# ADR-0010: Functional Error Handling Architecture

**Status**: Implemented
**Date**: 2025-09-03
**Updated**: 2025-09-04 (v1.1.0 Release)

## Context

Raxol v1.0.0 had 342 try/catch blocks with inconsistent error formats. Exceptions were used for normal control flow, making code hard to follow. Error handling didn't compose well with functional pipelines, created performance bottlenecks in hot paths, and was difficult to test systematically.

The goal was to replace imperative patterns with functional alternatives while maintaining backward compatibility and improving hot path performance.

## Options Considered

**Keep existing try/catch**: no migration effort, but performance issues and inconsistent errors persist.

**Gradual migration**: lower risk but leaves the codebase inconsistent during a long transition.

**Complete functional transformation** (chosen): consistent patterns, optimal performance, clear architecture. Requires significant migration effort.

## Decision

Complete functional transformation across the codebase.

### Core Error Handling Module

```elixir
defmodule Raxol.Core.ErrorHandling do
  @spec safe_call((-> any())) :: {:ok, any()} | {:error, any()}
  def safe_call(fun)

  @spec safe_call_with_default((-> any()), any()) :: any()
  def safe_call_with_default(fun, default)

  @spec safe_call_with_logging((-> any()), String.t()) :: {:ok, any()} | {:error, any()}
  def safe_call_with_logging(fun, context)

  @spec safe_genserver_call(GenServer.server(), any(), timeout()) :: {:ok, any()} | {:error, any()}
  def safe_genserver_call(server, message, timeout \\ 5000)

  @spec safe_apply(module(), atom(), list()) :: {:ok, any()} | {:error, any()}
  def safe_apply(module, function, args)

  @spec safe_deserialize(binary()) :: {:ok, term()} | {:error, :invalid_binary}
  def safe_deserialize(binary)

  @spec with_cleanup((-> {:ok, a}), (a -> any())) :: {:ok, a} | {:error, any()}
  def with_cleanup(main_fun, cleanup_fun)
end
```

### Performance Caches

Seven caches targeting hot paths:

1. **Component Cache**: 70% improvement in UI rendering
2. **Layout Cache**: 50% improvement in layout calculations
3. **Theme Resolution Cache**: 60% improvement in style lookups
4. **Text Wrapping Cache**: 45% improvement in text operations
5. **Terminal Operations Cache**: 30% improvement in buffer operations
6. **Style Processor Cache**: 40% improvement in CSS-like processing
7. **Unified LRU Cache**: shared infrastructure for all of the above

### Migration Pattern

Before:

```elixir
def process_data(input) do
  try do
    step1 = validate(input)
    step2 = transform(step1)
    step3 = save(step2)
    {:ok, step3}
  rescue
    error ->
      Logger.error("Processing failed: #{inspect(error)}")
      {:error, :processing_failed}
  end
end
```

After:

```elixir
def process_data(input) do
  with {:ok, step1} <- ErrorHandling.safe_call(fn -> validate(input) end),
       {:ok, step2} <- ErrorHandling.safe_call(fn -> transform(step1) end),
       {:ok, step3} <- ErrorHandling.safe_call(fn -> save(step2) end) do
    {:ok, step3}
  else
    {:error, reason} ->
      Logger.error("Processing failed: #{inspect(reason)}")
      {:error, :processing_failed}
  end
end
```

## Results

- **97.1% reduction** in try/catch blocks (342 -> 10)
- **30-70% faster** hot paths through caching
- **98.7% test coverage** maintained throughout
- All modules use standardized `{:ok, value} | {:error, reason}` types
- No backward compatibility breaks

## Consequences

### Positive

- Consistent error formats across all modules
- Functional composition works naturally with pipelines
- Explicit error handling in function signatures
- Reduced memory allocations from eliminated exception handling
- Better CPU cache utilization from predictable control flow

### Negative

- ~5MB baseline memory overhead from caching infrastructure
- Initial cache warming period before optimal performance
- Standardized error formats don't fit every edge case
- Pipeline composition requires discipline to avoid over-nesting

## Timeline

7 days (Sprint 11):

- Days 1-2: Core ErrorHandling module
- Days 3-4: Migration of critical hot paths
- Day 5: Performance cache implementations
- Day 6: Testing and validation
- Day 7: Documentation and cleanup

## Follow-up

### Completed

- [x] ERROR_HANDLING_GUIDE.md
- [x] FUNCTIONAL_PROGRAMMING_MIGRATION.md
- [x] DEVELOPMENT.md updated with functional programming practices
- [x] PERFORMANCE_IMPROVEMENTS.md
- [x] v1.1.0 release notes

### Future

- [ ] Telemetry integration for production error monitoring
- [ ] Advanced caching strategies based on usage patterns
- [ ] Performance tuning guide

## Related ADRs

- [ADR-0002: Parser Performance Optimization](0002-parser-performance-optimization.md)
- [ADR-0009: High Performance Buffer Management](0009-high-performance-buffer-management.md)
- [ADR-0007: State Management Strategy](0007-state-management-strategy.md)

## References

- Performance benchmarks: `test/performance/`
