[m-dev.] Introduce !! rather than ^elem() for indexing

[Ralph Becket]

Michael Day, Wednesday, 10 October 2001:
> > `A !! I' is syntactically more economical than the current Mercury
> > convention of using `A ^ elem(I)'.  However, to make substitution of
> > the form `A !! I := X' parse would require some work.
> 
> If you can have A !! I, why not have A ^ I ? Is the !! operator going to
> be specific to lists/arrays/maps and basically replace elem?

No, I think the field access syntax is good (although I have moaned
in the past about the parameter ordering after desugaring...)

I suppose A ^ I could work if one were prepared to allow overloading
of higher order application.  While I haven't thought it through,
that idea does set alarm bells ringing: it's reminiscient of the
time I watched someone debug C++ code in which the benighted original
author had seen fit to overload cast to void*.

- Ralph
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