RFC 184 (v1) Perl should support an interactive mode.

This and other RFCs are available on the web at
  http://dev.perl.org/rfc/

=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