Version history

We follow Semantic Versions since the 1.0.0 release. Versions before 1.0.0 are 0Ver-based: incremental in minor, bugfixes only are patches. See 0Ver.



  • Breaking: renames join to flatten, sorry!

  • Breaking: renames box to bind and moves it to returns.pointfree

  • Breaking: removes Maybe.rescue and Maybe.fix methods

  • Breaking: renames io_squash to squash_io and moves it to returns.converters

  • Breaking: moves all interfaces from returns.primitives.container to returns.primitives.interfaces

  • Adds rescue pointfree function

  • Adds ResultE alias for Result[..., Exception]

  • Adds RequiresContext container and Context helper class

  • Adds RequiresContext support for bind pointfree function

  • Adds RequiresContext support for flatten function

  • Adds RequiresContextResult container

  • Adds RequiresContextResultE alias

  • Adds ReaderResult and ReaderResultE aliases for RequiresContextResult[..., ..., Exception]

  • Adds RequiresContextResult support for bind and rescue

  • Adds RequiresContextResult support for flatten

  • Adds IOResult helper to work better with IO[Result[a, b]]

  • Adds IOResultE alias for IOResult[a, Exception]

  • Adds IOResult support for bind

  • Adds IOResult support for flatten

  • Adds IOResult support for @pipeline

  • Adds IOResult support for coalesce

  • Adds IOResult support for is_successful

  • Adds RequiresContextIOResult container

  • Adds RequiresContextIOResultE alias

  • Adds ReaderIOResult and ReaderIOResultE aliases for RequiresContextIOResult[..., ..., Exception]

  • Adds RequiresContextIOResult support for bind and rescue

  • Adds RequiresContextIOResult support for flatten

  • Adds Result.lift, Maybe.lift, RequiresContext.lift, and RequiresContextResult.lift functions in addition to IO.lift

  • Adds Immutable primitive type

  • Adds Unitable protocol and .from_success() and .from_failure() methods for all Result realted classes

  • Adds Instanceable protocol and .from_value() method for IO and RequiresContext

  • Adds flow function, which is similar to pipe

  • Adds swap coverter for Result and IOResult

  • Adds squash_context function to squash RequiresContext similar to IO


  • Now Success and Failure (both io and pure) return Any and not NoReturn

  • Fixes how flatten works, also adds more tests and docs about Failure case

  • Fixes Unwrappable type being parametrized with only one TypeVar

  • Changes Success and Failure to return Any instead of NoReturn


  • Updates poetry version in travis

  • Imporves pipe docs with lambda and Generic problem

  • Improves docs in several places

  • Now examples in docs tries to be docstests where possible

  • Changes how tests are checked with mypy in CI



  • Breaking: now @pipeline requires a container type when created: @pipeline(Result) or @pipeline(Maybe)

  • Maybe and Result now has success_type and failure_type aliases

  • Adds Result.unify utility method for better error type composition

  • We now support dry-python/classes as a first-class citizen

  • Adds io_squash to squash several IO containers into one container with a tuple inside, currently works with 9 containers max at a time

  • Adds untap function which does convert return type to None


  • Fixes that containers were not usable with multiprocessing

  • Changes the inheritance order, now BaseContainer is the first child

  • Fixes that Nothing had incorrect docstrings


  • Now generated package is protected

  • Updates poetry to 1.0



  • Breaking: now pipe() does not require argument to be the first value, instead it is required to use: pipe(f1, f2, f3, f4)(value)

  • Breaking: dropped everything from returns/, because we now have quite a lot of stuff

  • Breaking: dropped support of zero argument functions for Nothing.fix

  • Breaking: dropped support of zero argument functions for Nothing.rescue

  • Maybe now has .failure() to match the same API as Result

  • Adds identity function

  • Adds tap function

  • Now pipe allows to pipe 8 steps

  • Adds coalesce_result and coalesce_maybe coverters


  • Fixes that code inside .fix and .rescue of Maybe might be called twice


  • Now all methods have doctests

  • Updates docs about Success and _Success, Failure and _Failure

  • Updates docs about @pipeline

  • Typechecks async functions and decorators inside typesafety/ tests



  • Breaking: python>=3.7,<=3.7.2 are not supported anymore, because of a bug inside typing module

  • Breaking: Now bind does not change the type of an error

  • Breaking: Now rescue does not change the type of a value

  • Breaking: Renames map_failure to alt

  • Adds box() function with the ability to box function for direct container composition like: a -> Container[b] to Container[a] -> Container[b]

  • Adds IO.lift() function to lift a -> a to IO[a] -> IO[a]

  • Adds pipe() function to

  • Adds __hash__() magic methods to all containers


  • Changes Any to NoReturn in Success and Failure

  • Now all type parameters in Result, Maybe, and IO are covariant


  • Massive docs rewrite

  • Updates mypy version

  • Updates wemake-python-styleguide and introduces nitpick

  • Updates pytest-plugin-mypy, all tests now use yml



  • Provides a bunch of primitive interfaces to write your own containers

  • Adds .map_failure() method

  • Adds flatten() function to join nested containers


  • Fixes type of Maybe.fix and Maybe.rescue to work with both lambda: 1 and lambda _: 1


  • Improves README



  • Reintroduces the Maybe container, typed!

  • Introduces converters from one type to another

  • Adds mypy plugin to type decorators

  • Complete rewrite of Result types

  • Partial API change, now Success and Failure are not types, but functions

  • New internal types introduced: FixableContainer and ValueUnwrapContainer


  • Fixes issue when you could return IO container from Result.bind

  • Fixes @pipeline return type


  • Reapplied all types to .py files

  • Improved docs about IO and Container concept

  • Adds docs about container composition

  • Moves from Alpha to Beta



  • Adds IO marker

  • Adds unsafe module with unsafe functions

  • Changes how functions are located inside the project


  • Fixes container type in @pipeline

  • Now is_successful is public

  • Now raise_exception is public


  • Changes how str() function works for container types

  • Total rename to “container” in the source code

Version 0.6.0


  • safe and pipeline now supports asyncio

  • is_successful now returns Literal types if possible

Version 0.5.0


  • Adds compose helper function

  • Adds public API to import returns

  • Adds raise_exception helper function

  • Adds full traceback to .unwrap()


  • Updates multiple dev-dependencies, including mypy

  • Now search in the docs is working again

  • Relicenses this project to BSD

  • Fixes copyright notice in the docs

Version 0.4.0 aka Goodbye, containers!


  • Moves all types to .pyi files

  • Renames all classes according to new naming pattern

  • HUGE improvement of types

  • Renames fmap to map

  • Renames do_notation to pipeline, moves it to

  • Renames ebind to rescue

  • Renames efmap to fix

  • Renames container to Container

  • Removes Maybe container, since typing does not have NonNullable type

Version 0.3.1


  • Adds py.typed file to be PEP561 compatible

Version 0.3.0, Renamed to returns

The project is renamed to returns and moved to dry-python org.


  • Adds .pyi files for all modules, to enable mypy support for 3rd party users

Version 0.2.0


  • Adds Maybe container

  • Adds immutability and __slots__ to all containers

  • Adds methods to work with failures

  • Adds safe decorator to convert exceptions to Result container

  • Adds is_successful() function to detect if your result is a success

  • Adds failure() method to unwrap values from failed containers


  • Changes the type of .bind method for Success container

  • Changes how equality works, so now Failure(1) != Success(1)

  • Changes how new instances created on unused methods


  • Improves docs

Version 0.1.1


  • Changes how PyPI renders package’s page


  • Improves README with new badges and installation steps

Version 0.1.0

Initial release. Featuring only Result and do_notation.