There is really a somewhat deeper issue.  Like many who use LaTeX for more than superficial stuff (I wrote a class file newlfm and several sty files, most of which are not much used), I can read documentation and figger it out somewhat.  However, knowing of the EXISTENCE of a tool is really the key, not how it is used.  How are people to learn that specific primitives exist?  It's not in the LaTeX Companion, which is my starting point.
 
Paul Thompson

From: Joseph Wright <[log in to unmask]>
To: [log in to unmask]
Sent: Sun, January 2, 2011 5:45:58 AM
Subject: Re: LaTeX3 and engines

On 01/01/2011 21:16, Arno Trautmann wrote:
> Paul Thompson wrote:
>> What sort of documentation is available for \pdfstrcmp?  I confess that I have
>> never heard of this toolset.
>
> pdfTeX manual says:
>
> \pdfstrcmp<general text>  <general text>  (expandable)
> This command compares two strings and expands to 0 if the strings are
> equal, to -1 if the first string ranks before the second, and to 1
> otherwise. The primitive was introduced in pdfTEX 1.30.0.
>
> searching the internet may bring even more information.

Like many of the newer pdfTeX primitives, there is to some extend a need to 'suck it and see' to find out what happens. For LaTeX3, all we do is use the 'equal' test. The test does expansion, so to test two token lists as strings without any expansion, you do

\def\StringCompareTF#1#2{%
  \ifnum\pdfstrcmp{\unexpanded{#1}}{\unexpanded{#2}}=\z@
    \expandafter\@firstoftwo
  \else
    \expandafter\@secondoftwo
  \fi
}

or code to that effect.
--
Joseph Wright