fp-guide

name	fp-guide
description	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