Complementing sibling comments: Swift requires an enormous amount of syntactic ceremony in order to get things done and it lacks a powerful standard library to abbreviate common tasks. The generative tooling does so well here because Swift is designed for an IDE which provides generative tools of the sort invented in the 80s and 90s; when their editor already generates most of their boilerplate, predicts their types, and tab-completes their very long method/class names, they are already on auto-pilot.

The actual underlying algorithm should be a topological sort with either Kahn’s algorithm or Tarjan’s algorithm. It should take fewer than twenty lines total when ceremony is kept to a minimum; here is the same algorithm for roughly the same purpose in my Monte-in-Monte compiler, sorting modules based on their dependencies in fifteen lines. Also, a good standard library should have a routine or module implementing topological sorting and other common graph algorithms; for example, Python’s graphlib.TopologicalSorter was added in 2020 and POSIX tsort dates back to 1979. I would expect students to immediately memorize this algorithm upon grokking it during third-year undergrad as part of a larger goal of grokking graph-traversal algorithms; the idea of both Kahn and Tarjan is merely to look for vertices with no incoming edges and error if none can be found, not an easy concept to forget or to fail to rediscover when needed. Congrats, the LLM can do your homework.

If there’s any Swifties here: Hi! I love Taytay; I too was born in the late 80s and have trouble with my love life. Anyway, the nosology here is pretty easy; Swift’s standard library doesn’t include algorithms in general, only algorithms associated to data structures, which themselves are associated to standardized types. Since Swift descends from Smalltalk, its data structures include Collections, so a reasonable fix here would be to add a Graph collection and make topological sorting a method; see Python’s approach for an example. Another possibility is to abuse the builtin sort routine, but this will cost O(n lg n) path lookups and is much more expensive; it’s not a long-term solution.