ABSTRACT
This chapter discusses tools which are useful for writing complex A Programming Language (APL) functions and "programs". They include forms of branching, line labels, recursive functions, methods for input and output, format, and the powerful function execute. Computer programs lack the essential mathematical nature that those APL functions had; namely, those functions had explicit arguments and produced values or results that are automatically passed along as arguments for the next function. All user-defined functions previously considered produced a result or value that was passed on as an argument for the next function to the left or was displayed in the session log. A function might be constructed to invoke several other functions or effect an organized display of data. One of the powerful devices in user-defined functions is branching –the capacity for directing the subsequent flow of function execution depending on conditions at the branch point. A line label provides a name for a function line.
