General-purpose program synthesizers face a tradeoff between having a rich vocabulary for output programs and the time taken to discover a solution. One performance bottleneck is the construction of a space of possible output programs that is both expressive and easy to search. In this paper we achieve both richness and scalability using a new algorithm for constructing symbolic syntax graphs out of easily specified components to represent the space of output programs. Our algorithm ensures that any program in the space is type-safe and only mutates values that are explicitly marked as mutable. It also shares structure where possible and encodes programs in a straight-line format instead of the typical bushy-tree format, which gives an inductive bias towards realistic programs. These optimizations shrink the size of the space of programs, leading to more efficient synthesis, without sacrificing expressiveness. We demonstrate the utility of our algorithm by implementing \syncro, a system for synthesis of incremental operations. We evaluate our algorithm on a suite of benchmarks and show that it performs significantly better than prior work.