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Re: Why no macros in CAML Light ?
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Date: -- (:)
From: Pierre Weis <Pierre.Weis@i...>
Subject: Re: Why no macros in CAML Light ?

> I wonder why there is no macro facility in CAML.

In the past, this have been a very hot subject among the group of
implementors and designers of Caml. I guess this could be the
beginning of a long long flameware again, if we are not very careful
to keep cool!

In brief: there were a very powerful and complex macro facility in
Caml. But this facility is very hard to design if you want to provide
a true separate compilation module system. In effect, you certainly
would like to write your macros in Caml. Moreover, you want to use them
in other modules, and this implies that the {\em compiler} must call
your macros, hence the compiler should load them in core (either in
source code format or in compiled code format). This turns the
compiler into a definitively more complex system since it must be able
to evaluate arbitrary complex Caml expressions and deal with their
special typing and evaluation environment (to achieve this we must write 
an interpretor or a dynamic linker, and embbed it into the compiler).

The main design decision for Caml Light, apart from the ``keep it
simple and stupid'' guideline, was to enforce separate compilation
even if this means to completely abandon the macro facility. Hence
these very simple and handy modules with interface and implementation
files. Hence the complete absence of macros.

> it would be nice to have at least the equivalent of C macros.

No problem with this feature: you just have to write an equivalent of the C
preprocessor (for instance in Caml itself) and make it available to
the community! 

> Perhaps, there are implementation problems related with the type system of
> CAML, but I can't figure out what they could be.

Well, there are no problems with the type system, except that macro
functions will probably have several different incompatible types (for
instance a macro function that transforms an expression into another
expression will have type CAML_expression -> CAML_expression, while a
macro function that transforms a pattern into a pattern will have type
CAML_pattern -> CAML_pattern).

Finally, the problem of macros in Caml is still an active research
area ``in the background'', since it is definitively very powerful and
elegant (and completely safe in conjonction with the type system): I
hope it will be incoporated into Caml someday. In these lines, Daniel
de Rauglaudre and Michel Mauny have designed and implemented a
prototype of a new Caml dialect (we call it Chamau) with a very
powerful ``quotation'' facility, which provides a powerful macro
facility, which is still reasonably compatible with separate compilation.

> *****
> je me demande pourquoi il n'existe pas de macros en CAML.

Ca a existe' dans le passe'! Le proble`me c'est que les macros se
marient mal avec la compilation se'pare'e. En effet le compilateur
doit pouvoir appeler et charger vos macros ``au vol'', ce qui signifie
qu'il doit ge'rer un environnement spe'cial pour ces macros et
contenir un e'valuateur ou un chargeur dynamique de code.

Pour Caml Light, l'ide'e a toujours e'te' de privile'gier la
compilation se'pare'e et la simplicite'. C'est pourquoi le syste`me de
modules est simple et puissant, et les macros ont disparu.

> il serait agreable de disposer au moins de l'equivalent des macros C.

Il n'y a pas de proble`me pour cela: il suffit d'e'crire un
pre'processeur et de le rendre disponible a` tous.

> Il existe peut-etre des problemes d'implementation lies au systeme de
> verification de types de CAML, mais je ne vois pas trop lesquels.

Non pas de proble`mes, si ce n'est que les macros changent de type
selon les objets auquels elles s'appliquent, expressions, patterns,
de'finitions.

En conclusion, le proble`me des macros est toujours un actif sujet de
recherche dans notre projet, parceque c'est vraiment puissant et
e'le'gant (et su^r avec le syste`me de types de Caml): j'espe`re que
nous en aurons a` nouveau dans Caml un jour prochain. Par exemple Daniel
de Rauglaudre and Michel Mauny ont imple'mente' un dialecte de Caml
(Chamau) qui pre'sente un puissant me'canisme de ``quotation'' qui
permet des macros compatibles avec la compilation se'pare'e.

Pierre Weis
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