Strict Typing

Strict Typing

Overview

No any types. No unknown escapes. Everything fully typed.

Core principle: Types are documentation that the compiler verifies.

This skill applies to: TypeScript, Python (with type hints), Go, Rust, Java, C#, and any typed language.

The Rule

NEVER use any, unknown, or equivalent type escapes.
ALWAYS provide explicit, accurate types.
TAKE EXTRA TIME if needed to type correctly.

TypeScript Specifics

Forbidden Patterns

// NEVER
const data: any = fetchData();
const items: unknown[] = parseItems();
function process(input: any): any { }
const config = {} as any;
// @ts-ignore
// @ts-expect-error (unless truly necessary with documentation)

Required Patterns

// ALWAYS
interface UserData {
  id: string;
  name: string;
  email: string;
}

const data: UserData = fetchData();

function process<T extends Processable>(input: T): ProcessResult<T> {
  // ...
}

Configuration

Ensure tsconfig.json has strict mode:

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictBindCallApply": true,
    "strictPropertyInitialization": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "noUncheckedIndexedAccess": true
  }
}

Handling Third-Party Types

When library types are missing:

// Create type definitions
declare module 'untyped-library' {
  export interface Config {
    option1: string;
    option2: number;
  }

  export function init(config: Config): void;
}

Or contribute types to DefinitelyTyped.

Handling Dynamic Data

For API responses or parsed JSON:

// Define expected shape
interface ApiResponse {
  users: User[];
  pagination: Pagination;
}

// Use type guard for runtime validation
function isApiResponse(data: unknown): data is ApiResponse {
  return (
    typeof data === 'object' &&
    data !== null &&
    'users' in data &&
    Array.isArray((data as ApiResponse).users)
  );
}

// Use with validation
const response = await fetch('/api/users');
const data: unknown = await response.json();

if (!isApiResponse(data)) {
  throw new Error('Invalid API response');
}

// data is now typed as ApiResponse

Using Zod for Runtime Validation

import { z } from 'zod';

const UserSchema = z.object({
  id: z.string(),
  name: z.string(),
  email: z.string().email(),
});

type User = z.infer<typeof UserSchema>;

// Parse and validate
const user = UserSchema.parse(unknownData);
// user is now typed as User

Python Specifics

Forbidden Patterns

# NEVER
def process(data):  # Missing type hints
    pass

def fetch() -> Any:  # Using Any
    pass

from typing import Any
result: Any = compute()

Required Patterns

# ALWAYS
from typing import TypeVar, Generic, Protocol
from dataclasses import dataclass

@dataclass
class User:
    id: str
    name: str
    email: str

def process(data: User) -> ProcessResult:
    ...

T = TypeVar('T', bound='Processable')

def transform(items: list[T]) -> list[T]:
    ...

Configuration

Use strict mypy settings:

# mypy.ini
[mypy]
strict = True
disallow_any_generics = True
disallow_untyped_defs = True
disallow_incomplete_defs = True
check_untyped_defs = True
disallow_untyped_decorators = True
warn_redundant_casts = True
warn_unused_ignores = True

Go Specifics

Go is statically typed, but avoid:

// AVOID
interface{} // empty interface
any         // Go 1.18+ alias for interface{}

// PREFER
type specific interfaces or concrete types

When interface{} is truly needed, document why and add type assertions.

When Typing Is Hard

If typing seems impossible:

Step 1: Question the Design

Is the type hard to express because the design is complex?
→ Consider simplifying the design

Step 2: Use Generics

// Instead of any
function process<T>(input: T): T {
  return input;
}

Step 3: Use Union Types

// Instead of any for multiple types
type Input = string | number | User;

function process(input: Input): void {
  if (typeof input === 'string') {
    // input is string
  } else if (typeof input === 'number') {
    // input is number
  } else {
    // input is User
  }
}

Step 4: Create Type Guards

function isUser(value: unknown): value is User {
  return (
    typeof value === 'object' &&
    value !== null &&
    'id' in value &&
    'name' in value
  );
}

Step 5: Document and Justify (Last Resort)

If any is truly unavoidable (extremely rare):

// JUSTIFIED: Third-party library `foo` has no types and
// creating accurate types requires reverse-engineering
// the entire library. See issue #123 for type contribution.
// TODO(#456): Remove when @types/foo is available
const result: any = thirdPartyCall();

This should be exceptionally rare.

Time Investment

Proper typing takes time. That's acceptable.

If typing is taking longer, the design may need reconsideration.

Checklist

Before committing code:

• No any types
• No unknown without type guards
• No @ts-ignore or # type: ignore
• All functions have typed parameters
• All functions have typed return values
• All interfaces/types are exported if public
• Type configuration is strict

Common Excuses Rejected

Integration

This skill is applied by:

• issue-driven-development - Step 7

This skill ensures:

• Self-documenting code
• Compile-time error catching
• Refactoring safety
• Better IDE support