By name: effect-handlers-implementer description: 'Implements effect handlers for algebraic effects. Use when: (1) Building effect systems, (2) Implementing custom effects, (3) Creating extensible effects.' version: 1.0.0 tags:
- effects
- handlers
- control-flow
- algebraic-effects difficulty: advanced languages:
- python
- haskell
- ocaml dependencies:
- effect-type-system
Effect Handlers Implementer
Implements effect handlers for algebraic effects and resumable exceptions.
When to Use
- Building effect systems
- Implementing custom effects (I/O, state, nondeterminism)
- Creating extensible effect systems
- Implementing error handling
- Building async/await with effects
What This Skill Does
- Defines effects - Algebraic effect signatures
- Implements handlers - Effect interpretation
- Manages continuations - Resumable computations
- Composes handlers - Layered interpretations
Key Concepts
| Concept | Description |
|---|---|
| Effect | Operation signature (type of operations) |
| Handler | Interpretation of effects |
| Continuation | Remaining computation |
| Resumption | Continue after handling |
| Abort | Stop computation |
Common Effects
| Effect | Operations | Handler |
|---|---|---|
| State | get, put | StateHandler |
| Exceptions | throw, catch | FailHandler |
| I/O | read, write | IOHandler |
| Non-det | choose, fail | ChoiceHandler |
| Random | random | RandomHandler |
| Async | async, await | AsyncHandler |
Handler Composition
| Pattern | Description |
|---|---|
| Shallow | One-shot handling |
| Deep | Intercept nested effects |
| Scoped | Dynamic handler scope |
| Layered | Multiple handlers |
Tips
- Start with simple effects (state, fail)
- Use effect rows for polymorphism
- Handle resource cleanup with finally
- Test handlers in isolation
- Consider performance of continuations
Related Skills
effect-type-system- Effect type systemspolymorphic-effects- Effect polymorphismlambda-calculus-interpreter- Basic interpretation
Canonical References
| Reference | Why It Matters |
|---|---|
| Plotkin & Pretnar, "Handlers of Algebraic Effects" (2009) | Original effect handlers |
| Koka Language | Production effect system |
| Eff Language | Research effect language |
| Frank Language | Effect handlers in practice |
Tradeoffs and Limitations
Approaches
| Approach | Pros | Cons |
|---|---|---|
| Monadic | Simple, composable | Verbose |
| Effect handlers | Extensible, modular | Complex implementation |
| Delimited continuations | Powerful | Hard to use |
Limitations
- Performance overhead for continuations
- Complex to implement correctly
- Debugging is challenging
- Limited static analysis
- Some effects can't be handler
Research Tools & Artifacts
Real-world effect handler implementations:
| Tool | Why It Matters |
|---|---|
| Koka | Production language with effect types |
| Frank | Effect handlers research language |
| Eff | Effect handlers implementation |
| OCaml effects | Experimental effects in OCaml |
| Multicore OCaml | Effect handlers for OCaml |
Key Systems
- Koka: Microsoft research effect language
- Frank: Chameleon in the forest
Research Frontiers
Current effect handler research:
| Direction | Key Papers | Challenge |
|---|---|---|
| Typed handlers | "Handlers of Algebraic Effects" | Type-safe handlers |
| Continuations | "Delimited Control" | Efficient continuations |
| Effect polymorphism | "Polymorphic Effects" | Reusable handlers |
Hot Topics
- Effect handlers for web: Async via handlers
- Effects in Rust: Effects RFC discussion
Implementation Pitfalls
Common effect handler bugs:
| Pitfall | Real Example | Prevention |
|---|---|---|
| Resume after abort | Double resume | Track state |
| Stack overflow | Deep continuations | Tail-call opt |
| Handler order | Wrong handler catches | Order matters |