FP Principles
Core functional programming thinking patterns. Language-agnostic.
Cross-references: [fp-domain-modeling](../nw-fp-domain-modeling/SKILL.md) | [fp-hexagonal-architecture](../nw-fp-hexagonal-architecture/SKILL.md) | [fp-algebra-driven-design](../nw-fp-algebra-driven-design/SKILL.md)
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.
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.
2. Type-Driven Design
[STARTER]
Write the type signature before implementation. The type tells you what the function can and cannot do.
Process:
- Declare what the function consumes and produces
- Ask: "which type-specific operations do I actually use?"
- Replace concrete types with type variables for everything you don't inspect
- Add constraints only for capabilities you use (equality, ordering, display)
Design progression: Concrete types -> type variables -> constrained type variables. Each step increases reuse while documenting minimal assumptions.
Why: Function's type signature is a contract. Narrower types mean fewer possible implementations, fewer bugs.
3. Pattern Matching as Decision Decomposition
[STARTER]
Decompose decisions by data shape, not boolean conditions. Each clause handles one concrete case. Compiler verifies exhaustiveness.
When to use pattern matching: "What shape is this data?" When to use guards/conditions: "What property does this value have?" When to use named bindings: Intermediate results need a name to avoid repetition.
Heuristic: Prefer small extracted functions over giant match expressions. Pattern match on top-level shape, delegate to named functions for sub-decisions.
Exhaustiveness as safety net: When you add a new variant to a choice type, compiler flags every match that doesn't handle it.
4. Composition Patterns
[INTERMEDIATE]
Partial Application
Fix some arguments of a general function to create specialized version. Eliminates throwaway helper functions.
When: General function exists and you need specialized version for specific context.
Function Composition (Pipelines)
Chain functions where output of one feeds into next. Each function has single responsibility.
Why: Composition reveals architecture of computation. Pipelines read as sequence of steps, making business process visible.
Point-Free Style
Omit explicit argument when function is just a composition. Use when it reveals intent. Avoid when it obscures meaning.
5. Container Abstractions
[INTERMEDIATE] -> [ADVANCED]
Progressive hierarchy for working with values inside containers (nullables, lists, futures, results).
[INTERMEDIATE] Transformable Container (Functor)
What: Apply function to values inside container without changing structure. Plain English: "I have a value in a box. Transform the value without opening the box." When: You have nullable/optional/list/future and want to transform contents without inspecting the container. Guarantees: Transforming with identity does nothing. Can fuse or split transformations freely.
[INTERMEDIATE] Combinable Containers (Applicative)
What: Apply a function inside a container to values inside other containers. Plain English: "I have a function in a box AND values in boxes. Combine them." When: Validation -- check multiple fields independently, combine results only if all succeed. Doesn't short-circuit; collects all errors.
[INTERMEDIATE] Combinable Values (Monoid)
What: Combine two values of same type into one, with default element that changes nothing. Plain English: "I have many values. Smash them together into one." When: Folding/reducing collections. Combining operation must be associative, enabling parallelism. Examples: String concatenation with empty string | addition with zero | list append with empty list.
[ADVANCED] Chainable Operations (Monad)
What: Chain operations where each step produces wrapped value, next step depends on previous result. Plain English: "Step 1's output determines what step 2 does. Each step might fail/branch/have effects." When: Sequential dependent operations where each step can fail, branch, or produce effects.
Decision Tree: Which Abstraction Do I Need?
Do I need to transform values inside a container?
YES, one function, one container --> Transformable (Functor)
YES, combine multiple independent containers --> Combinable Containers (Applicative)
YES, chain dependent operations sequentially --> Chainable Operations (Monad)
Do I need to combine values of the same type?
YES --> Combinable Values (Monoid)Progression Summary
Each level adds a new kind of combination:
- Transformable: one function, one container
- Combinable Containers: one function, multiple containers (independent)
- Chainable: sequential dependent operations, each producing container
- Combinable Values: same-type values collapsed into one
Runnable Example: Map, Filter, Fold on Domain Objects
orders = [Order(100, "pending"), Order(250, "shipped"), Order(50, "pending")]
pendingTotals = orders
|> filter (o -> o.status == "pending") -- [Order(100, "pending"), Order(50, "pending")]
|> map (o -> o.amount) -- [100, 50]
|> fold 0 (acc, x -> acc + x) -- 1506. Specialized Chainable Patterns
[ADVANCED]
| Pattern | What It Manages | When to Use | |---------|----------------|-------------| | Optional (Maybe/Option) | Possible absence | Operations that can fail without explanation | | Result (Either) | Failure with context | Operations that fail with error details | | Environment (Reader) | Shared read-only config | Dependency injection, configuration threading | | Accumulator (Writer) | Side-channel output | Logging, auditing, collecting metadata | | Stateful (State) | Sequential state changes | Counters, parsers, accumulators |
These compose: real applications stack multiple patterns. See [fp-hexagonal-architecture](../nw-fp-hexagonal-architecture/SKILL.md) for DI patterns.
7. Lazy Evaluation as Design Pattern
[INTERMEDIATE]
Separate WHAT to compute from WHEN it gets computed. Define potentially infinite sequences and let consumer determine how much to evaluate.
When: Generating candidates then selecting results | pagination and streaming | decoupling producers from consumers | build systems that only rebuild what changed.
Separation principle: Generate all possibilities, then filter. Declarative style says WHAT you want, not HOW to search.
8. The FP Problem-Solving Method
[STARTER]
- Start with type signature: What does this function consume and produce?
- Identify traversal pattern: Map, filter, fold, or search?
- Recognize accumulator: If folding, what is the state and how does each element change it?
- Decompose by data shape: Pattern match on constructors, handle each case independently
- **Compose small