Re: Metaprogramming features

[Chung-chieh Shan <ccshan@post.harvard.edu>]

Jon Harrop <jonathandeanharrop@googlemail.com> wrote in article <200810061656.42903.jon@ffconsultancy.com> in gmane.comp.lang.caml.inria:
> On Monday 06 October 2008 15:06:12 you wrote:
> > I'm definitely missing something here.  How are the above examples
> > different from:
> > let f () = 1 + 2;;
> > f ();;
> > let f x = 1 + x;;
> >
> > ?
> >
> > This is a non-trivial question, as there is no clear delineation between
> > data structures and executable code.

Indeed the question is not trivial.  Because MetaOCaml (like OCaml) has
higher-order functions (which may operate on code values or on functions
on code values), it is not trivial at all to `simulate' MetaOCaml in
OCaml.  Our PEPM paper this year (Kameyama, Kiselyov, and Shan) needed
the full power of System F just to express a subset of MetaOCaml without
polymorphism (even let-bound) or nested quotation (such as .<.<42>.>.).

    http://www.cs.rutgers.edu/~ccshan/metafx/pepm66-kameyama.pdf

Section 2 of this paper gives a series of examples to illustrate the
non-triviality of the simulation.  You can try to skip to Section 2.5
to see the most challenging example there.

> Exactly. The difference is (only) the performance characteristics.

To the contrary, that is not the only difference; the timing of
side effects (including non-termination and exceptions) can also be
different.  We give an example at the beginning of our Section 2.

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