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From: cargo@TARDIS.CRAY.COM (David S. Cargo)
Newsgroups: comp.lang.icon
Subject: generation of procedures
Message-ID: <9002071557.AA01397@zk.cray.com>
Date: 7 Feb 90 15:57:47 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The Internet
Lines: 47

I was looking at the code for RSG (part of the Icon Program Library),
noticing how one part of the code takes advantage of the order of a
list of procedure variables to successively try evaluating a line of
input until one of the procedures succeeds.   My first reaction was
that this reminded me of searching an object hierarchy looking for a
handler for a particular type of message.  If a particular object
can't handle the message, it fails and lets the object next higher
in the hierarchy have a crack at it.  It sort of reminded me of
message passing, but with a distinctively Icon flavor to it.

My real question is that I can't figure out how to decide what the
symantics of the operation really are.  Normally I would expect to
say the !plist comes as element generation, which should succeed
when the first element is generated.  But then it is followed by
a parameter list and surrounded by parentheses.  This seems to
combine to make it an alternation expression equivelent to:

   (define | generate | grammar | source | comment | prompter | error)(line)


   plist := [define,generate,grammar,source,comment,prompter,error]
   :
   :
   while in := pop(ifile) do {		# process all files
      repeat {
         writes(\prompt)
         line := read(in) | break
         while line[-1] == "\\" do line := line[1:-1] || read(in) | break
         (!plist)(line)
# the line above is the interesting one!
         }
      close(in)
      }


I can't seem to find anything in the Icon book that spells out what is really
happening here.  It looked at first like !plist wasn't in a context that required
generation of all the list elements, but clearly that is not the case.

The confused snail,      o       o
                          \_____/
                          /-o-o-\     _______
DDDD      SSSS   CCCC    /   ^   \   /\\\\\\\\
D   D    S      C        \ \___/ /  /\   ___  \
D   D     SSS   C         \_   _/  /\   /\\\\  \
D   D        S  C           \  \__/\   /\ @_/  /
DDDDavid SSSS.   CCCCargo    \____\____\______/ cargo@tardis.cray.com