This paper presents a programming language that specifies a robust process for shape formation on a sheet of identicallyprogrammed agents, by combining local organization primitives from epithelial cell morphogenesis and Drosophila cell differentiation with combination rules from geometry. This work represents a significantly different approach to the design of self-organizing systems: the desired global shape is specified using an abstract geometry-based language, and the agent program is directly compiled from the global specification. The resulting self-assembly process is extremely reliable in the face of random agent distributions, random agent death and varying agent numbers, without relying on global coordinates or centralized control.