Use when writing functional-style code or reviewing for FP cleanliness. Triggers on prompts about pure functions, immutability, function composition, module-level functions, explicit context passing, avoiding inheritance, or stateless designs, even when the user doesn't say 'FP' or 'functional'.
Installation
Install with Codex or Claude Copy this prompt, paste it into Codex, Claude, or another assistant, and let it review the skill page and install it for you.
Use when writing functional-style code or reviewing for FP cleanliness. Triggers on prompts about pure functions, immutability, function composition, module-level functions, explicit context passing, avoiding inheritance, or stateless designs, even when the user doesn't say 'FP' or 'functional'.
General Functional Programming Guidelines
Core principles
Prefer module-level functions over classes; pass context explicitly; avoid inheritance.
Favor immutable data and pure functions; no global or shared mutable state.
Compose complex behavior from small, focused functions.
Order pattern matching from most specific to most general.
Gotchas
Most language map/filter are eager, not lazy — chaining many of them allocates intermediate arrays; use generators / streams for large data
Closures over mutable state look pure but aren't — referential transparency requires both inputs and captured environment to be immutable
Partial application order matters — flip exists because curry direction is opinionated, not arbitrary
Promise/IO/effects compose differently from pure values — naïve map over them runs the effect, not the value
