changes to __getitem__ and/or None alias

[rubik]

So we were discussing whether to change the current __getitem__ syntax or to just alias _ to None.

One idea is to allow the argument to !! to be a tuple. Like this:

a = (0..21)
a!!(1,14,2)  #=> range(1, 14, 2)
a !! slice 1 14 2 #=> range(1, 14, 2)

The !! would just check if its argument is a tuple and unpack the arguments into a slice object. Also, if _ is aliased to None, reversing a list would just be: xs!!(_,_,-1). But at this point the syntax isn't radically different or improved. So maybe just aliasing _ to None is the best change right now.

[pyos]

This problem with this is that a tuple can be a valid argument to __getitem__ in some cases; the most prominent example of that are numpy arrays. (xs !! (1, 2, 3) is equivalent to Python's xs[1, 2, 3], which is the same thing as xs[(1, 2, 3)].)

Either way, right now I'm busy rewriting the parser; you've probably noticed that the current one is awful. (So awful, in fact, I don't even know exactly what kind of parser that is.) Everything else will probably have to wait until I either finish the new one or give up.

[rubik]

Argh right. Or just simply a dictionary could accept tuples. So probably the best thing should just alias _ to None. As for the parser, I don't have a complete vision of the dg internals yet! :D Right now I'm reading the compiler source code.

[pyos]

On second thought, aliasing _ to None doesn't really work either. First, it would require reimplementing sys.displayhook, which for some reason is also responsible for updating _ with the result of an expression entered into the REPL; second, it'd be necessary to come up with some different variable for that purpose. And, well, I can't think of a good name.

Damn, language design is hard.