can you do:

git checkout 89075d281b9cd132c85d9de4596b85ee97b29b43
git tag 0.5.0
git push --tags
python3 setup.py bdist_wheel upload

I was experimenting a bit with how OSlash operates with Python's inherent mutability.

While you can't assign within a map, you can still mutate variables outside of the context, for instance a dict:

In [77]: y = Just(1)

In [78]: y.map(lambda x: d.pop('hoge'))
Out[78]: Just 10

In [79]: d
Out[79]: {}

This is troublesome in a context in which you would want to map with a function that relies on the mutability of an object - for instance splitting one dict in to two via popping the appropriate variables in to a second dict, while maintaining the structure of the initial dict.

Obviously this can be avoided by copying the dict, either outside the context or within the function. I would however expect that applying something in a functional context would absolutely not mutate anything. I don't know if this is avoidable in a way that isn't wasteful (simply implementing a copy within the library wouldn't be ideal) but it caught my interest.

1. def orElse(either1, either2) (try one either, or else ty the next one)

2. validation data type ==> Left of Either lifted to a List

3. Applicative for validation to accumulate errors.

Please excuse me if some of these are already available in some other way.

@dbrattli I am happy to contribute if you are ok with it.

An example is worth 1000 words:

import oslash

def operation_that_succeeds(input):
    return oslash.Right(input + 1)

def operation_that_fails(input):
    return oslash.Left("Failure")

def lift(m):
    def w(f):
        return lambda x: m.unit(f(x))
    return w

@lift(oslash.Right)
def pure_function(input):
    return input + 1

if __name__ == "__main__":
    print(operation_that_succeeds(45) | operation_that_succeeds | pure_function | operation_that_succeeds)

How can I rewrite this without needing to implement the lift decorator?

I can see the latest release being 0.5.1 which is on April 2015.
I find this project super useful, and would like to get a newer release. Please let me know if I am missing something.

type checking the follow program with mypy:

from oslash import List

IntList = List[int]

def f() -> IntList:
     return List.unit(0)

Yields error: Invalid type "StrList".

I can't work out what config to run mypy with to get this project to type check

OSlash is primarily meant to be a Monad tutorial for Python. If you want to use types like Maybe and Either in your Python code, you should also take a look at FSlash which uses the same concepts named as Option and Result, and aim to be a solid library for doing practical functional programming in Python.

Note that FSlash has now been renamed to Expression.

Hello @dbrattli, I tried to install oslash on python 3.7 and has some problems in runtime. (see full stacktrace).

I would like to fix that if you want.

These error is related List.__str__.

In [2]: oslash.List([1,2,3])                                                                   
Out[2]: ---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
~/.local/share/virtualenvs/tests-mSusHyBX/lib/python3.7/site-packages/oslash/list.py in __iter__(self)
     98             if xs.null():
---> 99                 raise StopIteration
    100 

StopIteration: 

The above exception was the direct cause of the following exception:

RuntimeError                              Traceback (most recent call last)
~/.pyenv/versions/3.7.3/lib/python3.7/site-packages/IPython/core/formatters.py in __call__(self, obj)
    700                 type_pprinters=self.type_printers,
    701                 deferred_pprinters=self.deferred_printers)
--> 702             printer.pretty(obj)
    703             printer.flush()
    704             return stream.getvalue()

~/.pyenv/versions/3.7.3/lib/python3.7/site-packages/IPython/lib/pretty.py in pretty(self, obj)
    400                         if cls is not object \
    401                                 and callable(cls.__dict__.get('__repr__')):
--> 402                             return _repr_pprint(obj, self, cycle)
    403 
    404             return _default_pprint(obj, self, cycle)

~/.pyenv/versions/3.7.3/lib/python3.7/site-packages/IPython/lib/pretty.py in _repr_pprint(obj, p, cycle)
    695     """A pprint that just redirects to the normal repr function."""
    696     # Find newlines and replace them with p.break_()
--> 697     output = repr(obj)
    698     for idx,output_line in enumerate(output.splitlines()):
    699         if idx:

~/.local/share/virtualenvs/tests-mSusHyBX/lib/python3.7/site-packages/oslash/list.py in __repr__(self)
    115         """Return string representation of List.
    116         """
--> 117         return str(self)
    118 
    119     def __eq__(self, other: 'List') -> bool:

