Effect handlers are a language feature not only popular among programming language researchers, but also among programming language implementors, as witnessed by the adoption by languages used in the industry. However, comparatively little is known about how to teach them effectively — both to seasoned programmers encountering them for the first time, and to undergraduate students with only basic programming experience. We report on two iterations of an elective practical course built entirely around effect handlers in the Effekt programming language, targeting late undergraduate students. After five lectures and corresponding homework assignments, students independently developed substantial software projects of their own chosing — ranging from games through compilers to web applications — using effect handlers as the primary abstraction mechanism. We discuss: (1) how we teach effect handlers, including the mental models, metaphors and examples we found effective for concepts such as delimited continuations and bidirectional effects; (2) which features and patterns students reach for naturally, and which require explicit guidance; and (3) technically interesting software designs utilizing effect handlers that emerged independently from student projects.

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