# Expression Functions

You can call a TERR script directly from the Spotfire expression language. Using a TERR expression function, you can perform an ad hoc analysis quickly and easily.

For ad-hoc expressions, you can embed the contents of a script directly in your expression language call. Alternatively, you can write a TERR script, and then register it as an expression function for use anytime from the Custom Expression dialog box in Spotfire.

After you have created an expression function, you can reopen it, edit it, and rerun it.