Yigal Chripun wrote:
On 12/10/2009 10:47, Don wrote:
Ah, OK. My cursory glance at Nemerle just screamed "hack". But first
impressions can be misleading.
No doubt as a C-family language, they have some useful ideas.
But if Christopher's analysis is correct, the "macro" bit is different
to the "plugin" bit. I think allowing the ASTs to be _modified_ by
plugins is the path to madness, but a read-only ABI is OK (it's hard to
see how compile-time reflection is possible without creating some kind
of API).
modifying the AST is dangerous but how would you do things like making a
class implement an interface without modifying the list of interfaces
the class implements ?
[serialize]
class Foo {...}
the Nemerle macro above transform this into:
class Foo : Serializable { ... }
what would be your design for this?
I don't think it should be done that way. Either it should be intrusive
(eg, require you to derive from Serializable), or else entirely external
(and operate via reflection). Eg, possibly by declaring
Serializable!(Foo) after the definition of Foo.
It's an excellent question though, we might need some language changes
to get a good solution. But I think it's very important to enforce that
the only way to modify the AST is indirect, through code.
One of the strengths I see of the string mixins, despite their syntactic
ugliness, is that metaprogramming transformations are only ever syntax
sugar: there is _always_ a source-code equivalent; moreover, it's
trivially available, simply by expanding all of the mixins (and Descent
will show it to you). It's not clear to me that that remains true if you
can manipulate the AST directly. Of course, if you allow direct access
to the AST, you have unlimited power. Capturing as much of the power as
possible, without the danger, is the challenge.
