# Skuld
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[Why Effects? >](docs/why.md)
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[](https://github.com/mccraigmccraig/skuld/actions/workflows/test.yml)
[](https://hex.pm/packages/skuld)
[](https://hexdocs.pm/skuld/)
Evidence-passing Algebraic Effects for Elixir.
## The problem
Between your pure business logic and your side-effecting infrastructure
sits the orchestration layer: "fetch the user, check permissions, load
their subscription, compute the price, write the invoice." This code
encodes your most important business rules, but it's tangled with
databases, APIs, and randomness - making it hard to test, hard to
refactor, and impossible to property-test.
## The insight
Skuld adds a layer between pure and side-effecting code: **effectful**
code. Domain logic *requests* effects (database access, randomness, error
handling) without performing them. Handlers decide what those requests
mean. The same orchestration code runs with real handlers in production
and pure in-memory handlers in tests - fully deterministic, trivially
testable.
## What Skuld solves
| Pain point | What Skuld does | More |
|----------------------------------------|----------------------------------------------------------------------------|----------------------------------------------------------------------------|
| Orchestration code is untestable | Swap handlers: same code runs in-memory with no DB, no network | [Testing](docs/pain-points.md#testing-orchestration-code) |
| Stateful test doubles for complex flows | Reusable in-memory handlers with read-after-write consistency | [Stateful testing](docs/pain-points.md#reusable-stateful-test-doubles) |
| Non-deterministic UUIDs / randomness | Fresh and Random have deterministic test handlers - same test, same values | [Determinism](docs/pain-points.md#deterministic-uuids-randomness-and-time) |
| N+1 queries | `deffetch` + `query` batch independent loads automatically | [Batching](docs/pain-points.md#automatic-query-batching) |
| Long-running workflows across restarts | EffectLogger serialises progress; resume from where you left off | [Durable workflows](docs/pain-points.md#long-running-computations) |
| LiveView multi-step operations | AsyncComputation bridges effects into LiveView's process model | [LiveView](docs/pain-points.md#liveview-multi-step-operations) |
| Hexagonal architecture plumbing | Port.Contract / Port.Adapter.Effectful - typed boundaries, no parameter threading | [Hex arch](docs/pain-points.md#clean-architecture-boundaries) |
See [What Skuld Solves](docs/pain-points.md) for worked examples of each.
## Quick example
```elixir
defmodule Onboarding do
use Skuld.Syntax
defcomp register(params) do
# Read configuration from the environment
config <- Reader.ask()
# Generate a deterministic ID
id <- Fresh.fresh_uuid()
# Use a port for the database call
user <- UserRepo.EffectPort.create_user!(%{id: id, name: params.name, tier: config.default_tier})
# Accumulate domain events
_ <- EventAccumulator.emit(%UserRegistered{user_id: id})
{:ok, user}
end
end
```
Run with production handlers:
```elixir
Onboarding.register(%{name: "Alice"})
|> Reader.with_handler(%{default_tier: :free})
|> Fresh.with_uuid7_handler()
|> Port.with_handler(%{UserRepo => UserRepo.Ecto})
|> EventAccumulator.with_handler(output: fn r, events ->
MyApp.EventBus.publish(events)
r
end)
|> Throw.with_handler()
|> Comp.run!()
```
Run with test handlers - same code, no database, fully deterministic:
```elixir
Onboarding.register(%{name: "Alice"})
|> Reader.with_handler(%{default_tier: :free})
|> Fresh.with_test_handler()
|> Port.with_test_handler(%{
UserRepo.EffectPort.key(:create_user, %{id: _, name: "Alice", tier: :free}) =>
{:ok, %User{id: "test-uuid", name: "Alice", tier: :free}}
})
|> EventAccumulator.with_handler(output: fn r, events -> {r, events} end)
|> Throw.with_handler()
|> Comp.run!()
```
## Installation
Add `skuld` to your dependencies in `mix.exs` (see [Hex](https://hex.pm/packages/skuld) for the current version):
```elixir
def deps do
[
{:skuld, "~> 0.3"}
]
end
```
## What can it do?
### Foundational effects
Effects that solve problems every Elixir developer recognises. They feel
like well-structured Elixir code with better testability.
