How Developers Extract Functions: An Experiment

Abstract: Creating functions is at the center of writing computer programs. But there has been little empirical research on how this is done and what are the considerations that developers use. We design an experiment in which we can compare the decisions made by multiple developers under exactly the same conditions. The experiment is based on taking existing production code, "flattening" it into a single monolithic function, and then charging developers with the task of refactoring it to achieve a better design by extracting functions. The results indicate that developers tend to extract functions based on structural cues, such as 'if' or 'try' blocks. And while there are significant correlations between the refactorings performed by different developers, there are also significant differences in the magnitude of refactoring done. For example, the number of functions that were extracted was between 3 and 10, and the amount of code extracted into functions ranged from 37% to 95%.