aleff 0.2.0

Algebraic effects for Python — deep and shallow, stateful, composable, multi-shot handlers

aleff

Algebraic effects for Python — deep and shallow, stateful, composable, multi-shot handlers.

Features

• Deep handlers — effects propagate through nested function calls without annotation
• Shallow handlers — handle an effect once, then delegate re-installation to the handler function; enables state machines, shift0/reset0, and strategy changes between invocations
• Stateful handlers — handler functions can execute code after resume, enabling patterns like transactions and reverse-mode AD
• Multi-shot continuations — resume can be called multiple times in a single handler, enabling backtracking search, non-determinism, and other advanced patterns
• Sync and async — both synchronous (Handler) and asynchronous (AsyncHandler) handlers are supported, with transparent bridging between the two
• Effect composition — handler functions can perform effects themselves, dispatched to enclosing handlers; enables layered architectures and modular effect stacking
• Effect annotation — @effect(step1, step2) collects effect sets transitively from decorated functions
• Introspection — effects(fn) and unhandled_effects(fn, h) for querying and validating effect coverage
• Typed — effect parameters and return types are checked by type checkers (pyright, ty)
• No macros, no code generation — pure Python library built on greenlet and a small CPython C extension

Requirements

• CPython >=3.12
  • Tested versions:
    • 3.12.13
    • 3.13.12
    • 3.14.3
    • 3.14.3t (free-threaded)
• greenlet >= 3.3.2
• Linux / macOS / Windows

Installation

# uv
uv add aleff

# pip
pip install aleff

Quick start

from aleff import effect, Effect, Resume, create_handler

# Define effects
read: Effect[[], str] = effect("read")
write: Effect[[str], int] = effect("write")

# Write business logic using effects
def run():
    s = read()
    return write(s)

# Provide handler implementations
h = create_handler(read, write)

@h.on(read)
def _read(k: Resume[str, int]):
    return k("file contents")

@h.on(write)
def _write(k: Resume[int, int], contents: str):
    print(f"writing: {contents}")
    return k(len(contents))

result = h(run)
print(result)  # 13

Multi-shot example

from aleff import effect, Effect, Handler, Resume, create_handler

choose: Effect[[int], int] = effect("choose")

h: Handler[list[int]] = create_handler(choose)

@h.on(choose)
def _choose(k: Resume[int, list[int]], n: int):
    # Resume once for each choice and collect all results
    results: list[int] = []
    for i in range(n):
        results += k(i)
    return results

def computation():
    x = choose(3)  # 0, 1, or 2
    y = choose(2)  # 0 or 1
    return [x * 10 + y]

result = h(computation)
print(result)  # [0, 1, 10, 11, 20, 21]

Effect composition

Handler functions can perform effects that are handled by enclosing handlers:

from aleff import effect, Effect, Resume, create_handler

log: Effect[[str], None] = effect("log")
parse: Effect[[str], int] = effect("parse")

# Outer handler: logging
h_log = create_handler(log)

@h_log.on(log)
def _log(k: Resume[None, int], msg: str):
    print(f"[LOG] {msg}")
    return k(None)

# Inner handler: parsing with logging
h_parse = create_handler(parse)

@h_parse.on(parse)
def _parse(k: Resume[int, int], s: str):
    log(f"parsing: {s}")       # handled by the outer handler
    return k(int(s))

result = h_log(lambda: h_parse(lambda: parse("42") + 1))
# prints: [LOG] parsing: 42
print(result)  # 43

How it works

Effects are declared as typed values and invoked like regular function calls. A Handler intercepts these calls via greenlet-based context switching:

1. Business logic runs in a greenlet
2. When an effect is invoked, control switches to the handler
3. The handler processes the effect and calls resume(value) to return a value
4. If resume is called multiple times, each call restores a snapshot of the continuation's frames (multi-shot)
5. If the handler returns without calling resume, the computation is aborted (early exit)

Because handlers use greenlets (not exceptions), the control flow is:

• Transparent — no yield, await, or special syntax in business logic
• Stateful — code after resume runs after the rest of the computation completes, enabling reverse-order execution (useful for backpropagation, transactions, etc.)

Multi-shot continuations are implemented via a CPython C extension (aleff._multishot.v1._aleff) that snapshots and restores interpreter frame chains.

Package structure

Package	Description
aleff	Default: re-exports aleff.multishot (multi-shot handlers)
aleff.multishot	Multi-shot handlers with frame snapshot/restore
aleff.oneshot	One-shot handlers (no C extension required)

Examples

See examples/ for demonstrations:

• N-Queens — backtracking search via multi-shot continuations
• Amb / Logic puzzle — Scheme-style amb operator and constraint solving (SICP Exercise 4.42)
• Probability — exact discrete probability distributions via weighted multi-shot
• Dependency injection — swap DB/email/logging implementations
• Record/Replay — record effect results, replay without side effects
• Transactions — buffer writes, commit on success, rollback on failure
• Automatic differentiation — forward-mode (dual numbers) and reverse-mode (backpropagation) with the same math expressions
• Shallow state machine — mutable state (get/put) and traffic light controller via shallow handler re-installation
• shift/reset, shift0/reset0 — delimited continuations: deep = shift/reset, shallow = shift0/reset0, with generator example

API reference

Function / Class	Description
effect("name")	Create a new Effect
@effect(e1, e2, ...)	Decorate a function to declare its effects
create_handler(*effects, shallow=False)	Create a synchronous handler
create_async_handler(*effects, shallow=False)	Create an asynchronous handler
h.on(effect)	Register a handler function (decorator)
h(caller)	Run caller with the handler active
effects(fn)	Get the declared effect set of a function
unhandled_effects(fn, *handlers)	Get effects not covered by the given handlers
Effect[P, R]	Effect protocol (parameters P, return type R)
Handler[V]	Sync handler protocol
AsyncHandler[V]	Async handler protocol
Resume[R, V]	Sync continuation (k(value) -> V)
ResumeAsync[R, V]	Async continuation (await k(value) -> V)

Development

From source:

git clone https://github.com/hnmr293/aleff.git
cd aleff
uv sync

# Run tests
uv run pytest

# Run tests on all supported Python versions
./run_tests.sh

# Format
uv run ruff format

# Lint
uv run pyright

License

Apache-2.0