1. A computer program product encoding a computer program for executing on a computer system a computer process for creating a user-defined function in a spreadsheet application, the user-defined function being callable by a calling label in the spreadsheet application, the computer process comprising: receiving a formula in the spreadsheet application; defining an interface identifying the calling label, an input parameter and a result parameter of the user-defined function; transferring the formula to a function sheet of the spreadsheet application to define at least a portion of a function body, wherein the function body operates on an argument corresponding to the input parameter and generates a result corresponding to the result parameter; and storing the interface in association with the function body to create the user-defined function.