[mercury-users] lisp macros

[Richard A. O'Keefe]

I wrote:
	> (defmacro x-define (Id . Body)
	>   `(define (,Id Element Ancestors) ,(list 'quasiquote Body)))
	> (defmacro apply-templates ()
	>   `(process-children Element Ancestors))

Michael Day <mikeday at bigpond.net.au> replied:
	Any tips on how this kind of thing would look in Mercury for
	people who don't know lisp?  :)
	
Different.

It isn't really relevant, because the point was what macros can do for
you.  They can hide "plumbing"; they can define new binding forms (like
with-open-file, for example); they can make your code a lot shorter.

But for an example of the *kind* of thing that might be done this way,
consider Definite Clause Grammar rules, which are precisely the kind of
"hide the plumbing" technique I was describing.  Come to think of it,
the functional sublanguage is also an example.  (I and many other people
have term_expansion/2-based plugins that let you use functional syntax
in Prolog.)
	
The key reasons that macros for Mercury would look different are
 - Mercury already has unification-based pattern-matching; 
 - Lisp and Scheme are imperative/functional hybrids; they can easily
   have macros that introduce mutable variables without requiring them
   to be coupled to an IO state, World, or IO monad.
 - Lisp and Scheme have run-time types; they don't have to couple macro
   expansions to a type environment.

What would Mercuy with macros look like then?

Shorter.

I'm sorry to say this, but after Haskell, Mercury is just so overwhelmingly
BULKY.  (Even while I was working at Quintus, I had the same feeling about
Prolog.)  Part of the reason is that Prolog syntax is pretty heavy on
punctuation marks, nearly as heavy as C and Java.  Part of the reason is
that you can't hide the plumbing.  Take a look at HaXML (which uses
combinators, not macros).  What would that look like in Mercury?  Bigger.

Different applications will have different plumbing to hide.

--------------------------------------------------------------------------
mercury-users mailing list
post:  mercury-users at cs.mu.oz.au
administrative address: owner-mercury-users at cs.mu.oz.au
unsubscribe: Address: mercury-users-request at cs.mu.oz.au Message: unsubscribe
subscribe:   Address: mercury-users-request at cs.mu.oz.au Message: subscribe
--------------------------------------------------------------------------