Re: [Caml-list] [ANN] visitors

["François Pottier" <francois.pottier@inria.fr>]

Hello,

Le 27/01/2017 23:55, Drup a écrit :
> Very nice! I wanted this for quite a while and the documentation is
> excellent.

Thanks! And thanks for your comments and pointers.

> A big difference is that it load type definition from .cmi files directly.

Indeed, a limitation of the ppx approach is that it is purely syntatic:
a syntax tree (the type definition) is turned into a syntax tree (the
definition of the visitor classes). In particular, at the moment, "visitors"
cannot generate visitor classes for a pre-existing type. Perhaps one could
write a tool that reads a .cmi file, but I suppose it would have to be
explicitly invoked as an external tool. I will think about it.

> What do you think of visitor that translates one datatype into another ?

Well, the current package already allows this.

If the two data types are just two distinct instances of a single
parameterized data type, then a "map" visitor can do this, with very little
effort. See the examples in the manual when "ordinary expressions" are
converted to "hash-consed expressions" and back.

If the two data types are unrelated, then, in order to translate the type
"foo" to the type "bar", you need a "fold" visitor for the type "foo". You
then have to manually implement each of the "build_" methods, so as to 
explain
how each data constructor of the type "foo" should be translated.

> I wonder if, given some annotations, it would be doable to use similar
> techniques to generate visitors between different datatypes.

I think you have it already, but instead of placing "annotations" in the
type definition, you have to implement the "build_" methods.

--
François Pottier
francois.pottier@inria.fr
http://gallium.inria.fr/~fpottier/