Type-Driven Development

Type-Driven Development Approach

• Structure programs around types rather than control flow
• Think about data types first, then functions
• Let types guide the design, use compiler to check for consistency
• Gradually refine the design and implementation with compiler validation

Steps in Type-Driven Development

1. Define types that model the problem domain
2. Sketch function prototypes
3. Let compiler check design consistency
4. Gradually fill in implementation details

Benefits of Type-Driven Development

• Types constrain behavior and make it obvious
• Types are machine-checkable documentation
• Nonsensical operations don’t cause runtime crashes, they fail to compile
• Debugging becomes checking the design for correctness

Design Patterns for Type-Driven Design

• Specific Numeric Types: Replace generic numeric types with domain-specific ones

  • Example: Creating Celsius and Fahrenheit types instead of using raw numbers
  • Prevents errors like adding temperatures in different units
  • In Type Systems and Rust Programming Language, can be implemented with no runtime overhead

• Enums for Alternatives: Use enumerated types for options, results, and flags

  • Avoid “string typing” (using strings for structured data)
  • Avoid boolean flags as function arguments (use enums for clarity)

State Machines in Rust Programming Language

• Two implementation strategies:
  1. Enum-based approach:
     • Define enums for states and events
     • Create state transition function
     • Good for complex state machines with many states/transitions
     • Makes state transition table explicit
  2. Struct-based approach:
     • One struct per state
     • Events as methods on structs that return new states
     • State transitions enforced by ownership rules
     • Better for state machines managing complex resources

Ownership System in Rust Programming Language

• Compiler tracks ownership of all data
• Every value has a single owner at any time
• Three cases for data transfer:
  1. Function takes ownership of passed value (consumes it)
  2. Function borrows a value (temporary use, ownership remains with caller)
  3. Function returns ownership of a value
• State transitions can be modeled using ownership rules (see Resource Ownership and Memory Management)
• Resource cleanup is automatic when values go out of scope