Roger K.W. Hui posted a transcribed copy of a 1981 letter from computer‑science legend Edsger W. Dijkstra to Dr. A. Caplin, responding to observations about the APL “cult”. Dijkstra argues that the language’s appeal lies not in usability but in how it reshapes the programmer’s thinking, likening it to a violinist’s instrument. He also thanks Bob Bernecky, Nicolas Delcros, Jay Foad and Eric Iverson for their manuscript comments.

He notes that enthusiasts often cling to specific implementations, such as terminals, and that teaching APL without a dedicated machine feels like trying to discuss a natural language without a native speaker. Dijkstra describes devotees reciting a prayer, “Dear Lord, don’t let me become like them,” illustrating the cultural resistance. The letter also reveals his frustration with the medium, apologizing for a left‑hand signature after breaking his right arm.

Hui points out the irony that Ken Iverson created the notation precisely to aid communication, yet Dijkstra encountered it years before any interpreter existed. The post supplies two APL demonstrations—a formal proof of Ackermann’s function and an inverted table index—showing that the language can express sophisticated algorithms without requiring execution, reinforcing Dijkstra’s point about mental rigor. It also references Dijkstra’s own EWD series, where he derived provably correct programs using formal methods.