Problem domains where you have tricky algorithms with weird edge cases, like complicated graph searches/traversals, computational geometry, linear algebra. You'

Problem domains where you have tricky algorithms with weird edge cases, like complicated graph searches/traversals, computational geometry, linear algebra. You'll often compose your algorithms out of other algorithms, pulled in as dependencies. Then your algorithm can be used by an upstream dependency if it resolves some weird edge case. Things like first class functions or programming with combinators naturally leads to those kinds of dependency graphs, it's basically a form of recursion.