Need help with a macro

Need help with a macro

[Josef Wolf]

Hello,

I am trying to work through the little schemer book. In order to make it
easier to go through the examples, I've come up with the following macro:

  ;;; Helper to visualize

  (define-macro (disp exp)
    (display exp)   (newline)
    `(display ,exp)
  ;  (newline)  (newline)
  )

This works great, except that when I uncomment the (newline) expressions,
the result of the evaluation is omitted:

  guile> (define-macro (disp exp)
  ...   (display exp)   (newline)
  ...   `(display ,exp)
  ... ;  (newline)  (newline)
  ... )
  guile> (disp (+ 3 4)) (newline)
  (+ 3 4)
  7
  guile> (define-macro (disp exp)
  ...   (display exp)   (newline)
  ...   `(display ,exp)
  ...   (newline)  (newline)
  ... )
  guile> (disp (+ 3 4)) (newline)
  (+ 3 4)
  
  
  
  guile>

Any hints?

Re: Need help with a macro

[Andy Wingo]

Hello,

On Sun 15 Nov 2009 17:33, Josef Wolf <jw@raven.inka.de> writes:

> Hello,
>
> I am trying to work through the little schemer book. In order to make it
> easier to go through the examples, I've come up with the following macro:
>
>   ;;; Helper to visualize
>
>   (define-macro (disp exp)
>     (display exp)   (newline)
>     `(display ,exp)
>   ;  (newline)  (newline)
>   )

Does the little schemer book actually use defmacro?

Anyway the issue is that defmacro needs to return Scheme code as an
s-expression. The last expression in a function is its return value.
That would be the `(display ,exp) bit there. But if you uncomment the
newlines, well, you return whatever newline returns, which is actually
unspecified.

Good luck,

Andy
-- 
http://wingolog.org/

Re: Need help with a macro

[Neil Jerram]

Josef Wolf <jw@raven.inka.de> writes:

> Hello,
>
> I am trying to work through the little schemer book. In order to make it
> easier to go through the examples, I've come up with the following macro:
>
>   ;;; Helper to visualize
>
>   (define-macro (disp exp)
>     (display exp)   (newline)
>     `(display ,exp)
>   ;  (newline)  (newline)
>   )
>
> This works great, except that when I uncomment the (newline) expressions,
> the result of the evaluation is omitted:
>
>   guile> (define-macro (disp exp)
>   ...   (display exp)   (newline)
>   ...   `(display ,exp)
>   ... ;  (newline)  (newline)
>   ... )
>   guile> (disp (+ 3 4)) (newline)
>   (+ 3 4)
>   7
>   guile> (define-macro (disp exp)
>   ...   (display exp)   (newline)
>   ...   `(display ,exp)
>   ...   (newline)  (newline)
>   ... )
>   guile> (disp (+ 3 4)) (newline)
>   (+ 3 4)
>   
>   
>   
>   guile>
>
> Any hints?

The body of a define-macro definition is supposed to return the code
that should be substituted in place of the original macro call.  With
the newlines there, the body returns *unspecified*, because that is what
(newline) returns.

Do you want those newlines to happen at macro-expansion time or eval
time?  If the latter (which I would guess), the code that you want is

(define-macro (disp exp)
  (display exp)   (newline)
  `(begin
     (display ,exp)
     (newline)  (newline)))

Regards,
        Neil

Re: Need help with a macro

[Josef Wolf]

On Sun, Nov 15, 2009 at 08:26:58PM +0000, Neil Jerram wrote:
> Josef Wolf <jw@raven.inka.de> writes:
> > I am trying to work through the little schemer book. In order to make it
> > easier to go through the examples, I've come up with the following macro:
> > [ ... ]
> 
> The body of a define-macro definition is supposed to return the code
> that should be substituted in place of the original macro call.  With
> the newlines there, the body returns *unspecified*, because that is what
> (newline) returns.

OK, I think I see the problem now...

> Do you want those newlines to happen at macro-expansion time or eval
> time?  If the latter (which I would guess), the code that you want is
> 
> (define-macro (disp exp)
>   (display exp)   (newline)
>   `(begin
>      (display ,exp)
>      (newline)  (newline)))

Works great. Thanks!

Re: Need help with a macro

[Josef Wolf]

On Sun, Nov 15, 2009 at 07:18:33PM +0100, Andy Wingo wrote:

> On Sun 15 Nov 2009 17:33, Josef Wolf <jw@raven.inka.de> writes:
> > I am trying to work through the little schemer book. In order to make it
> > easier to go through the examples, I've come up with the following macro:
> > [ ... ]
> 
> Does the little schemer book actually use defmacro?

It does not. But I use it only to avoid typing everything twice. I think it
is allowed for such usage ;-)

> Anyway the issue is that defmacro needs to return Scheme code as an
> s-expression. The last expression in a function is its return value.
> That would be the `(display ,exp) bit there. But if you uncomment the
> newlines, well, you return whatever newline returns, which is actually
> unspecified.

Thanks!