new Bar from the given Foo'''
ret = cls()
# ...

If you want to convert something into another, for example a string to an integer, the method is to be defined on the receiver, and hence its class is clear, so you should not put the receiver class as part of the method name: String#to_i, not String#string_to_i. This in one of the core ideas of object oriented programming (polymorphism).
If the receiver is too general to be assigned such method, for example if user_id is a mere string, and defining a method on String to convert it to a User does not look right, then you should define a constructor method on the class that you expect the return value to be: User.new or User.from_id.

I think it depends a lot on context and choosing a meaningful metaphor. ActiveRecord for instance uses the class method "find" for finding records in the database, a more meaningful idea than "input a user_id, output a user". For example:
User.find(user_id)
User.find_by_email(user_email)
For conversions, I usually like to write the conversion methods to make it easy to use in higher order functions. For example in ruby, conversions are often done with to_* instance methods, for example to convert a Foo to a Bar it would make sense to have a to_bar method for all foos, so you could write:
foo = Foo.new(...) # make a new Foo
bar = foo.to_bar # convert it to a Bar
And then to convert a bunch of foos, you could simply:
bars = foos.map(&:to_bar)
Ruby also tends to have Foo.parse(str) for converting a string to the object.
For javascript, I like having class methods (which I got from standard ml), for example:
Foo.toBar = function(foo) {
return new Bar(...);
};
And then you can map over it as well (using underscore in this example):
var bars = _.map(foos, Foo.toBar);
the Standard ML convention is structure (class) methods. Example fn types:
Foo.toBar : foo -> bar
Foo.fromBar : bar -> foo
And you'd use it like:
val bar = Foo.toBar foo;
val bars = map Foo.toBar foos;

Why placing the type of the parameter the name of the function? I think it would be clearer something like
a_bar = bar_from(a_foo)
then you can rely on dynamic dispatch or overload in many languages... for example in Python the implementation could try to call x.toBar() if present or it could check for a global registry like Bar_builders[type(x)](x); in Lisp you could rely on methods; in C++ on overloads...

Prototypal programming in Python

Javascript uses a prototype-based model for its objects. Nevertheless, the language is very flexible, and it is easy to write in a few lines functions which replace other kind on constructs. For instance, one can make a class function, emulating the standard class behaviour, including inheritance or private members. Or one can mimcìic functional tools by writing, for instance, a curry function which will take a function and some of its arguments and return the partially applied function.
I was wondering whether it is possible to do the reverse and imitate the prototypal method in more classical languages. In particular I have been thinking a bit whether it is possible to imitate prototypes in Python, but the lack of support for anonymous functions (more general than lambdas) leaves me stuck.
Is it possible to write some functions to mimic propotypes in class-based languages, in particular in Python?
EDIT Let me give some example of how one could go implementing such a thing (but I'm not really able to do it all).
First, the thing which most closely resembles Javascript objects is a Python dictionary. So we could have simple objects like
foo = {
'bar': 1,
'foobar': 2
}
Of course we want to add method, and this is not a problem as long as the method fits in a lambda
foo = {
'bar': 1,
'foobar': 2,
'method': lambda x: x**2
}
So now we can call for instance
foo['method'](2)
>>> 4
Now if we had arbitrary functions as methods we could go on like this. First we need the functions inside foo to have access to foo itself; otherwise they are just ordinary functions and not methods.
I guess one could do this by applying a makeObject function to foo, which loops through foo values and, whenever finds a value that is callable, modifies its __call__ attribute to pass foo as its first argument.
At this stage we would have self standing objects, which can be declared without the need of creating classes.
Then we need to be able to give foo a prototype, which can be passed as a second argument of the makeObject function. The function should modify foo.__getattr__ and foo.__setattr__ as follows: whenever the attribute is not found in foo, it should be searched in foo.prototype.
So, I think I would be able to implement this, expect for one thing: I cannot think any ways to declare methods more complicated than lambdas, except for declaring them beforehand and attaching them to my object. The problem is the lack of anonymous functions. I asked here because maybe some Python guru could find some clever way to circumvent this.

