Mathematica is an expression rewriting language. Rules and definitions basically say "See this? Rewrite as that.". And then, once it has done the rewrite, it re-evaluates the resulting expression. So, what you want doesn't seem to be anything special. If (shudder), you want the rewritten expression to have side effects, you can arrange it.
Perhaps what you're looking for is RuleDelayed.
RuleDelayed
I'm still not clear on exactly what you want but look at Reap and Sow (or related, Throw and Catch). You can use them in a "myWhen" function that triggers your action. Note that you can use the optional extra argument to Reap and Sow to gather up different results in different lists.
I hope this helps.
Regards,
Neil
Joshua,
I think you want Cases. Modifying their first documentation example a bit:
In[2]:= Cases[{1, 1, f[a], 2, 3, y, f[8], 9, f[10]}, i_Integer -> i^2] Out[2]= {1, 1, 4, 9, 81}
Thank you for your response Neil, however this isn't what I'm looking for. I'm in need of something that will either look for trueness in the left hand side of the > or a structure match and then evaluate the right hand side of the > that way I can do conditional tasks that partially relate. Again, thank you for taking your time to respond!
