Code Implementer Skill

Code Implementer Skill

Expert implementation guidance ensuring code follows FP principles, DDD patterns, and is designed for maximum testability.

This is an IMPLEMENTATION skill - write production code following the architectural patterns in the imported rules. For design decisions and architectural review, use /​architecture-tech-lead instead.

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Pre-Implementation Checklist

Before writing code, verify:

• Boundaries clear: Where does I/​O happen vs pure logic?
• Invariants identified: What must always be true?
• Types designed: Records/​sealed types for domain? Discriminated unions?
• Parse, don't validate: Return validated types? Invalid states unrepresentable?
• Error handling: Using Either/​Result? What errors are possible?
• Testability: Can business logic be unit tested without mocks?

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During Implementation

Structure Code as Functional Core + Imperative Shell

Imperative Shell (thin):

• Fetches data (DB, APIs)
• Calls pure functions
• Persists results
• Handles I/​O errors

Functional Core (where logic lives):

• Pure functions, no side effects
• Receives all data as parameters
• Returns new data, never mutates
• Trivially unit testable

Apply Type Design

Apply patterns from imported rules:

• Java: Records, sealed types, pattern matching (see java-patterns.md)
• TypeScript: Discriminated unions, ts-pattern, Zod (see typescript-patterns.md)

Handle Errors Properly

Use Either/​Result for all expected failures (see java-patterns.md for Either examples):

• Chain operations with flatMap
• Collect validation errors with Eithers.allFailures()
• Never throw for expected failures - return typed errors

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Post-Implementation Checklist

After writing code, verify:

• No business logic in shell: All logic in pure functions?
• Immutability: No mutations? Defensive copies where needed?
• Invariants enforced: Constructors validate? Invalid states unrepresentable?
• Error paths typed: Using Either/​Result? No hidden throws?
• Testable without mocks: Can test core logic with plain data?

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Implementation Patterns Quick Reference

Data Transformation Flow

fetch (I/​O) -> transform (pure) -> persist (I/​O)

Service Method Structure

1. Fetch required data (shell - I/​O)
2. Call pure function with data (core - testable)
3. Persist result (shell - I/​O)
4. Return result

Invariant Enforcement

Constructor validates -> Object always valid -> No defensive checks elsewhere

Error Handling Chain

validate -> process -> transform -> (all via flatMap/​map)

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Test Strategy

For every implementation, consider:

Unit Tests (core logic)

• Test pure functions with plain data
• No mocks needed
• Cover edge cases

Property Tests (invariants)

• What must ALWAYS be true?
• Round-trip properties
• Idempotence where applicable

Integration Tests (shell only)

• Minimal - just verify I/​O works
• Use real DB/​APIs in test containers
• Don't test business logic here

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Quality Standards

• Complete: Implement full feature, not partial
• Testable: Every function testable without mocks
• Typed Errors: Either/​Result for all failures
• Immutable: No mutations without justification
• Validated: Parse, don't validate - return validated types

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Context Awareness

Tailor to detected stack. See imported rules for patterns:

• Java: java-patterns.md (records, sealed types, Either)
• TypeScript: typescript-patterns.md (discriminated unions, ts-pattern)
• Testing: property-testing.md (jqwik patterns)