coding-effectively

name	coding-effectively
description	ALWAYS use this skill when writing or refactoring code. Includes context-dependent sub-skills to empower different coding styles across languages and runtimes.
user-invocable	false

Coding Effectively

Required Sub-Skills

ALWAYS REQUIRED:

• howto-functional-vs-imperative - Separate pure logic from side effects
• defense-in-depth - Validate at every layer data passes through

CONDITIONAL: Use these sub-skills when applicable:

• howto-code-in-typescript - TypeScript code
• howto-code-in-rust - Rust code
• howto-develop-with-postgres - PostgreSQL database code
• programming-in-react - React frontend code
• writing-good-tests - Writing or reviewing tests
• property-based-testing - Tests for serialization, validation, normalization, pure functions

Property-Driven Design

When designing features, think about properties upfront. This surfaces design gaps early.

Discovery questions:

Question	Property Type	Example
Does it have an inverse operation?	Roundtrip	decode(encode(x)) == x
Is applying it twice the same as once?	Idempotence	f(f(x)) == f(x)
What quantities are preserved?	Invariants	Length, sum, count unchanged
Is order of arguments irrelevant?	Commutativity	f(a, b) == f(b, a)
Can operations be regrouped?	Associativity	f(f(a,b), c) == f(a, f(b,c))
Is there a neutral element?	Identity	f(x, 0) == x
Is there a reference implementation?	Oracle	new(x) == old(x)
Can output be easily verified?	Easy to verify	is_sorted(sort(x))

Common design questions these reveal:

• "What about deleted/deactivated entities?"
• "Case-sensitive or not?"
• "Stable sort or not? Tie-breaking rules?"
• "Which algorithm? Configurable?"

Surface these during design, not during debugging.

Core Engineering Principles

Correctness Over Convenience

Model the full error space. No shortcuts.

• Handle all edge cases: race conditions, timing issues, partial failures
• Use the type system to encode correctness constraints
• Prefer compile-time guarantees over runtime checks where possible
• When uncertain, explore and iterate rather than assume

Don't:

• Simplify error handling to save time
• Ignore edge cases because "they probably won't happen"
• Use any or equivalent to bypass type checking

Error Handling Philosophy

Two-tier model:

1. User-facing errors: Semantic exit codes, rich diagnostics, actionable messages
2. Internal errors: Programming errors that may panic or use internal types

Error message format: Lowercase sentence fragments for "failed to {message}".

Good: failed to connect to database: connection refused
Bad:  Failed to Connect to Database: Connection Refused

Good: invalid configuration: missing required field 'apiKey'
Bad:  Invalid Configuration: Missing Required Field 'apiKey'

Lowercase fragments compose naturally: "operation failed: " + error.message reads correctly.

Pragmatic Incrementalism

• Prefer specific, composable logic over abstract frameworks
• Evolve design incrementally rather than perfect upfront architecture
• Don't build for hypothetical future requirements
• Document design decisions and trade-offs when making non-obvious choices

The rule of three applies to abstraction: Don't abstract until you've seen the pattern three times. Three similar lines of code is better than a premature abstraction.

File Organization

Descriptive File Names Over Catch-All Files

Name files by what they contain, not by generic categories.

Don't create:

• utils.ts - Becomes a dumping ground for unrelated functions
• helpers.ts - Same problem
• common.ts - What isn't common?
• misc.ts - Actively unhelpful

Do create:

• string-formatting.ts - String manipulation utilities
• date-arithmetic.ts - Date calculations
• api-error-handling.ts - API error utilities
• user-validation.ts - User input validation

Why this matters:

• Discoverability: Developers find code by scanning file names
• Cohesion: Related code stays together
• Prevents bloat: Hard to add unrelated code to string-formatting.ts
• Import clarity: import { formatDate } from './date-arithmetic' is self-documenting

When you're tempted to create utils.ts: Stop. Ask what the functions have in common. Name the file after that commonality.

Module Organization

• Keep module boundaries strict with restricted visibility
• Platform-specific code in separate files: unix.ts, windows.ts, posix.ts
• Use conditional compilation or runtime checks for platform branching
• Test helpers in dedicated modules/files, not mixed with production code

Cross-Platform Principles

Use OS-Native Logic

Don't emulate Unix on Windows or vice versa. Use each platform's native patterns.

Bad: Trying to make Windows paths behave like Unix paths everywhere.

Good: Accept platform differences, handle them explicitly.

// Platform-specific behavior
if (process.platform === 'win32') {
  // Windows-native approach
} else {
  // POSIX approach
}

Platform-Specific Files

When platform differences are significant, use separate files:

process-spawn.ts        // Shared interface and logic
process-spawn-unix.ts   // Unix-specific implementation
process-spawn-windows.ts // Windows-specific implementation

Document Platform Differences

When behavior differs by platform, document it in comments:

// On Windows, this returns CRLF line endings.
// On Unix, this returns LF line endings.
// Callers should normalize if consistent output is needed.
function readTextFile(path: string): string { ... }

Test on All Target Platforms

Don't assume Unix behavior works on Windows. Test explicitly:

• CI should run on all supported platforms
• Platform-specific code paths need platform-specific tests
• Document which platforms are supported

Common Mistakes

Mistake	Reality	Fix
"Just put it in utils for now"	utils.ts becomes 2000 lines of unrelated code	Name files by purpose from the start
"Edge cases are rare"	Edge cases cause production incidents	Handle them. Model the full error space.
"We might need this abstraction later"	Premature abstraction is harder to remove than add	Wait for the third use case
"It works on my Mac"	It may not work on Windows or Linux	Test on target platforms
"The type system is too strict"	Strictness catches bugs at compile time	Fix the type error, don't bypass it

Red Flags

Stop and refactor when you see:

• A utils.ts or helpers.ts file growing beyond 200 lines
• Error handling that swallows errors or uses generic messages
• Platform-specific code mixed with cross-platform code
• Abstractions created for single use cases
• Type assertions (as any) to bypass the type system
• Code that "works on my machine" but isn't tested cross-platform

Commit Hygiene

Applies to all languages. Commits are the unit of review and bisect; treat them with the same care as the code they contain.

• Each commit is a logical, atomic unit of change.
• Every commit must build and pass all checks (bisect-able history).
• Separate concerns: formatting fixes and refactoring go in separate commits from feature changes.
• Use simple past and present tense in bodies: "Previously X happened. With this commit, Y now happens."
• Commit message bodies use markdown. Do not use backticks in commit titles, but do use them in bodies.