Recursive lexically-scoped subroutines

[p5pRT]

Migrated from rt.perl.org#1005 (status was 'resolved')

Searchable as RT1005$

[p5pRT]

From garethr@cre.canon.co.uk

If I run the program

  use strict;
  sub factorial {
  my $fact = sub {
  my ($n) = @​_;
  $n > 0 ? $n * $fact->($n-1) : 1;
  };
  $fact->(@​_);
  }

I get the error

  Global symbol "$fact" requires explicit package name at fact.pl line 9.

The `perlsub' manpage states that

  A "my" declares the listed variables to be confined
  (lexically) to the enclosing block, conditional
  (if/unless/elsif/else), loop
  (for/foreach/while/until/continue), subroutine, eval, or
  do/require/use'd file.

so I think that either Perl's behavour should be changed to match the
documentation, or the documentation should be made clearer.

(I know that I can rewrite `factorial' as

  sub factorial {
  my $fact; $fact = sub {
  my ($n) = @​_;
  $n > 0 ? $n * $fact->($n-1) : 1;
  };
  $fact->(@​_);
  }

in order to get it to work as intended. That's not what this report is
about.)

Perl Info

Site configuration information for perl 5.00502:

Configured by abw at Thu Nov 19 10:50:08 GMT 1998.

Summary of my perl5 (5.0 patchlevel 5 subversion 2) configuration:
  Platform:
    osname=solaris, osvers=2.6, archname=sun4-solaris
    uname='sunos bandanna 5.6 generic_105181-03 sun4u sparc sunw,ultra-1 '
    hint=previous, useposix=true, d_sigaction=define
    usethreads=undef useperlio=undef d_sfio=undef
  Compiler:
    cc='gcc', optimize='-O', gccversion=2.8.0
    cppflags='-I/opt/gnu/include -I/user/perl/build/include'
    ccflags ='-I/opt/gnu/include -I/user/perl/build/include'
    stdchar='unsigned char', d_stdstdio=define, usevfork=false
    intsize=4, longsize=4, ptrsize=4, doublesize=8
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    alignbytes=8, usemymalloc=n, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags ='-L/opt/gnu/lib -L/user/perl/build/lib'
    libpth=/opt/gnu/lib /lib /usr/lib /usr/ccs/lib /user/perl/build/lib
    libs=-lsocket -lnsl -lgdbm -ldl -lm -lc -lcrypt -ldb
    libc=/lib/libc.so, so=so, useshrplib=false, libperl=libperl.a
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags=' '
    cccdlflags='-fPIC', lddlflags='-G -L/opt/gnu/lib -L/user/perl/build/lib'

Locally applied patches:
    


@INC for perl 5.00502:
    /apps/perl5/lib/5.00502/sun4-solaris
    /apps/perl5/lib/5.00502
    /apps/perl5/lib/site_perl/5.005/sun4-solaris
    /apps/perl5/lib/site_perl/5.005
    .


Environment for perl 5.00502:
    HOME=/user/garethr
    LANG=C
    LD_LIBRARY_PATH=/usr/dt/lib:/usr/openwin/lib:/apps/X11R5/lib:/usr/lib
    LOGDIR (unset)
    PATH=/usr/dt/bin:/usr/openwin/bin:/bin:/usr/bin:/usr/ucb:/bin:/usr/bin:/cre/bin:/apps/gcc-2.8.0/bin:/apps/frame5/bin:/apps/emacs/bin:/apps/tex/bin:/apps/perl5/bin:/user/web/bin/scripts:/user/web/bin/SunOS-5.6:/apps/gnu/bin:/apps/mysql/bin:/usr/ccs/bin:/apps/X11R5/bin:/usr/openwin/bin
    PERL_BADLANG (unset)
    SHELL=/cre/bin/bash

[p5pRT]

@iabyn - Status changed from 'open' to 'resolved'

[p5pRT]

From @iabyn

Date​: Fri, 16 Jul 1999 15​:24​:38 +0100
If I run the program

use strict;
sub factorial \{
  my $fact = sub \{
    my \($n\) = @​\_;
    $n > 0 ? $n \* $fact\->\($n\-1\) : 1;
  \};
  $fact\->\(@​\_\);
\}

I get the error

Global symbol "$fact" requires explicit package name at fact\.pl

line 9.

The `perlsub' manpage states that

A "my" declares the listed variables to be confined
\(lexically\) to the enclosing block\, conditional
\(if/unless/elsif/else\)\, loop
\(for/foreach/while/until/continue\)\, subroutine\, eval\, or
do/require/use'd file\.

so I think that either Perl's behavour should be changed to match the
documentation, or the documentation should be made clearer.

What you are seeing here is a more glorified version of

  my $x = $x;

From perlsub.pod​:

  The declared variable is not introduced (is not visible) until after
  the current statement. Thus,

  my $x = $x;

  can be used to initialize a new $x with the value of the old $x, and
  the expression

  my $x = 123 and $x == 123

  is false unless the old $x happened to have the value C<123>.

So the lexical $fact doesnt come into scope until the end of the
assigment, so the '$fact' seen within the anon sub is the global
$​::fact,
which doesn't exist.

Dave M.

[p5pRT]

From @iabyn

notabug.

[p5pRT]

@iabyn - Status changed from 'open' to 'resolved'