in reply to Using a paramter value to call a module

I have faced this issue enough times to construct the following function: 

sub require_if_present
  {
    # So here is a trick for discovering if the module is 
    # available (and loading it if it is).  This function returns 
    # True if the module is there and this generally sets a 
    # flag to tell the rest of the program how to behave
    my($module_name,$loaded) = @_;    
    my($module_found);
    
    $module_found = 0;
    
    eval(<<"LoadModule");
        # Use require so we don't load the module if it is not there
        require $module_name;
        \$module_found = 1;
LoadModule

    return($module_found);
  }

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So now I can load different modules depending on run time features such as the OS being used or the value of a command line option. 

    # Load different modules depending on OS
    if($operating_system =~ /^MSWin/i)
      {
        if(!&require_if_present("Win32"))
          {
            $no_win32 = 1;
          }
      }

    # The $no_tk etc vars are set by command line
    # options before I get here
    if(!$no_tk)
      {
        if(&require_if_present("Tk"))
          {
            &require_if_present("Tk::DialogBox");
            if($no_fileselect || 
                         !&require_if_present("Tk::FileSelect"))
              {
                $no_fileselect = 1;
              }
            if($no_progress || 
                         !&require_if_present("Tk::ProgressBar"))
              {
                $no_progress = 1;
              }
            if($no_dirtree || !&require_if_present("Tk::DirTree"))
              {
                $no_dirtree = 1;
              }
          }
        else
          {
            $no_tk = 1;
          }
      }

    # Then later I can...
    if($no_tk)
      {
        # Run without an interface
        ...
      }
    else
      {
        $main_win = new Tk::MainWindow(
            -title   => "My App",
          );
        ...
      }

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Re: Re: Using a paramter value to call a module
by pike (Monk) on Feb 07, 2003 at 11:13 UTC
    Interesting solution, but could you please explain the eval statement in the require_if_present sub? It looks as if you are using a here document, but would a try-catch-block eval not be more appropriate here? E.g.

    sub require_if_present {
   my $mod = shift;
   eval {
      require $mod;
      return 1;
   }
   return 0 if $@;
}

    [download]

    pike

[reply]
[d/l]

      require() has different meanings depending on the argument. See perlfunc. It even has an example of what you're trying to do, and why it's helps to use eval EXPR.

      <nit>eval requires a semi-colon after it.</nit>

      ihb
[reply]
[d/l]
[select]

      Interesting solution, but could you please explain the eval statement in the require_if_present sub? It looks as if you are using a here document, but would a try-catch-block eval not be more appropriate here?

      TMTOWTDI: That just happens to be the way I implemented it. A here document is convenient for some of the other things I do this way, but that is not a good reason to use it here. Using an eval as you suggest sounds like a cleaner approach. But against that this works (I have used it many times in the last year) and I try not to fix things that work.

      Update: Thinking about this last night I realised why I used a here document to do an eval, its because this was a very common construct in Perl 4 (now that is a really bad reason for doing it here).

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