Despite the existence of extensive literature on functions, fewer studies used sociocultural views to explore the development of student learning about the concept. This study uses a discursive lens to examine whether an instructional approach that specifically attends to particular metalevel rules in the mathematical discourse on functions supports students' learning of the concept in a postsecondary mathematics classroom. The findings suggest that such instruction has the potential to foster learning as indicated by the changes in the ways students talked about functions, and their awareness and modifications of the assumptions shaping their thinking about functions.