.. contents::
PyMonad implements data structures typically available in pure
functional or functional first programming languages like Haskell and
F#. Included are Monad and Monoid data types with several common
monads included - such as Maybe and State - as well as some useful
tools such as the @curry decorator for defining curried
functions. PyMonad 2.0.x represents and almost complete re-write of
the library with a simpler, more consistent interface as well as type
annotations to help ensure correct usage.
1 Getting Started
-----------------
These instructions will get you a copy of the project up and running
on your local machine for development and testing purposes.
1.1 Prerequisites
~~~~~~~~~~~~~~~~~
PyMonad requires Python 3.7+. If installing via ``pip`` then you
will also need `Pip <https://pypi.org/project/pip/>`_ and `Wheel <https://pypi.org/project/wheel/>`_ installed. See those projects for
more information on installing them if necessary.
Potential contributors should additionally install `pylint <https://pypi.org/project/pylint/>`_ and
`pytype <https://pypi.org/project/pytype/>`_ to ensure their code adheres to common style conventions.
1.2 Installing
~~~~~~~~~~~~~~
1.2.1 From the Python Package Index (PyPI) with pip
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
From a command line run:
.. code:: bash
pip install PyMonad
1.2.2 Manual Build from PyPI
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Download the project files from
`https://pypi.org/project/PyMonad/#files <https://pypi.org/project/PyMonad/#files>`_ and from the project
directory run:
.. code:: bash
python setup.py install
If that doesn't work you may need to run the following instead.
.. code:: bash
python3 setup.py install
1.2.3 From github
^^^^^^^^^^^^^^^^^
Clone the project repository:
.. code:: bash
git clone https://github.com/jasondelaat/pymonad.git
Then from the project directory run ``setup.py`` as for the manual
build instructions above.
1.2.4 Example Usage
^^^^^^^^^^^^^^^^^^^
The following example imports the ``tools`` module and uses the
``curry`` function to define a curried addition function.
.. code:: python
import pymonad.tools
@pymonad.tools.curry(2) # Pass the expected number of arguments to the curry function.
def add(x, y):
return x + y
# We can call add with all of it's arguments...
print(add(2, 3)) # Prints '5'
# ...or only some of them.
add2 = add(2) # Creates a new function expecting a single arguments
print(add2(3)) # Also prints '5'
1.2.5 Next Steps
^^^^^^^^^^^^^^^^
The PyMonad documentation is a work in progress. For tutorials,
how-to, and more head over to the `PyMonad Documentation Project <https://jasondelaat.github.io/pymonad_docs/>`_.
If you'd like to contribute visit the documentation repository
`here <https://github.com/jasondelaat/pymonad_docs>`_.
1.3 Upgrading from PyMonad 1.3
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you've used the 1.x versions of PyMonad you'll notice that
there are a few differences:
1.3.1 Curried functions
^^^^^^^^^^^^^^^^^^^^^^^
Currying functions in PyMonad version 1.x wrapped a function in
an instance of the Reader monad. This is no longer the case and
currying simply produces a new function as one might expect.
The signature of ``curry`` has changed slightly. The new ``curry``
takes two arguments: the number of arguments which need to be
curried and the function.
.. code:: python
from pymonad.tools import curry
def add(x, y):
return x + y
curried_add = curry(2, add)
# add = curry(2, add) # If you don't need access to the uncurried version.
``curry`` is itself a curried function so it can be used more
concisely as a decorator.
.. code:: python
from pymonad.tools import curry
@curry(2)
def add(x, y):
return x + y
1.3.2 Operators
^^^^^^^^^^^^^^^
Version 2 of PyMonad discourages the use of operators (>>, \\\*, and
&) used in version 1 so old code which uses them will
break. Operators have been removed from the default monad
implementation but are still available for users that still wish
to use them in the ``operators`` package. To use operators:
.. code:: python
# Instead of this:
# import pymonad.maybe
# Do this:
import pymonad.operators.maybe
While it's unlikely operators will be removed entirely, it is
strongly suggested that users write code that doesn't require
them.
1.3.3 Renamed Methods
^^^^^^^^^^^^^^^^^^^^^
The ``fmap`` method has been renamed to simply ``map`` and ``unit`` is now called ``insert``.
.. code:: python
from pymonad.maybe import Maybe
def add2(x):
return x + 2
m = (Maybe.insert(1)
.map(add2)
)
print(m) # Just 3
1.3.4 Applicative Syntax
^^^^^^^^^^^^^^^^^^^^^^^^
Previously applicative syntax used the ``&`` operator or the ``amap``
method. ``amap`` still exists but there's now another way to use
applicatives: ``apply().to_arguments()``
.. code:: python
from pymonad.tools import curry
from pymonad.maybe import Maybe, Just
@curry(2)
def add(x, y):
return x + y
a = Just(1)
b = Just(2)
c = Maybe.apply(add).to_arguments(a, b)
print(c) # Just 3
If the function passed to ``apply`` accepts multiple arguments then
it *must* be a curried function.
1.3.5 New ``then`` method
^^^^^^^^^^^^^^^^^^^^^^^^^
The ``then`` method combines the functionality of both ``map`` and
``bind``. It first tries to ``bind`` the function passed to it and,
if that doesn't work, tries ``map`` instead. It will be slightly
less efficient than using ``map`` and ``bind`` directly but frees
users from having to worry about specifically which functions are
being used where.
