TITLE

Perl should not abort when a required file yields a false value

VERSION

  Maintainer: Dominus <mjd@plover.com>
  Date: 21 Sep 2000
  Last Modified: 23 Sep 2000
  Mailing List: perl6-language@perl.org
  Number: 269
  Version: 2
  Status: Withdrawn
  See Also: RFC 55

STATUS

This proposal is withdrawn because it duplicates RFC 55, "Compilation: Remove requirement for final true value in require-d and do-ed files"

ABSTRACT

Modules should not have to end with 1;. It is silly and confusing.

DESCRIPTION

Modules typically contain subroutine definitions. A module may contain initialization code also. If the initialization code fails, the module can return a false value to its caller, which aborts the compilation.

In Perl 5, a module that contains nothing but subroutine definitions will return false by default, necessitating a

1;

at the bottom. If the 1; is omitted, Perl emits the error

        Foo.pm did not return a true value...

In spite of plenty of documentation, people Frequently Ask what this error means. Some languages like to have the compiler emit annoying messages to announce you forgot to include some pointless code whose only purpose is to stop the compiler from emitting the annoying message. Perl is mostly free of such nonfeatures.

I propose that this unfeature be dropped entirely. No useful functionality is lost. If a Perl 6 module wants to indicate an initialization failure by throwing a fatal exception, it can simply call die. If the calling module wants to abort when a required file returns a false value, it is free to do that.

The 'module initialization' feature is little-used. 99 the of 102 files in Perl 5.6 lib/*.{pl,pm} end with 1;. AnyDBM_File invokes 'die' explicitly. The only real exceptions are diagnostics.pm and timelocal.pl.

IMPLEMENTATION

'require' should execute code in a file and return the result, as before, but it should not call Perl_die when the result is false.

However, see below.

MIGRATION

In 98% of cases, no translation is necessary. The first version of the translator can ignore the issue entirely. Strategies to cover the other 2% follow:

Is general, direct source translation of this feature of Perl 5 modules would probably be impossible.

It's tempting to say that the translator should simply translate the last statement or block in the module from this:

        STATEMENT

to this:

        unless (do {STATEMENT}) {
          require Carp;
          Carp::croak "... did not return a true value";
        }

However, I think that is impractical. The module might contain code that looks like this:

        if (something()) {
          return $v1;
        }

        ...

        $v2;

In this case the 'return $v1' statement would also have to be translated. In general, there might be many, many statements that would need to be translated. This would look awful.

I think that if complete coverage is desired, the best choice would be to introduce a new pragma, which would enable the old behavior. A translated module would begin with

        package Foo;
        use perl5 'require/use semantics'; 

        ...

When this file was required, the pragma would set a flag. The pp_require opcode would check the flag after compiling the file, and would call Perl_die as before if the file returned a false value and if the flag was set. If Foo required any other modules, the flag would be cleared before loading them, and restored again afterwards. (That is, the flag would have file scope.)

REFERENCES

Perl on-line manuals

This file is part of the Perl 6 Archive