~/.local/share/virtualenvs/tests-mSusHyBX/lib/python3.7/site-packages/oslash/list.py in __str__(self)
    110         """Return string representation of List.
    111         """
--> 112         return "[%s]" % ", ".join([str(x) for x in self])
    113 
    114     def __repr__(self) -> str:

~/.local/share/virtualenvs/tests-mSusHyBX/lib/python3.7/site-packages/oslash/list.py in <listcomp>(.0)
    110         """Return string representation of List.
    111         """
--> 112         return "[%s]" % ", ".join([str(x) for x in self])
    113 
    114     def __repr__(self) -> str:

RuntimeError: generator raised StopIteration

Related #4

When trying to run hello.py under folder examples, something goes wrong with following error message:

» python3 ./hello.py                                                                                                                      feng@DDM05959
Traceback (most recent call last):
  File "./hello.py", line 1, in <module>
    from oslash import put_line, get_line
  File "/usr/lib/python3.7/site-packages/oslash/__init__.py", line 10, in <module>
    from .identity import Identity
  File "/usr/lib/python3.7/site-packages/oslash/identity.py", line 13, in <module>
    class Identity(Generic[A], Monad, Applicative, Functor):
  File "/usr/lib/python3.7/abc.py", line 126, in __new__
    cls = super().__new__(mcls, name, bases, namespace, **kwargs)
TypeError: Cannot create a consistent method resolution
order (MRO) for bases Generic, Monad, Applicative, Functor

Python version

 » python3                                                                                                                                 feng@DDM05959
Python 3.7.0 (default, Jul 15 2018, 10:44:58)
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version
'3.7.0 (default, Jul 15 2018, 10:44:58) \n[GCC 8.1.1 20180531]'
>>>

(lambda x: x+3) % Just(2)

Traceback (most recent call last):
File "", line 1, in
TypeError: unsupported operand type(s) for %: 'function' and 'Just'

Thanks

Hello, I am going through a book by Martin McBride about functional programming in Python.
I know this is not production-class library, but I am trying to instantiate a List functor (from oslash.list) as below and I end up raising an Exception...

from oslash import List as ListFunctor
a = ListFunctor([1,2,3])

Ends up with:

---------------------------------------------------------------------------  
TypeError                                 Traceback (most recent call last)
Cell In[3], line 1
----> 1 ListFunctor([1,2,3])

TypeError: List() takes no arguments

I looked into the code of the library and it seems that we're supposed to use the class method/factory from_iterable right now? Is this true?
While I understand this is not a repository about this book, but it also mentioned possibility of mapping functions using % operator on Functors, which seemed to be working for Just and Nothing Functors, but does not work for this List Functor. I have to use List.from_iterable([1,2,3]).map(operator.neg), the "old way" (if ever supported) neg % List.from_iterable(1,2,3) ends up raising an Exception

from oslash import List as ListFunctor


a = ListFunctor.from_iterable([1,2,3])

neg % a
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[9], line 1
----> 1 neg % a

TypeError: unsupported operand type(s) for %: 'builtin_function_or_method' and 'Cons'

The mypy-lang package was obsoleted: https://pypi.org/project/mypy-lang/ and seemingly it not necessary for runtime operation of the oslash.
Can this dependency be removed as when mypy is installed with obsoleted mypy-lang, then mypy does not run because of conflict with old mypy-lang.

q = Just(5) | (lambda x: Just(x + 1) | (lambda x: Just(x * 2) | (lambda x: Just(x - 4))))
TypeError: unsupported operand type(s) for |: 'Just' and 'function'

I have python,
Python 3.10.7 (v3.10.7:6cc6b13308, Sep 5 2022, 14:02:52) [Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.

Hi, the next error appears on python3.12.2:

 File "/venv/lib/python3.12/site-packages/oslash/_init_.py", line 11, in <module>
    from .state import State
  File "/venv/lib/python3.12/site-packages/oslash/state.py", line 78, in <module>
    assert issubclass(State, Functor)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/typing.py", line 1841, in _subclasscheck_
    cls._non_callable_proto_members_
AttributeError: type object 'Functor' has no attribute '_non_callable_proto_members_'

with 3.11 everything is fine

oslash-0.6.3 (latest)

Hello
For the reason I need typing extensions, I get a Protocol error when import oslash.
Thanks.