In functional programming, a fold is a higher-order function that analyzes a recursive data structure and, through use of a given combining operation, recombines the results of recursively processing its constituent parts, building up a return value. Fold is also termed as reduce, accumulate, aggregate, compress, or inject. Typically, a fold is presented with a combining function, a top node of a data structure, and possibly some default values to be used under certain conditions. The fold then proceeds to combine elements of the data structure's hierarchy, using the function in a systematic way.