Chapter 4

The only aspect of file naming that File::Spec doesn't treat is the constraints on 

the actual names themselves. The rule here is keep them short and simple. To be 

maximally portable, your filenames shouldn't be longer than 11 characters and 

should contain only one dot (.). Also, you can't count on a case sensitive file 

system, so having both test.pl and TEST.pl in the same directory is out. Spaces in 

filenames are generally verboten, as are punctuation characters aside from under 

scores and hyphens.

Portable modules should never contain hard coded paths. If you need to refer 

to an external file, you should provide a way for users of the module to specify it. Of 

course, when they do, they'll be using the filenaming conventions of their platforms, 

so use of File::Spec is necessary.

On Self Reliance

Think of a user's system as a strange and alien land you don't know where any 

thing is, you don't know who to ask for help, and you don't speak the language well 

enough to understand the user's reply if you did. In Perl terms, don't expect to be 

able to call external programs. Even if you could reliably locate the program you 

want, there's probably a version out there that produces output you aren't expecting. 

A classic example is the ps utility found on many UNIX systems. Every ps accom 

plishes the same basic task: displaying information about running processes. 

Unfortunately, every flavor of ps is different each takes different options and 

produces different output formats.

If you need a particular service in a portable module, you have two options 

code it yourself in pure Perl, or find a module that has it. The latter solution is def 

initely preferable portability problems are common ground shared by the entire 

CPAN community. The chances are quite good that the service you need is already 

available on CPAN.

Coding Around the Problem

One way to address portability is to build it into your module's design. A good example 

of this is in the way the File::Spec module works. File::Spec provides a standard API 

across all platforms using an object oriented inheritance strategy. File::Spec is con 

ceptually a virtual base class from which the concrete File::Spec subclasses inherit.

24

When you perform a use File::Spec, code inside File::Spec determines which 

24. Notice that I said  conceptually.  The implementation is actually the reverse File::Spec 

dynamically chooses one of the  subclasses  as its parent class. Perl OO is fantastic!

124