using-recursion-and-lazinesslisted

# Using Recursion and Laziness ## Overview Recursion replaces the loop when a computation is defined in terms of itself, and it is the natural way to consume the recursive types a domain model produces. Laziness delays a computation until its result is needed, which turns infinite sequences into ordinary values and lets a pipeline process more data than fits in memory. Both are mechanics rather than design principles, but getting them wrong produces the two most common runtime failures in functional code: a stack that overflows on real data, and a computation repeated a thousand times because nobody noticed it was not cached. ## When to use - Consuming a tree, an expression, or any recursive type - A stack overflow on input larger than the test data - The same pure computation runs repeatedly with the same arguments - A collection is too large to hold in memory - Generating a sequence with no natural end Not for: reducing a collection to a value, which is [folding-over-data](../folding-over-data/SKILL.md). ## Core rules 1. **Write the base case first.** Then the recursive case, and check that it always moves towards the base. 2. **Make the recursive call the last thing.** A tail call can be turned into a loop; a call whose result is then modified cannot. 3. **Know whether your language eliminates tail calls.** If it does not, bound the depth, use an explicit accumulator loop, or trampoline. 4. **Prefer an existing traversal.** Fold, map, and unfold cover most