We present a framework for the unifying high-level synthesis of tree-structured and iterative combinational networks. Based on the theory of list homomorphisms, we develop a standard implementation for tree-structured modules processing the input digits in parallel. The design is systematically specialized to iterative networks processing the input sequentially from the least resp. from the highest significant positions. Throughout the paper, we explicate functional methods for the transformational design of combinational circuits. We illustrate the approach with a parity generatormodule, a comparatormodule, and a priority resolution module.