I was chugging right along. I had a File access class which had all the methods of our FTP access class. You could call size(), exists(), etc. And it didn't matter whether you where accessing an FTP server or a local file.

Then all my fun came to an end.

With files, the option to read at on offset is exercised after the file is open via a seek. While FTP servers, the option to read at an offset is exercised via the REST command prior to opening the file.

Usually polymorphism, just means calling the same method and having the instance do the right thing. Here, polymorphism must span a series of API calls to work right. Ie, 

$file->open;
$file->seek;
$file->read;

## must translate to open, seek, read for files
## must translate to REST, RETR for FTP

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In reply to Polymorphic Abstraction of Disk versus FTP File Access: A Stumbling Block by princepawn

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