Software Architect

“Make things as simple as possible, but no simpler.” — Albert Einstein

This skill is for architecting and implementing software that is:

• Highly cohesive (each module has a clear purpose).
• Loosely coupled (modules depend on stable interfaces, not internal details).
• Composition-first (prefer wiring small parts together over inheritance trees).
• Functional-style by default (pure core, explicit effects, data-oriented design).
• Disciplined about quality (simplicity over cleverness; high standards).

TigerStyle-inspired ideas are used as defaults and heuristics (apply when they improve safety, correctness, clarity, or performance; relax when they add accidental complexity).

North Star

• Optimize for correctness through understanding, then performance.
• Prefer simple, explicit control flow over implicit behavior.
• Choose the smallest design that fully solves the problem.
• Treat “clever” as a code smell; prefer boring, obvious solutions.
• Pay down confusion immediately (rename, simplify, split, delete).

Architecture Goals

High Cohesion

• Each module has exactly one job and one reason to change.
• Keep related data and the code that manipulates it together (ownership is a design tool).
• Create APIs that reflect the domain (nouns/verbs that fit), not the implementation.

Low Coupling

• Depend on interfaces/ports and data contracts, not concrete implementations.
• Make dependencies explicit at construction time (dependency injection over hidden globals).
• Keep the public surface area small and stable; keep internals private and replaceable.
• Avoid “action at a distance” (side effects and shared mutable state that surprise callers).

Composition Over Inheritance / Dynamic Polymorphism

Use inheritance and deep OO polymorphism only when it is clearly the simplest tool. Default to:

• Composition: build behavior by wiring small components together.
• Parameterization: pass functions/strategies/config instead of subclassing.
• Data + functions: plain data structures and pure transformations.
• Sum types / tagged unions + pattern matching (when available) instead of subtype webs.
• Small interfaces (ports) that describe capabilities, not “class identity”.

Rule of thumb: prefer “assemble behavior at the boundary” over “embed behavior in types”.

Functional-Style Defaults

Functional Core, Imperative Shell

• Put domain logic in pure functions (deterministic, testable, no I/O).
• Keep I/O, time, randomness, and external services in the imperative shell.
• Make boundaries explicit: parsing/validation, domain decisions, side effects.

Pseudo-structure:

imperative shell:
  load inputs (I/O)
  validate/parse -> domain types
  call pure domain functions
  persist/emit effects (I/O)

Immutability and Data Flow

• Prefer immutable data and persistent transformations.
• Minimize mutable state; if state must exist, confine it (single owner, clear lifecycle).
• Prefer returning new values over mutating shared ones.
• Avoid hidden mutation via global caches/singletons (unless carefully isolated and justified).

Errors as Values

• Model failures explicitly (Result/Either/Option) instead of implicit exceptions where possible.
• Separate programmer errors (bugs, invalid invariants) from operational errors (timeouts, disk full, network failures).
• Fail fast on programmer errors (assert/throw/panic) and handle operational errors deliberately.

TigerStyle-Inspired Defaults (Apply When Applicable)

These are high-leverage defaults for quality and safety. Apply them when they reduce risk and improve clarity; relax them when the surrounding ecosystem makes them counterproductive.

Prefer Explicitness

• Avoid magic, reflection-heavy control flow, and spooky metaprogramming.
• Prefer iteration over recursion; use recursion only when clearly bounded and simpler.
• Make non-obvious defaults explicit at call sites (especially for safety/performance knobs).
• Keep control flow in one place (“push ifs up and fors down”):
  • Parent functions own branching and orchestration.
  • Leaf helpers do straight-line work and stay pure.

Put Limits on Everything

• Ensure loops/queues/retries have clear upper bounds and timeouts.
• Bound work per request/event to avoid tail-latency spikes.
• Avoid unbounded memory growth (caches need eviction and explicit sizing).

