Re: [h-e-w] info

["Lennart Borgman (gmail)" <lennart.borgman@gmail.com>]

David sent this as a bug report to h-e-w (because bug reports for my 
patched versions may be sent there).

David, I think what you want is some better explanations for those 
points you have marked below, right?


David R wrote:
> File: elisp, Node: Surprising Local Vars, Next: Eval During Expansion, 
> Prev: Argument Evaluation, Up: Problems with Macros
> 
> 13.6.3 Local Variables in Macro Expansions
> ------------------------------------------
> 
> In the previous section, the definition of `for' was fixed as follows
> to make the expansion evaluate the macro arguments the proper number of
> times:
> 
> (defmacro for (var from init to final do &rest body)
> "Execute a simple for loop: (for i from 1 to 10 do (print i))."
>  `(let ((,var ,init)
> (max ,final))
> (while (<= ,var max)
> ,@body
> (inc ,var))))
> 
> The new definition of `for' has a new problem: it introduces a local
> variable named `max' which the user does not expect.  This causes
> trouble in examples such as the following:
> 
> (let ((max 0))
> (for x from 0 to 10 do
> (let ((this (frob x)))
> (if (< max this)
> (setq max this)))))
> 
> The references to `max' inside the body of the `for', which are
> supposed to refer to the user's binding of `max', really access the
> binding made by `for'.
> 
>  The way to correct this is to use an uninterned symbol instead of
> `max' (*note Creating Symbols::).  The uninterned symbol can be bound
> and referred to just like any other symbol, but since it is created by
> `for', we know that it cannot already appear in the user's program.
> Since it is not interned, there is no way the user can put it into the
> program later.  It will never appear anywhere except where put by
> `for'.  Here is a definition of `for' that works this way:
> 
> (defmacro for (var from init to final do &rest body)
> "Execute a simple for loop: (for i from 1 to 10 do (print i))."
>  (let ((tempvar (make-symbol "max")))
> `(let ((,var ,init)
> (,tempvar ,final))
> (while (<= ,var ,tempvar)
> ,@body
> (inc ,var)))))
> 
> This creates an uninterned symbol named `max' and puts it in the
> ^^^^^
> tempvar
> 
> expansion instead of the usual interned symbol `max' that appears in
> expressions ordinarily.
> 
> 
> File: elisp,  Node: Compiling Macros,  Next: Defining Macros,  Prev: 
> Expansion,  Up: Macros
> 
> 13.3 Macros and Byte Compilation
> ================================
> 
> You might ask why we take the trouble to compute an expansion for a
> macro and then evaluate the expansion.  Why not have the macro body
> produce the desired results directly?  The reason has to do with
> compilation.
> 
>   When a macro call appears in a Lisp program being compiled, the Lisp
> compiler calls the macro definition just as the interpreter would, and
> receives an expansion.  But instead of evaluating this expansion, it
> compiles the expansion as if it had appeared directly in the program.
> As a result, the compiled code produces the value and side effects
>                                          ^^^^^ value of what?
> 
> intended for the macro, but executes at full compiled speed.  This would
>     ^^^ as the expansion of the macro, or passed to the macro?
> 
> not work if the macro body computed the value and side effects
>                             ^^^^^^^^ computed = evaluated or expanded?
> 
> itself--they would be computed at compile time, which is not useful.
>                   ^^^^^^^^
> 
>   In order for compilation of macro calls to work, the macros must
> already be defined in Lisp when the calls to them are compiled.  The
> compiler has a special feature to help you do this: if a file being
> compiled contains a `defmacro' form, the macro is defined temporarily
> for the rest of the compilation of that file.  To make this feature
> work, you must put the `defmacro' in the same file where it is used,
> and before its first use.
> 
>   Byte-compiling a file executes any `require' calls at top-level in
> the file.  This is in case the file needs the required packages for
> proper compilation.  One way to ensure that necessary macro definitions
> are available during compilation is to require the files that define
> them (*note Named Features::).  To avoid loading the macro definition
> files when someone _runs_ the compiled program, write
> `eval-when-compile' around the `require' calls (*note Eval During
> Compile::).
> 
> 
> 
> 
> 
> 
> In GNU Emacs 22.0.990.1 (i386-mingw-nt5.1.2600)
> of 2007-05-23 on LENNART-69DE564 (patched)
> Windowing system distributor `Microsoft Corp.', version 5.1.2600
> configured using `configure --with-gcc (3.4) --cflags -Ic:/g/include'
> 
> Important settings:
> value of $LC_ALL: nil
> value of $LC_COLLATE: nil
> value of $LC_CTYPE: nil
> value of $LC_MESSAGES: nil
> value of $LC_MONETARY: nil
> value of $LC_NUMERIC: nil
> value of $LC_TIME: nil
> value of $LANG: ENG
> locale-coding-system: cp1252
> default-enable-multibyte-characters: t
> 
> Major mode: Info
> 
> Minor modes in effect:
> server-mode: t
> show-paren-mode: t
> encoded-kbd-mode: t
> tooltip-mode: t
> tool-bar-mode: t
> mouse-wheel-mode: t
> noticeable-minibuffer-prompts-mode: t
> menu-bar-mode: t
> file-name-shadow-mode: t
> global-font-lock-mode: t
> font-lock-mode: t
> blink-cursor-mode: t
> unify-8859-on-encoding-mode: t
> utf-translate-cjk-mode: t
> auto-compression-mode: t
> line-number-mode: t
> transient-mark-mode: t
> 
> Recent input:
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1> <mouse-1>
> <mouse-1> SPC <help-echo> <mouse-1> <mouse-1> [ SPC
> ] <down-mouse-1> <mouse-1> <down> <right> <down> <up>
> <up> <down> <down> <left> <left> <left> <left> <right>
> <right> <left> <right> <left> <down> <down> <down>
> <down> <down> <down> <down> <down> <down> <down> <down>
> <down> <down> <down> <down> <down> <down> <down> <down>
> <down> <down> <up> <up> <up> <up> <up> <down> <up>
> <right> <right> <right> <right> <right> <right> <right>
> <right> <right> <right> <right> <left> <down> <up>
> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
> <up> <up> <up> <up> <up> <up> <up> <up> <up> <down>
> [ SPC ] SPC <up> <down> <up> <up> <up> <up> <up> <up>
> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up> <up>
> <up> <up> <up> <up> <up> <up> <down> <down> <down>
> <down> <down> <down> <down> <down> <down> <down> <down>
> <down> <down> <down> <down> <down> <down> <down> <down>
> <down> <up> <prior> <next> <up> <down-mouse-1> <mouse-movement>
> <mouse-movement> <help-echo> <mouse-movement> <mouse-movement>
> <mouse-movement> <mouse-movement> <drag-mouse-1> C-x
> a <help-echo> <help-echo> <help-echo> M-w <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo>
> <help-echo> <menu-bar> <help-menu> <report-emacs-b
> ug>
> 
> Recent messages:
> (No files need saving)
> nil
> call-interactively: End of buffer
> call-interactively: Beginning of buffer
> Mark set [2 times]
> (2 3)
> (1 2 3 4 2 3) [2 times]
> (1 (2 3) 4 (2 3))
> (2 3)
> Loading emacsbug...done
> *** E-Mail body has been placed on clipboard, please paste them here! ***
> 
> 
>