Include result of function call within WHERE clause in results

Hi all -

I need to select the rows of a table that satisfy a condition that
includes a function call, and I want to include the result of the
function call as a column in the resulting table. For example, this
pseudo-code produces the result I'm looking for:

SELECT field1, some_function(...) FROM my_table
WHERE some_function(...) < some_number

However, I would like to avoid calling the function in two places,
especially since embedded in the parameters to the function I have
another SELECT command. Not to mention it just looks messy...

I've already tried the following without luck:

SELECT field1, some_function(...) AS result FROM my_table
WHERE result < some_number

Is there a way to cache the result of the function call so it can be
used in both places without two separate calls?

Thanks in advance,
Dallas Morisette