Re: session macros

[David Kastrup]

Han-Wen Nienhuys <address@hidden> writes:

> I've been looking at (auto)compilatoin for a bit. It fails with
>
> ;;; WARNING: compilation of
> /home/hanwen/vc/lilypond/lily/share/lilypond/current/scm/chord-entry.scm
> failed:
> ;;; unhandled constant #<procedure add-session-variable (name value)>
>
> I'm puzzled by your macro:
>
> (defmacro define-session (name value)
>   ..
>   (define (add-session-variable name value)
>     (set! lilypond-declarations
>           (cons (make-session-variable name value) lilypond-declarations)))
>   `(,add-session-variable ',name ,value))
>
> I understand why we need sessions, but I don't understand why this has
> to be a macro, and why the macro has to define add-session-variable.
>
> Why can't this be something like
>
>   (define lilypond-declarations '())
>   (define (define-session name value)
>      (set! lilypond-declarations
>           (cons (make-session-variable name value) lilypond-declarations)))
>
>
> When calling the macro twice, we'll define add-session-variable twice,
> which seems fishy.

I think you got confused here.  A macro is just an ordinary function on
its inside, it only differs in its calling conventions during
evaluation: instead of evaluating its arguments and calling the function
body like with ordinary functions, it is called with unevaluated
arguments and the result is then evaluated in its expression.

add-session-variable is a local function defined inside of
define-session.  The effect is that it is not callable externally, and
not modifiable externally.  It is just putting stuff near to where it is
being used (it is used as a function in the expansion of the macro).

define-session is a macro since its use parallels the use of define ,
and the first argument of define is the symbol to be defined which must
not be evaluated before define-session is being called.

-- 
David Kastrup