nw-fp-principles

FP Principles

Core functional programming thinking patterns. Language-agnostic.

Cross-references: fp-domain-modeling | code-craftsmanship: FP Hexagonal Architecture | fp-algebra-driven-design

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1. Higher-Order Functions as Problem Decomposition

[STARTER]

Three operations replace most loops:

Operation	Purpose	Replaces
Map	Transform each element, preserve structure	Loop building new collection
Filter	Keep elements matching condition	Loop with conditional
Fold	Accumulate elements into single result	Loop with running total

When to use Map: Transform every element without changing collection shape. Nested maps handle nested structures. When to use Filter: Select elements without changing their values. When to use Fold: Reduce collection to single value. Accumulator IS your state. Combining function IS your state transition. Folds make state machines explicit.

Dependent streams: Fold over the evolving descriptive state, not the initial state for every item. Approve then archive an invoice uses the post-approval invoice; snapshots project only the final register. Law: prepare(s, xs ++ ys) = prepare(prepare(s, xs), ys) on admitted transitions. When refusal forbids construction activity, load nw-certainty-by-construction for its readiness boundary before execution. Basis: Sandy Maguire, Algebra-Driven Design, Designing Algebras → Scavenger Hunt → A Unified Observation; Thinking with Types, Indexed Monads → Definition and Necessary Machinery.

Decision: "Am I transforming, selecting, or accumulating?" Pick matching operation. If none fit, compose two.

Why: These operations communicate intent. Map says "same shape, different values." Fold says "many inputs, one output." Loops say nothing about intent until you read every line.

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2. Type-Driven Design