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Concept: Iterators and the Cartesian Product

Kind: Concept.

The idea

Generating vignettes means taking every combination of parameter values — 3 confidences × 2 time pressures × 2 reversibilities × 2 info-request states — and producing one vignette per valid combination. That "every combination" is a cartesian product, and Rust's iterator ecosystem expresses it cleanly.

itertools::iproduct! gives you the product as an iterator. You then .filter() out invalid combinations and .map() each surviving combination into a Vignette. The whole generation is a single iterator chain: product → filter → map → collect.

Why iterators, not loops

You could write four nested for loops. The iterator chain is preferred here for reasons that matter beyond style: it is lazy (nothing computes until you collect), it is composable (each stage is independent and testable), and it makes the shape of the computation legible — "the cartesian product, minus invalid combos, each turned into a vignette" reads directly off the code.

Rust iterators vs. Python generators If you know Python generators, iterators will feel familiar: both are lazy sequences. The difference is that Rust iterators are statically typed and monomorphized — the compiler often turns a chain into code as tight as the hand-written loop, so you pay nothing for the abstraction.

Questions to lock

  1. What is a cartesian product, and why is it the right description of "all parameter combinations"?
  2. What does "lazy" mean for an iterator chain, and why is collect() the moment the work actually happens?
  3. Why is a filter → map chain easier to test than the equivalent nested loops?