A functional programming library for Python. Katharos brings algebraic abstractions — Functor, Applicative, Monad, Semigroup, Monoid — together with concrete types like Maybe, Result, ImmutableList, and IO, so you can model effects, errors, and data transformations as composable, type-safe pipelines.
pip install katharosOr using uv
uv add katharosBefore — scattered None checks and exception handling:
user = find_user(user_id)
if user is None:
return None
account = find_account(user)
if account is None:
return None
return account.discountAfter — do-notation that short-circuits cleanly on Nothing:
from katharos.types import Maybe
from katharos.syntax_sugar import do, DoBlock
@do(Maybe)
def lookup_discount(user_id: int) -> DoBlock[Maybe, float]:
user = yield find_user(user_id)
account = yield find_account(user)
return account.discount # Just(0.15) or Nothing()Before — nested try/except to propagate errors:
def process(raw: str) -> int:
try:
n = parse_int(raw)
except ValueError as e:
raise RuntimeError("bad input") from e
try:
return validate_positive(n)
except ValueError as e:
raise RuntimeError("bad value") from eAfter — errors as values, chained with |:
from katharos.types import Result
def process(raw: str) -> Result[Exception, int]:
return parse_int(raw) | validate_positive # Failure short-circuits automaticallyHandle optional values without None checks:
from katharos.types import Maybe
result = Maybe[int].Just(5) | (lambda x: Maybe[int].Just(x * 2)) # Just(10)
nothing = Maybe[int].Nothing() | (lambda x: Maybe[int].Just(x * 2)) # Nothing()Model errors as values instead of exceptions:
from katharos.types import Result
def parse_int(s: str) -> Result[ValueError, int]:
try:
return Result.Success(int(s))
except ValueError as e:
return Result.Failure(e)
parse_int("42").fmap(lambda n: n * 2) # Success(84)
parse_int("??").fmap(lambda n: n * 2) # Failure(...)Combine values with the Semigroup operator:
from katharos.types import ImmutableList
ImmutableList([1, 2]) @ ImmutableList([3, 4]) # ImmutableList([1, 2, 3, 4])do-notation works with any monad — Maybe, Result, IO, ImmutableList and your custom monads. Each yield unwraps the monadic value:
from katharos.syntax_sugar import do, DoBlock
from katharos.types import Result
def parse_positive(x: int) -> Result[ValueError, int]:
return Result.Success(x) if x > 0 else Result.Failure(ValueError(f"{x} is not positive"))
# Clean, imperative-style monadic code
@do(Result)
def do_block() -> DoBlock[Result, int]:
x: int = yield parse_positive(5)
y: int = yield parse_positive(3)
return x + y
print(do_block()) # Success(8)Full tutorials, how-to guides, API reference, and explanations of the mathematical foundations are at katharos.readthedocs.io.
MIT