Path: utzoo!mnetor!tmsoft!torsqnt!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!uakari.primate.wisc.edu!ames!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Are only simple scalars allowed in "do SUBROUTINE (LIST)" ?
Message-ID: <7269@jpl-devvax.JPL.NASA.GOV>
Date: 5 Mar 90 18:03:12 GMT
References: <1211@frankland-river.aaii.oz.au>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 50

In article <1211@frankland-river.aaii.oz.au> pem@frankland-river.aaii.oz.au (Paul E. Maisano) writes:
: The manual section for "do SUBROUTINE (LIST)" says:
: 	SUBROUTINE may be a scalar variable, ... The parentheses are
: 	required to avoid confusion with the "do EXPR" form.
: 
: I always assumed that an array reference could be considered a scalar.

A scalar value, but not a scalar variable.

: In particular I thought the following was allowed.
: 
: #!/usr/bin/perl
: sub sub1 {print "1\n";}
: sub sub2 {print "2\n";}
: @subs = ('sub1', 'sub2');
: do $subs[1]();
: 
: Unfortunately it gives a syntax error.
: 
: I think this is a pity. It is a useful thing to be able to do.
: Allowing an arbitrary EXPR instead of a scalar would be even better :-)

I might be able to get away with it in this part of the grammar, but
there are other similar parts of the grammar that I couldn't, and I
have to watch out for non-orthogonalities...

On top of which, there's the readability issue.

I also have people arguing for ('foo','bar')[$which].  Maybe.  I'm not
done thinking about it.  It's certainly more powerful to have [] and ()
as first class operators, but I'm not sure that such conciseness is
always a virtue.

: Of course, we could always just use eval instead.
: Not as elegant though (or as efficient, maybe).

More efficient than eval would be to use a temp variable:
    
    @subs = ('sub1', 'sub2','sub3','sub4');

    $whichsub = $subs[index($blather{$token},pack("c",$current_state))];
    do $whichsub(*my_stack);

I almost like that better than

    do $subs[index($blather{$token},pack("c",$current_state))](*my_stack);

There's something to be said for naming your intermediate products.

Larry