It's much easier in Python than in JS. Your JS code could be replaced with this in Python:
>>> class Foo(object):
... pass
>>> foo = Foo()
>>> foo.bar = 1
>>> foo.foobar = 2
Then you can add methods dynamically as well
>>> foo.method = lambda x: x**2
>>> foo.method(2)
4
For methods more complicated than lambdas, you declare them as functions, and they will have access to foo itself, no problems:
>>> def mymethod(self, bar, foobar):
... self.bar = bar
... self.foobar = foobar
>>> foo.mymethod = mymethod
>>> foo.mymethod(1,2)
>>> foo.bar
1
>>> foo.foobar
2
Or for that matter:
>>> mymethod(foo, 3, 4)
>>> foo.bar
3
>>> foo.foobar
4
Same thing.
So as you see, doing what your example is in Python is almost ridiculously simple. The question is why. :) I mean, this would be better:
>>> class Foo(object):
... def __init__(self, bar, foobar):
... self.bar = bar
... self.foobar = foobar
... def method(self, x):
... return x**2

Each time you read some property of Python object, method __getattribute__ is called, so you can overload it and completely control access to object's attributes. Nevertheless, for your task a bit different function - __getattr__ - may be used. In contrast to __getattribute__ it is called only if normal lookup for an attribute failed, i.e. at the same time, as prototype lookup in JavaScript starts. Here's usage:
...
def __getattr__(self, name):
if hasattr(prototype, name)
return getattr(prototype, name)
else:
raise AttributeError
Also pay attention to this question, since it has some notes on old and new style objects.

This implementation contains one important enhancement over otherwise similar implementations in other answers and on the internet: the correct inheritance of methods from the prototype. If a value obtained from the prototype is a a bound instance method -- and to cover weird corner cases the method is also actually bound to that prototype -- then the method's underlying function is extracted and a new method binding that function to the calling object is returned
import types
import inspect
class Proto(object):
def __new__(self, proto, *args, **kw):
return super(Proto, self).__new__(self, *args, **kw)
def __init__(self, proto, *args, **kw):
self.proto = proto
super(Proto, self).__init__(*args, **kw)
def __getattr__(self, name):
try:
attr = getattr(self.proto, name)
# key trick: rebind methods from the prototype to the current object
if (inspect.ismethod(attr) and attr.__self__ is self.proto):
attr = types.MethodType(attr.__func__, self)
return attr
except AttributeError:
return super(Proto, self).__getattr__(name)
This should fully implement prototypal inheritance as I understand it.
One restriction is that classes inheriting from Proto must have Proto first in their MRO because __new__ and __init__ have the prototype as the first parameter, which they drop when they delegate to super.
Here is a use example:
from prototypal import Proto
class A(Proto):
x = "This is X"
def getY(self):
return self._y
class B(Proto):
_y = "This is Y"
class C(object):
def __getattr__(self, name):
return "So you want "+name
class D(B,C):
pass
a = A(None) # a has no proto
b = B(a) # a is the proto for b
print b.x
print b.getY() # this will not work in most implementations
d = D(a) # a is the proto for d
print d.x
print d.getY()
print d.z
Here is the gist

Short version, yes but it's a bit more complicated than JS.
From Metaclass programming in Python :
>>> class ChattyType(type):
... def __new__(cls, name, bases, dct):
... print "Allocating memory for class", name
... return type.__new__(cls, name, bases, dct)
... def __init__(cls, name, bases, dct):
... print "Init'ing (configuring) class", name
... super(ChattyType, cls).__init__(name, bases, dct)
...
>>> X = ChattyType('X',(),{'foo':lambda self:'foo'})
Allocating memory for class X
Init'ing (configuring) class X
>>> X, X().foo()
(<class '__main__.X'>, 'foo')
Also check What is a metaclass in Python.
Edit : Check Prototype based OO, which is the closest thing you will get, but it will always come down to either using a lambda or just defining the function outside and adding a pointer to it to the class object.

I know this is quite old, but I though I'd add my $.02:
https://gist.github.com/4142456
The only problematic thing here is adding methods. JavaScript has a far more elegant syntax with its function literals, and it's truly hard to beat that. Lambda's don't quite cut it. So my solution was to add an awkward method method for adding methods.
Doctests are included in the gist, and it all works.
EDIT:
Updated gist to no longer use method instance method. However, we still need to define the function beforehand.
At any rate, the most important part of prototypal object model is implemented in that gist. Other things are simply syntactic sugar (would be nice to have them, but it doesn't mean prototypal object model cannot be used).