sumtypes 0.1a1

Sum Types, aka Tagged Unions, for Python

sumtypes

Sum Types (aka Tagged Unions) for Python. Documentation is at https://sumtypes.readthedocs.org/

Example

Decorate your classes to make them a sum type:

@sumtype
class MyType(object):
    MyConstructor = constructor(int)

    @constructor
    def AnotherConstructor(x, y):
        assert type(x) is str
        assert type(y) is int
        return (x, y)

Then construct them by calling the constructors:

v = MyType.MyConstructor(1)
v2 = MyType.AnotherConstructor('foo', 2)

You can get the value from the tagged objects:

assert v.value == 1
assert v2.value == ('foo', 2)

The type of the tagged objects is the type you declared:

assert type(v) is MyType

You can also introspect the constructor used:

assert v.constructor is MyType.MyConstructor

And the tagged objects have equality semantics, which you can use to do pattern matching:

assert v == MyType.MyConstructor(1)
assert v != MyType.MyConstructor(2)

See Also

Over the past few years, the ecosystem of libraries to help with functional programming in Python has exploded. Here are some libraries I recommend:

• effect - a library for isolating side-effects

• pyrsistent - persistent (optimized immutable) data structures in Python

• toolz - a general library of pure FP functions

• fn.py - a Scala-inspired set of tools, including a weird lambda syntax, option type, and monads

Thanks

Thanks to Rackspace for allowing me to work on this project, and having an excellent open source employee contribution policy

License

sumtypes is licensed under the MIT license:

Copyright (C) 2014 Christopher Armstrong

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.