Defined functions

Algorithms written in APL are called defined functions. A defined function consists of a header line and zero or more body lines. The header line indicates the name and syntax class of the function and gives a list of names to be localised and, in the case of argument names, initialised. Each body line consists of an optional label followed by an APL line to be evaluated. Body lines are evaluated in the order in which they occur in the defined function unless a statement beginning with a right arrow is evaluated.A defined function is established in a workspace by the system function □ FX, by actions in function definition mode, or by the system command ) COPY. In this standard, a defined-function is represented as an object with three attributes: canonical-representation, trace-vector, and stop-vector. A defined-function is called when its name occurs in a prefix of the current-stack matching one of the patterns in the phrase-table. A defined-function is called by the operation call defined function. This operation prefixes the state-indicator with a new context, localises and initialises any local names in the header-line, and calls the operation defined-function-control. A defined-function ends when defined-function-control finds current-line-number set to a value not in the closed-interval-between one and last-line-number. At this point, the first item of the state-indicator is discarded and a token of class result is returned, via call defined function, to the phrase-evaluator that called it.The returned token may be constant (for functions returning a value), nil (for functions that return no value), unwind (for functions that end through the evaluation of an escape arrow), or reset (as the result of an ) SIC command).If a line in a defined-function signals an error, immediate-execution is called to report the error and suspend the defined-function. Immediate-execution may return either a branch to indicate that evaluation of the defined-function should continue, or an escape or reset to indicate that the defined-function context should be removed from the state-indicator.