On 12 Apr 2011, at 10:46, Mark Cox wrote: > On 12/04/2011, at 7:45 AM, Matthew Swank wrote: > >> I am current reading an interesting paper on Racket's macro system >> (referenced >> here: http://lambda-the-ultimate.org/node/4196 >> ), and some of the objects they expose quack like CLTL-2 Environments: >> http://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node102.html. I have read that >> having write access to environments makes it hard for the compiler to >> optimize >> code. However, even portable read only access would be helpful. Does anyone >> besides me and Cyrus Harmon (http://cyrusharmon.org/blog/display?id=113) >> miss Environments? > > Alas, I wish I had been exposed to them before. Until Cyrus' post I had no > knowledge of them. I thank Cyrus for making that post. Using the function > CLTL2 VARIABLE-INFORMATION function, I produced the following higher level > functionality as a proof of concept for an idea I have for a mathematical > optimisation library.
[...] > Given the possibilities it offers, I am interested to know why it isn't part > of the standard. There are several possible answers to this. Ideally, you would want a data structure that is based either on structs or on CLOS classes that gives you information about the lexical environment. However, the compiler must exist already at a very early phase of bootstrapping a Lisp system, and the more advanced types are probably not ready yet at that stage. Secondly, there are quite a few namespaces (not just 2) that all need to handled in some way in lexical environments. Ensuring a good API to inspect all of them was apparently not trivial. Thirdly, different compilers have different ways how to organize their compilation phases - making them all agree on how to provide the information is maybe also not trivial. And then, from a different angle, Common Lisp already provides environment objects and local macro definitions, and they already allow you to go a very long way. For example, I have found the discussion at http://www.lispworks.com/documentation/HyperSpec/Issues/iss066_w.htm very illuminating. (See the two local-type-declare examples - I particularly like the version using symbol macros.) I have implemented an experimental hygienic-compatible macro system for Common Lisp using just these ingredients. See http://dx.doi.org/10.3217/jucs-016-02-0271 for a description. I would strongly recommend considering using such techniques, instead of holding your breath and waiting for wide adoption of fully specified lexical environments. ;) (Just to avoid misunderstandings: such environments would definitely be more expressive than just opaque environment objects + local macros, it's just that environment objects + local macros are already a lot more powerful than some may expect!) Pascal -- Pascal Costanza The views expressed in this email are my own, and not those of my employer. _______________________________________________ pro mailing list pro@common-lisp.net http://common-lisp.net/cgi-bin/mailman/listinfo/pro
