type-safety

Type Safety

Purpose: Use type systems to catch errors at compile/analysis time rather than runtime. Goal: Prevent null errors, type mismatches, and invalid states. Note: For implementation, see C# Development or Python Development.

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When to Use This Skill

• Adding type annotations to existing code
• Implementing nullable reference types
• Building type-safe APIs with value objects
• Configuring static analysis tools
• Designing strongly-typed data transfer objects

Prerequisites

• Language with type system support

Decision Tree

Type safety concern?
+-- Adding types to existing code?
|   +-- Greenfield? -> Enable strict mode from day one
|   +-- Legacy codebase? -> Enable strict incrementally (file-by-file)
|   +-- Third-party input? -> Validate at boundary, trust internally
+-- Choosing a type strategy?
|   +-- Primitive obsession? -> Introduce value objects (Email, Money, UserId)
|   +-- Magic strings/numbers? -> Replace with enums or constants
|   +-- Nullable confusion? -> Enable nullable reference types, annotate everything
+-- Handling unknown data?
|   +-- API response -> Parse and validate into typed model (Pydantic, Zod, etc.)
|   +-- User input -> Validate at boundary, reject invalid shapes early
|   +-- Config values -> Bind to typed config class at startup
+-- Compiler/analyzer errors?
    +-- Too many `any`/`object`? -> Replace with generics or specific types
    +-- Null warnings? -> Add explicit null checks or use non-null assertions with care
    +-- False positive? -> Suppress with inline comment explaining why

Why Type Safety Matters

Runtime Error (Bad):
 function getUser(id):
 return database.find(id) # What type? Nullable?
 
 user = getUser(123)
 print(user.email) # NullReferenceException at runtime

Type-Safe (Good):
 function getUser(id: int) -> User | null:
 return database.find(id)
 
 user = getUser(123)
 if user != null:
 print(user.email) # [PASS] Compiler ensures null check