Push ifs up and fors down
A programming blog post argues that code is often clearer and faster when conditional branches (`if`s) are moved “up” toward the caller and loops (`for`s) are moved “down” into lower-level, batch-style functions. Commenters largely agree this can improve performance and data-oriented design, but stress that it’s only a heuristic: rigidly applying it can harm readability, violate encapsulation, or duplicate validation logic, especially in languages without strong type systems. Many emphasize understanding responsibilities, types, and context—pushing conditions to boundaries, using types or contracts for preconditions, and optimizing only after profiling—rather than following any control-flow rule dogmatically.
Overall reception of “push ifs up, fors down”
- Many see it as a useful heuristic that articulates an intuition they already had, especially around simplifying control flow and improving performance.
- Others think it’s too slogan-like, risks becoming dogma, and lacks enough context to be safe as a general rule.
Heuristics vs. dogma and teaching
- Several comments stress that rules of thumb are valuable starting points, especially for less-experienced programmers, but must be taught with their “why” and limits.
- Others argue that advice like this leads to PR bikeshedding and dogmatic juniors who apply rules blindly without understanding responsibilities and context.
- There is a recurring theme that engineering is about intentional design, not rote application of rules.
Readability, maintainability, and architecture
- Some prefer precondition checks “down” in the callee so requirements are visible in one place, not duplicated at every call site.
- Others like pushing decisions “up” to centralize branching, reduce repeated checks, and keep hot paths straight and branch-free.
- Guard-clauses / “sad ifs” and early returns are advocated as a way to avoid deep nesting and keep “happy path” code linear.
Performance and compilers
- Proponents emphasize fewer branches in hot loops, better vectorization, and reduced function-call overhead, especially in data‑oriented or performance‑critical code.
- Skeptics note that modern compilers and branch predictors often hoist invariant conditions and optimize obvious patterns; micro-optimizing control flow is rarely the main bottleneck.
- Several warn that compilers can’t use domain knowledge; algorithm and data layout still dominate real performance.
Types, validation, and contracts
- In Rust and other typed languages, “push ifs up” is linked to encoding preconditions in types (typestate, newtypes, branded types), so invalid states are unrepresentable.
- In languages without strong types, many argue for defensive checks at boundaries and in callees; pushing all validation up can harm safety and reuse.
- Some propose context/contract systems (dynamic contexts, specs, monads) to express “this function only runs under condition X” without scattering ifs.
Language and paradigm dependence
- Several note the advice is more natural in Rust and data‑oriented designs, less so in C with raw pointers, Python/JS with dynamic types, or typical OOP business apps.
- Others argue the “fors down” idea clearly applies to batch operations and avoiding N+1 database queries, even in line‑of‑business software.