Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!rutgers!rochester!pt.cs.cmu.edu!dsl.pitt.edu!pitt!willett!ForthNet
From: ForthNet@willett.pgh.pa.us (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: Vocabularies
Message-ID: <1649.UUL1.3#5129@willett.pgh.pa.us>
Date: 31 Aug 90 02:54:00 GMT
Organization: String, Scotch tape, and Paperclips.  (in Pgh, PA)
Lines: 82

Category 18,  Topic 92
Message 44        Thu Aug 30, 1990
D.RUFFER [Dennis]            at 00:28 EDT
 
Re: R.BERKEY [Robert]

 >       IF(
 >          c1 e1
 >          ...
 >          ck ek
 >       )IF

 > So, no.  None of the posted solutions work.  At least to the
 > problem for which a solution was sought.

 > Another closely related issue is whether comments are to be
 > allowed within the range of the custom compiler.

I believe I could solve the multiple line problem easily (if it isn't already
solved by using blocks), but adding comments does invalidate both Mitch's and
my solutions (not to mention invalidating John's original solution).  So, we
just need to think about it a little more closely.  (no one said it was going
to be easy :-).

What we really have here is a case statement and the "problem" is that John
has typically written it without including "markers" to separate the cases. 
This organization makes it essentually impossible to put anything between the
start of the case statements and the end except pairs of logical tests and
execution phrases. The fact remains that John's code allows multiple lines and
excludes intervening comments.  Therefore I contend that my solution works
just as well as his version, and Mitch's is probably also correct.

However, since you have added an additional requirement to the problem, I will
suggest an alternate solution.  Even if I were to solve the problem without
adhearing to the ANS Standard, I really don't see any way to add comments
without adding in the missing markers.  So, if you don't mind, I will change
the format to allow the requirements to be satisfied.  Here is the new format
that I will try to implement

        CASE(  ...  OF  ...  ELSE
               ...  OF  ...  ELSE
                .
                .
                .
               ...  )CASE

Now, without even looking at all the ways case statements have been
implemented before, here is a way that will work within the standard:

 VARIABLE #CASES
 : CASE(   0 #CASES ! ;    IMMEDIATE
 : )CASE   BEGIN   #CASES @
           WHILE   POSTPONE THEN  -1 #CASES +!
           REPEAT ;    IMMEDIATE
 : OF   1 #CASES +!  POSTPONE IF ;    IMMEDIATE

Now, ain't that easy enough?  Of course, there is the multi-user implications
with using a variable to count the cases, but that can be easily solved by
making the variable a stack.  That would even allow nesting the case
structures.  Although this solution to John's problem does not look as
"pretty", the added functionality more than makes up for the the loss of
asthetics.  Here is how John's original problem would look:

 : Perm  ( n -- t )  DUP  2 =  \ terminal condition
      IF    DROP  1
      ELSE  DUP  Next-Pos  ( n pos )
            CASE(    <n?   OF   Ascending     ELSE
                     =n?   OF   p(n-1)        ELSE
                     <2n?  OF   Descending    ELSE
                     true  OF   p(n-1)+1     )CASE
      THEN  ;

Of course, I should also mention the range problems that are inherant with the
"embedded IF" method on some Forths (16 bit polyFORTH for one), but the ANS
Stanadard hasn't touched that problem so I will ignore it also.  <grin>

Now, can I say again, YES IT DOES!

DaR
-----
This message came from GEnie via willett through a semi-automated process.
Report problems to: uunet!willett!dwp or dwp@willett.pgh.pa.us