Fixes for list monad (#25)

* Fixes for list monad

* Do not try to instantiate List directly. Use empty instead.

.flake8

@@ -1,2 +1,3 @@
 [flake8]
+ignore =  E731 # Do not assign a lambda expression, use a def
 max-line-length = 120

README.md

@@ -2,12 +2,13 @@
 
 ![Python package](https://github.com/dbrattli/OSlash/workflows/Python%20package/badge.svg)
 
-OSlash (Ø) is a library for playing with functional programming in
-Python 3.8+. It's an attempt to re-implement some of the code from [Learn You
-a Haskell for Great Good!](http://learnyouahaskell.com/) in Python 3.8.
-OSlash unifies functional and object oriented paradigms by grouping related
-functions within classes. Objects are however never used for storing
-values or mutable data, and data only lives within function closures.
+OSlash (Ø) is a library for playing with functional programming in Python 3.8+. It's an attempt to re-implement some of
+the code from [Learn You a Haskell for Great Good!](http://learnyouahaskell.com/) in Python 3.8. OSlash unifies
+functional and object oriented paradigms by grouping related functions within classes. Objects are however never used
+for storing values or mutable data, and data only lives within function closures.
+
+OSlash is intended to be a tutorial. For practical functional programming in Python in production environments you
+should use [FSlash](https://github.com/dbrattli/fslash) instead.
 
 ## Install
 
@@ -49,17 +50,14 @@ The project currently contains implementations for:
 
 ## But why?
 
-Yes, I know there are other projects out there like
-[PyMonad](https://bitbucket.org/jason_delaat/pymonad/),
-[fn.py](https://github.com/kachayev/fn.py). I'm simply doing this in order to
-better understand the [book](http://learnyouahaskell.com/). It's so much easier to learn when you implement
-things yourself. The code may be similar to PyMonad in structure, but is
-quite different in implementation.
-
-Why is the project called OSlash? OSlash is the Norwegian character called
-[Oslash](http://en.wikipedia.org/wiki/Ø). Initially I wanted to create a
-project that used Ø and ø (unicode) for the project name and modules. It didn't
-work out well, so I renamed it to OSlash.
+Yes, I know there are other projects out there like [PyMonad](https://bitbucket.org/jason_delaat/pymonad/),
+[fn.py](https://github.com/kachayev/fn.py). I'm simply doing this in order to better understand the
+[book](http://learnyouahaskell.com/). It's so much easier to learn when you implement things yourself. The code may be
+similar to PyMonad in structure, but is quite different in implementation.
+
+Why is the project called OSlash? OSlash is the Norwegian character called [Oslash](http://en.wikipedia.org/wiki/Ø).
+Initially I wanted to create a project that used Ø and ø (unicode) for the project name and modules. It didn't work out
+well, so I renamed it to OSlash.
 
 ## Examples
 

oslash/cont.py

@@ -6,7 +6,7 @@
 
 """
 
-from typing import Any, Callable, Generic, TypeVar
+from typing import Callable, Generic, TypeVar
 
 
 from .util import identity, compose
@@ -83,5 +83,6 @@ def __call__(self, comp: Callable[[T], TResult]) -> TResult:
     def __eq__(self, other) -> bool:
         return self(identity) == other(identity)
 
+
 assert isinstance(Cont, Functor)
-assert isinstance(Cont, Monad)
+assert isinstance(Cont, Monad)

oslash/do.py

@@ -23,11 +23,11 @@ def guard(M, test):
 
     What it does::
 
-        return M.pure(Unit) if test else M()
+        return M.pure(Unit) if test else M.empty()
 
     https://en.wikibooks.org/wiki/Haskell/Alternative_and_MonadPlus#guard
     """
-    return M.pure(Unit) if test else M()
+    return M.pure(Unit) if test else M.empty()
 
 
 # The kwargs syntax forces name to be a valid Python identifier.

oslash/either.py

@@ -1,4 +1,4 @@
-from abc import ABCMeta, abstractmethod
+from abc import abstractmethod
 from functools import partial
 
 from typing import Callable, TypeVar, Generic

oslash/identity.py

@@ -1,4 +1,4 @@
-from functools import partial  # type: ignore
+from functools import partial
 from typing import TypeVar, Generic, Callable
 
 from .typing import Functor, Monad, Applicative

oslash/ioaction.py

@@ -124,7 +124,6 @@ def __call__(self, world: int = 0) -> TSource:
         return self.run(world)
 
     def __str__(self, m: int = 0, n: int = 0) -> str:
-        assert self._value is not None
         s, io = self._value
         a = io.__str__(m + 1, n)
         return '%sPut ("%s",\n%s\n%s)' % (ind(m), s, a, ind(m))