We introduce a lambda calculus λFGT for transformations of finite graphs by generalizing and extending an existing calculus UnCAL. Whereas UnCAL can treat only unordered graphs, λFGT can treat a variety of graph models: directed edge-labeled graphs whose branch styles are represented by monads T. For example, λFGT can treat unordered graphs, ordered graphs, weighted graphs, probability graphs, and so on, by using the powerset monad, list monad, multiset monad, probability monad, respectively. In λFGT, graphs are considered as extension of tree data structures, i.e. as infinite (regular) trees, so the semantics is given with bisimilarity. A remarkable feature of UnCAL and λFG T is structural recursion for graphs, which gives a systematic programming basis like that for trees. Despite the non-well-foundedness of graphs, by suitably restricting the structural recursion, UnCAL and λFGT ensures that there is a termination property and that all transformations preserve the finiteness of the graphs. The structural recursion is defined in a "divide-and-aggregate" way
"aggregation" is done by connecting graphs with ε-edges, which are similar to the -transitions of automata. We give a suitable general definition of bisimilarity, taking account of ε-edges
then we show that the structural recursion is well defined with respect to the bisimilarity. © 2013 ACM.