How to write a custom function/method

Hi. I was wondering how to write a custom user function eg. a binary to decimal converter. The function may look like this:

function binaryToDecimalConverter (String binary)
{
do some calculation;
return the decimal value;
}

Also, how to call such custom functions. A help on this topic would be much appreciated.

Anyone willing to help does not have to write the function for me, just the basic syntax is welcomed . Please let me know if any documentation is available on this topic.

The manual documents User Commands and syntax, which are also good search terms for this forum.