User-created keywords

[pzinn]

Following #3584, compare

i1 : symbol ×

o1 = ×

o1 : Symbol

i2 : symbol ⧢

o2 = ⧢

o2 : Keyword

Why the difference? because an arbitrary list of symbols has been declared as a keyword (something that I'd rather call a binary operator, but sadly the word Binary is used for something completely different in M2. I digress.), i.e., something that's used like this:

i3 : 1 ⧢ 2
stdio:3:2:(3): error: no method for binary operator ⧢ applied to objects:
            1 (of class ZZ)
      ⧢     2 (of class ZZ)

But realistically most users are never going to use ⧢ so why this symbol rather than another?
I think it'd make much more sense for users/packages to be able to define their own Keywords. Then the Core should stick to a minimal set of Keywords.
Incidentally note that new Keyword from × will not produce an error (though currently, it probably should, since the result is not what you would expect).

[pzinn]

As pointed out in the comments of #3584, this also works but messes up things:

i1 : a⊗b
stdio:1:1:(3): error: no method for binary operator ** applied to objects:
            a (of class Symbol)
     **     b (of class Symbol)

i2 : getSymbol "⊗"

o2 = ⊗

o2 : Keyword

i3 : a⊗b

i4 : 

[pzinn]

Some relevant remarks:

• at the d level, there's no difference in "type" between Keywords and Symbols, they're both SymbolClosure, and class determines on the fly which is which thanks to this line in classes.dd:

     is s:SymbolClosure do if s.symbol.word.parse == parseWORD then symbolClass else keywordClass

so the difference is made using parsing information.

• on the other hand new Keyword from xxx produces a SpecialExpr (which it probably shouldn't)
• finally, getSymbol "⊗" creates a new symbol which is correctly determined to be a Keyword, except it doesn't have any methods associated to it (and in fact, we can't define any: error: can't assign a method for this binary operator) so it calls the dummy binary function, which is supposed to throw an error, except AFAICT err_error does nothing, so it doesn't.

[pzinn]

I intend to rewrite binding.d and actors5.d which have become a mess, due to the overabundance of keywords/symbols.