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=head1 TITLE
Perl should support an interactive mode.
=head1 VERSION
Maintainer: Ariel Scolnicov <[EMAIL PROTECTED]>
Date: 31 Aug 2000
Mailing List: [EMAIL PROTECTED]
Version: 1
Number: 184
Status: Developing
=head1 ABSTRACT
Perl5 does not have an interactive mode. The debugger is fine for
testing a single line, but it is inadequate for running a set of
commands interactively. The Perl6 parser (and possibly the language)
should contain hooks to allow full interactive environments to be
written.
=head1 DESCRIPTION
Perl does not have an interactive mode. It has C<perl -de 42>, but
that is not the same. An interactive mode is useful not only for a
debugger, but also for exploring the capabilities of a module, or even
for performing simple "one-off" programming tasks.
The most serious obstacle to easy interaction is the impossibility of
typing multiple line commands to a Perl debugger (see below).
However, the Perl debugger also limits this use in other ways, notably
by evaluating each line in a separate C<eval>. This too has
unfortunate consequences.
Languages which include better interactive capabilities than Perl's
include Python and zsh.
=head2 Example
Observe an interaction with another language whose name begins with a
`P':
Python 1.5.1 (#1, Jul 28 1998, 22:02:27) [GCC 2.7.2.3] on sunos5
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> def fact(x):
... if x <= 1:
... return 1
... else:
... return x*fact(x-1)
...
>>> fact(10)
3628800
>>> ^D
Note in particular the definition of C<fact>, which spans multiple
lines.
With Perl5, it doesn't work:
<bioserv 108 [13:31] ~ >perl -de 42
Loading DB routines from perl5db.pl version 1.0402
Emacs support available.
Enter h or `h h' for help.
main::(-e:1): 42
DB<1> sub fact {
Missing right bracket at (eval 5) line 4, at end of line
syntax error at (eval 5) line 4, at EOF
[ oops... must fit it all on one line ]
< shift; if ($x < 2) { return 1 } else { return $x*fact($x-1) }
Missing right bracket at (eval 12) line 4, at end of line
syntax error at (eval 12) line 4, at EOF
[ can't see the beginning of the line I'm editing, and forgot a close
brace; might as well forget about any indentation to help remind me ]
<if ($x < 2) { return 1 } else { return $x*fact($x-1) } }
[ Finally! But I can't even see what I typed! ]
DB<4> print fact(10)
3628800
=head2 Variable scoping
C<my> and C<local> variables don't work in the debugger as one would
expect; their scope does not propagate between lines:
<bioserv 112 [14:08] ~ >perl -de 42
Loading DB routines from perl5db.pl version 1.0402
Emacs support available.
Enter h or `h h' for help.
main::(-e:1): 42
DB<1> $x = 17
DB<2> my $x=5
DB<3> x $x
0 17
The ability to be able to create variables is essential for serious
interactive use of Perl.
=head2 Possible uses
=over 4
=item *
The Perl debugger (and other Perl debuggers)
=item *
Interaction environments (e.g. C<perldl>)
=item *
"Super" calculators
=item *
Perl shell
=back
=head1 IMPLEMENTATION
This would require considerable assistance from the parser, I think.
At the very least, it should be possible to feed Perl multi-line input
from the terminal. Perl should read whole lines, and respond I<only>
when it has parsed a complete statement at the end of a line or when
it has read enough to identify a syntax error.
There are proposals to allow programmatic access to the Perl parser.
Such access might allow "interactive Perl" (C<iPerl>?) to be written
in Perl.
User input will need to be interpreted in a "continuously interrupted"
context of the Perl interpreter. Use of C<eval> is insufficient, as
the second example show. Being able to create, run and interrogate a
secondary Perl interpreter (from within Perl) could help.
As much as possible of these interaction environments should be
outside of the core, preferably written in Perl.
=head1 REFERENCES
