Re: gsoc: grabbing clisp environment info, cltl2

[Bruno H. <br...@cl...>]

Hi Charles,

> 'front-end' compilation needs to take place in a lexical environment
> in order to get variable, function, optimize information etc as well
> as to hook into macroexpanders. However, I have found that CLISP does
> not adhere to the CLtL2 environment interface.

The environment interface is not available as a "modern", "abstract"
interface, only as a data structure with accessors:

  - The concepts of these environments are described in lispbibl.d
    around line 14000.

  - These environments mix lexical and dynamic information; if you
    want to ignore the dynamic information, do so carefully.

  - The variables environment has Lisp code that accesses it in:
    venv-assoc (init.lisp), venv-search (compiler.lisp).

  - The functions/macros environment has Lisp code that accesses it in:
    fenv-assoc (init.lisp), fenv-search (compiler.lisp).

  - The block environment has Lisp code that accesses it in:
    benv-search (compiler.lisp).

  - The tagbody/go environment has Lisp code that accesses it in:
    genv-search (compiler.lisp).

  - The declarations environment has Lisp code that accesses it in:
    declared-notinline, declared-constant-notinline, declared-declaration,
    declared-optimize (all compiler.lisp).

Note: These environment structures are also accessed from the interpreter
(C code in eval.d and control.d). This means that if you extend these
structures to accommodate objects built by SICL, the C code may need an
update as well. This is because the compiler has to access interpreter
environments, as in

  (let ((x 5))
    (declare (special x)) ; This declaration has an effect on the function!
    (locally (declare (compile))
      (lambda () (incf x))))

With the '(declare (special x))':
0     (GETVALUE&PUSH 0)                   ; X
2     (CALLS2 177)                        ; 1+
4     (SETVALUE 0)                        ; X
6     (SKIP&RET 1)

Without the '(declare (special x))':
0     (CONST&PUSH 0)                      ; #(X 5 NIL)
1     (CONST 1)                           ; 1
2     (SVREF)
3     (PUSH)
4     (CALLS2&PUSH 177)                   ; 1+
6     (CONST&PUSH 0)                      ; #(X 5 NIL)
7     (CONST 1)                           ; 1
8     (SVSET)
9     (SKIP&RET 1)

> Additionally,
> (ext:the-environment) does not seem to give compile time lexical
> environment information, only runtime environment information.

Yes, (ext:the-environment) has the same mix between lexical and dynamic
information.

> I couldn't find any interface to tell whether
> symbols were declared special with defvar.

There is SYS::SPECIAL-VARIABLE-P, defined in eval.d, and
CONSTANTP, defined in control.d. Both of these access special bits in
the symbol (cf. constant_var_p, special_var_p in lispbibl.d).

> Any way of getting CLtL2-esque information here? Maybe buried in a codewalker?

It is surely possible to implement the functions from
https://www.cs.cmu.edu/Groups/AI/html/cltl/clm/node102.html
- except for augment-environment - in a straightforward manner.

augment-environment, however, is not so adapted for clisp, because it would
be consing more than is desirable.

Note that this interface did not make it into ANSI CL, see
http://www.ai.mit.edu/projects/iiip/doc/CommonLISP/HyperSpec/Issues/iss343.html

Bruno