[CM] Syntactic changes to snd-rt

[Kjetil S. Matheussen]

I've added a few improvements to the syntax of snd-rt.
What I like most is the use of the keywords ":where" and
":when" which can be placed anywhere in code. (inspired
by various functional languages)

":where" is a postfix operator to add local variables,
and ":when" is a postfix operator for doing conditional tests.



Personally, I think code can becomes more gentle when using
these operators.

For the use of ":where", it is nice to know what you want to do with a 
variable, before it is created.

And similarly, for the use of ":when', it is nice to know the action you
may want to do, before checking whether you want to do it.



Example below is the (usual) midi synthesizer using this new
syntax: (It looks much better in emacs with the "rt-DotEmacs"
file loaded!)

(<rt-stalin>
   (define (midi-synth)
     (while #t
       (wait-midi :command note-on
 	(sound
 	  (out (* (-> adsr next) (midi-vol) (oscil :freq (midi-to-freq (midi-note)))))
 	  (stop :when (-> adsr is-finished)))
 	(spawn
 	  (wait-midi :command note-off :note (midi-note)
 	    (-> adsr stop)))
 	:where adsr (make-adsr :a 20:-ms :d 30:-ms :s 0.2 :r 70:-ms))))
   (define (reverb input)
     (+ (* 0.5 dry)
        (* 0.1 wet)
        :where dry input
        :where wet (+ (comb :scaler 0.742 :size  9601 allpass-sum)
 		     (comb :scaler 0.733 :size 10007 allpass-sum)
 		     (comb :scaler 0.715 :size 10799 allpass-sum)
 		     (comb :scaler 0.697 :size 11597 allpass-sum)
 		     :where allpass-sum (send input :through
 					      (all-pass :feedback -0.7 :feedforward 0.7)
 					      (all-pass :feedback -0.7 :feedforward 0.7)
 					      (all-pass :feedback -0.7 :feedforward 0.7)
 					      (all-pass :feedback -0.7 :feedforward 0.7)))))
   (sound
     (out (reverb (in (midi-synth))))))



The same code, but without using :when and :where:

(<rt-stalin>
   (define (midi-synth)
     (while #t
       (wait-midi :command note-on
 	(define adsr (make-adsr :a 20:-ms :d 30:-ms :s 0.2 :r 70:-ms))
 	(sound
 	  (out (* (-> adsr next) (midi-vol) (oscil :freq (midi-to-freq (midi-note)))))
 	  (if (-> adsr is-finished)
 	      (stop)))
 	(spawn
 	  (wait-midi :command note-off :note (midi-note)
 	    (-> adsr stop))))))
   (define (reverb input)
     (define dry input)
     (define wet (let ((allpass-sum (send input :through
 					 (all-pass :feedback -0.7 :feedforward 0.7)
 					 (all-pass :feedback -0.7 :feedforward 0.7)
 					 (all-pass :feedback -0.7 :feedforward 0.7)
 					 (all-pass :feedback -0.7 :feedforward 0.7))))
 		  (+ (comb :scaler 0.742 :size  9601 allpass-sum)
 		     (comb :scaler 0.733 :size 10007 allpass-sum)
 		     (comb :scaler 0.715 :size 10799 allpass-sum)
 		     (comb :scaler 0.697 :size 11597 allpass-sum))))
     (+ (* 0.5 dry)
        (* 0.1 wet)))
   (sound
     (out (reverb (in (midi-synth))))))