Explanation of macros; Haskell macros

[Robert Klemme]

"Lex Spoon" <lex at cc.gatech.edu> schrieb im Newsbeitrag
news:m3wuau4owd.fsf at logrus.dnsalias.net...
> This thread has seemed to miss the biggest advantage of macros.  Most
> examples so far are cases where the macro buys you faster code.  This
> is not extremely exciting, IMHO, because a better compiler can often
> accomplish the things that are described.  If you care that much about
> performance then surely you want to be using a good optimizing
> compiler, and a good compiler will surely know about, e.g., for loops
> that go across constant ranges.  I tend to think of macros as bad
> style for this kind of thing, unless it's very clearly a performance
> problem you are having.  Macros often look like functions, but they
> don't act like them, and it's very easy to introduce bugs with them.
>
> A much bigger advantage of macros is that you can literally extend
> the language.  For example, it is nice to be able to type something
> like this:
>
>      (regex (| h H)  "ello, "  (| w W)  orld  (? "!"))
>
>
> To allow this in the nicest way requires using a macro for 'regex.
> Without macros, you can still do it:
>
>      (regex '(| h H)  "ello, "  (| w W)  orld  (? "!"))
>
> but now this guy is going to get compiled at runtime, and you get into
> the issue of trying to save the compiled regex somewhere ahead of
> time.  And while regexes themselves compile pretty quickly, suppose
> it's something more like:
>
>      (parser
>           (grammer
>              ;;... a BNF grammer ...
>
>
>
> I also loved the example of both printing a statement and executing
> it, which is extremely useful for assertion checking.  You can't
> really do this properly without macros.

Just for the sake of my understanding: could one summarize this as "macros
make control of *when* something is evaluated easier"?  I mean, marco
arguments are not evaluated before the macro "call" as opposed to function
arguments.  With that, you can decide inside the macro, how you treat
those arguments and if you evaluate them at all.  Thx!

Regards

    robert