| Side-effecting operation | Effectful equivalent |
|---------------------------------|------------------------------|
| Configuration / environment | Reader |
| Process dictionary / state | State, Writer |
| Random values | Random |
| Generating IDs (UUIDs) | Fresh |
| Transactions | Transaction |
| Blocking calls to external code | Port, Port.Contract |
| Effectful code from plain code | Port.Adapter.Effectful |
| Mutation dispatch | Command |
| Domain event collection | EventAccumulator |
| Fork-join concurrency | Parallel |
| Thread-safe state | AtomicState |
| Effects from LiveView | AsyncComputation |
| Raising exceptions | Throw |
| Resource cleanup (try/finally) | Bracket |
| Effectful list operations | FxList, FxFasterList |
### Advanced effects
Effects that use cooperative fibers and continuations to do things that
aren't possible with standard BEAM patterns. You don't need these to
get value from Skuld - the foundational effects stand on their own.
| Pattern | Effectful equivalent |
|---------------------------------|------------------------------|
| Coroutines / suspend-resume | Yield |
| Cooperative fibers | FiberPool |
| Bounded channels | Channel |
| Streaming with backpressure | Brook |
| Automatic N+1 query batching | query, deffetch, Query.Cache |
| Serializable coroutines | EffectLogger |
## Documentation
**New to algebraic effects?** Start with
[Why Effects?](docs/why.md) - the problem, explained without jargon.
**Ready to code?** Jump to
[Getting Started](docs/getting-started.md) for your first computation.
### Full documentation
| Layer | Topic | Description |
|-------|-------|-------------|
| 1 | [Why Effects?](docs/why.md) | The problem effects solve |
| 2 | [The Concept](docs/what.md) | How algebraic effects work |
| | [What Skuld Solves](docs/pain-points.md) | Concrete problems, worked examples |
| 3 | [Getting Started](docs/getting-started.md) | Your first computation |
| 4 | [Syntax In Depth](docs/syntax.md) | `comp`, `else`, `catch`, `defcomp` |
| 5 | **Foundational Effects** | |
| | [State & Environment](docs/effects/state-environment.md) | State, Reader, Writer |
| | [Error Handling](docs/effects/error-handling.md) | Throw, Bracket |
| | [Value Generation](docs/effects/value-generation.md) | Fresh, Random |
| | [Collections](docs/effects/collections.md) | FxList, FxFasterList |
| | [Concurrency](docs/effects/concurrency.md) | Parallel, AtomicState, AsyncComputation |
| | [Persistence](docs/effects/persistence.md) | Transaction, Command, EventAccumulator |
| | [External Integration](docs/effects/external-integration.md) | Port, Port.Contract, Port.Adapter.Effectful |
| 6 | **Advanced Effects** | |
| | [Yield](docs/advanced/yield.md) | Coroutines |
| | [Fibers & Concurrency](docs/advanced/fibers-concurrency.md) | FiberPool, Channel, Brook |
| | [Query & Batching](docs/advanced/query-batching.md) | Automatic N+1 prevention |
| | [EffectLogger](docs/advanced/effect-logger.md) | Serializable coroutines |
| 7 | **Recipes** | |
| | [Testing](docs/recipes/testing.md) | Property-based testing with effects |
| | [Hexagonal Architecture](docs/recipes/hexagonal-architecture.md) | Port.Contract + Port.Adapter.Effectful |
| | [Decider Pattern](docs/recipes/decider-pattern.md) | Event-sourced domain logic |
| | [Handler Stacks](docs/recipes/handler-stacks.md) | Composing production & test stacks |
| | [LiveView](docs/recipes/liveview.md) | Multi-step wizards |
| | [Durable Workflows](docs/recipes/durable-workflows.md) | Persist-and-resume with EffectLogger |
| | [Data Pipelines](docs/recipes/data-pipelines.md) | Streaming with Brook |
| | [Batch Loading](docs/recipes/batch-loading.md) | N+1-free data access |
## Demo Application
See [TodosMcp](https://github.com/mccraigmccraig/todos_mcp) - a
voice-controllable todo application built with Skuld. It demonstrates
command/query structs with algebraic effects for LLM integration and
property-based testing. Try it live at
https://todos-mcp-lu6h.onrender.com/
## License
MIT License - see [LICENSE](LICENSE) for details.
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[Why Effects? >](docs/why.md)
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