Assert and Enforce Invariants

• Use assertions/contracts to encode invariants at boundaries (inputs, outputs, state transitions).
• When an invariant is critical, assert it in two places (before crossing a boundary and after crossing it) when practical.
• Prefer multiple small assertions over one compound assertion (easier to debug and reason about).

Design First, Then Build

• Spend thought upfront: define invariants, data model, interfaces, failure modes.
• Do performance “back-of-the-envelope” sketches early (big-O and resource costs).
• Avoid premature abstraction; but also avoid “we’ll fix it later” debt.

Zero-Technical-Debt Attitude

• Do not ship known footguns (missing error handling, unbounded loops, fragile concurrency, unclear ownership, undocumented invariants).
• If you must compromise, make it explicit: capture the rationale and a concrete follow-up.

How to Design (Before Writing Code)

1. Clarify the problem

   • What are the inputs/outputs?
   • What must always be true (invariants)?
   • What are the failure modes and recovery expectations?

2. Choose boundaries and dependencies

   • Identify the pure domain core.
   • Identify external systems (DB, network, filesystem, clock, random, UI).
   • Define ports/interfaces for external dependencies.

3. Model the domain

   • Use types and naming to prevent invalid states.
   • Prefer explicit units/qualifiers in names (timeout_ms, limit_items_max).

4. Plan for testability

   • Pure functions get unit tests.
   • Adapters get integration tests.
   • Edge cases include invalid/negative tests, not only happy paths.

Implementation Guidelines

• Keep functions small enough to fit on a screen; split by responsibility.
• Keep mutable state tightly scoped; prefer local variables over shared fields.
• Keep APIs narrow: fewer parameters, simpler return types, fewer states.
• Prefer readable naming over abbreviations; choose nouns/verbs carefully.
• Delete dead code and unnecessary indirection.
• Structure by feature/domain, not by technical role. Avoid folders like commands/, components/, utils/ at the top level. Instead, group by domain: auth/, calendar/, settings/. Technical groupings (if needed) should be leaves within feature folders, not roots.

Checklists

Architecture Checklist

• Clear module boundaries and ownership
• High cohesion within modules (no “misc” grab bags)
• Low coupling across modules (interfaces/ports, no internal leaks)
• Composition-first design (no unnecessary inheritance or frameworks-as-architecture)
• Pure core identified; effects isolated at boundaries
• Explicit error model and failure handling strategy
• Work/memory bounded where relevant
• Performance sketch done for hot paths

Implementation Checklist

• Core logic is testable without I/O
• Side effects are explicit and centralized
• Invariants asserted at boundaries
• Error cases handled (not silently ignored)
• Naming is precise; no misleading abstractions
• Complexity reduced (simplify after it works)
• Docs/comments explain why for non-obvious decisions

Review / Simplification Checklist

• Can a new engineer explain the design in 5 minutes?
• Are there fewer concepts than necessary? If yes, delete/merge.
• Are there more concepts than necessary? If yes, simplify.
• Does each module have one reason to change?
• Are dependencies pointing the right way (domain does not depend on infrastructure)?
• Are there hidden side effects or shared mutable state?
• Is any abstraction “clever”? Replace with something obvious.

Examples (Language-Agnostic)

Composition via Dependency Injection (no inheritance)

def process_order(deps, command):
  validated = validate(command)
  decision  = decide(validated)         # pure
  effects   = plan_effects(decision)    # pure
  deps.outbox.publish(effects.events)   # I/O
  deps.repo.save(effects.state)         # I/O

deps is explicit (repo/outbox/clock/etc.), domain decisions stay pure, and behavior is composed by passing dependencies rather than subclassing.

Functional Core / Imperative Shell Boundary

shell:
  raw_input -> parse/validate -> domain types
  domain types -> pure transform -> new domain types
  new domain types -> serialize/persist/emit

If you keep this boundary crisp, the system stays cohesive, low-coupled, and easy to test.