.. code:: python
from pymonad.tools import curry
from pymonad.maybe import Maybe, Just, Nothing
@curry(2)
def add(x, y):
return x + y
@curry(2)
def div(y, x):
if y == 0:
return Nothing
else:
return Just(x / y)
m = (Maybe.insert(2)
.then(add(2)) # Uses map
.then(div(4)) # Uses bind
)
print(m) # Just 1.0
1.3.6 Getting values out of ``Maybe`` and ``Either``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Previously, if you need to get a value out of a ``Maybe`` or an
``Either`` after a series of calculations you would have to access
the ``.value`` property directly. By the very nature of these two
monads, ``.value`` may not contain valid data and checking whether
the data is valid or not is the problem these monads are supposed
to solve. As of PyMonad 2.3.0 there are methods -- ``maybe`` and
``either`` -- for properly extracting values from these
monads.
Given a ``Maybe`` value ``m``, the ``maybe`` method takes a default
value, which will be returned if ``m`` is ``Nothing``, and a function
which will be applied to the value inside of a ``Just``.
.. code:: python
from pymonad.maybe import Just, Nothing
a = Just(2)
b = Nothing
print(a.maybe(0, lambda x: x)) # 2
print(b.maybe(0, lambda x: x)) # 0
The ``either`` method works essentially the same way but takes two
functions as arguments. The first is applied if the value is a
``Left`` value and the second if it's a ``Right``.
.. code:: python
from pymonad.either import Left, Right
a = Right(2)
b = Left('Invalid')
print(a.either(lambda x: f'Sorry, {x}', lambda x: x)) # 2
print(b.either(lambda x: f'Sorry, {x}', lambda x: x)) # Sorry, Invalid
1.4 Note on efficiency in versions <2.3.5
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In pymonad versions 2.3.4 and earlier, an error in the
implementation of ``then`` meant that some monad
types executed ``then`` with exponential complexity. As of version
2.3.5 this has been corrected. All monad types now execute ``then``
in linear time. A similar problem occured with the ``map`` and
``bind`` methods for the State monad which have also been fixed in
2.3.5
If you're using an earlier version of pymonad upgrading to 2.3.5
is highly recommended.
2 Running the tests
-------------------
2.1 Unit Tests
~~~~~~~~~~~~~~
These tests primarily ensure that the defined monads and monoids
obey the required mathematical laws.
On most \*nix systems you should be able to run the automated tests
by typing the following at the command line.
.. code:: bash
./run_tests.sh
However, ``run_tests.sh`` is just a convenience. If the above doesn't
work the following should:
.. code:: bash
python3 -m unittest discover test/
2.2 Style Tests
~~~~~~~~~~~~~~~
Contributors only need to run ``pylint`` and ``pytype`` over their
code and ensure that there are no glaring style or type
errors. PyMonad (mostly) attempts to adhere to the `Google Python Style Guide <https://google.github.io/styleguide/pyguide.html>`_
and includes type hinting according to `PEP 484 <https://www.python.org/dev/peps/pep-0484/>`_.
In general, don't disable ``pylint`` or ``pytype`` errors for the
whole project, instead disable them via comments in the code. See
the existing code for examples of errors which can be disabled.
3 Authors
---------
**Jason DeLaat** - *Primary Author/Maintainer* - `https://github.com/jasondelaat/pymonad <https://github.com/jasondelaat/pymonad>`_
4 License
---------
This project is licensed under the 3-Clause BSD License. See
`LICENSE.rst <./LICENSE.rst>`_ for details.
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"description": "\n.. contents::\n\nPyMonad implements data structures typically available in pure\nfunctional or functional first programming languages like Haskell and\nF#. Included are Monad and Monoid data types with several common\nmonads included - such as Maybe and State - as well as some useful\ntools such as the @curry decorator for defining curried\nfunctions. PyMonad 2.0.x represents and almost complete re-write of\nthe library with a simpler, more consistent interface as well as type\nannotations to help ensure correct usage.\n\n1 Getting Started\n-----------------\n\nThese instructions will get you a copy of the project up and running\non your local machine for development and testing purposes.\n\n1.1 Prerequisites\n~~~~~~~~~~~~~~~~~\n\nPyMonad requires Python 3.7+. If installing via ``pip`` then you\nwill also need `Pip <https://pypi.org/project/pip/>`_ and `Wheel <https://pypi.org/project/wheel/>`_ installed. See those projects for\nmore information on installing them if necessary.\n\nPotential contributors should additionally install `pylint <https://pypi.org/project/pylint/>`_ and\n`pytype <https://pypi.org/project/pytype/>`_ to ensure their code adheres to common style conventions.\n\n1.2 Installing\n~~~~~~~~~~~~~~\n\n1.2.1 From the Python Package Index (PyPI) with pip\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\nFrom a command line run:\n\n.. code:: bash\n\n pip install PyMonad\n\n1.2.2 Manual Build from PyPI\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\nDownload the project files from\n`https://pypi.org/project/PyMonad/#files <https://pypi.org/project/PyMonad/#files>`_ and from the project\ndirectory run:\n\n.. code:: bash\n\n python setup.py install\n\nIf that doesn't work you may need to run the following instead.\n\n.. code:: bash\n\n python3 setup.py install\n\n1.2.3 From github\n^^^^^^^^^^^^^^^^^\n\nClone the project repository:\n\n.. code:: bash\n\n git clone https://github.com/jasondelaat/pymonad.git\n\nThen from the project directory run ``setup.py`` as for the manual\nbuild instructions above.\n\n1.2.4 Example Usage\n^^^^^^^^^^^^^^^^^^^\n\nThe following example imports the ``tools`` module and uses the\n``curry`` function to define a curried addition function.\n\n.. code:: python\n\n import pymonad.tools\n\n @pymonad.tools.curry(2) # Pass the expected number of arguments to the curry function.\n def add(x, y):\n return x + y\n\n # We can call add with all of it's arguments...\n print(add(2, 3)) # Prints '5'\n\n # ...or only some of them.\n add2 = add(2) # Creates a new function expecting a single arguments\n print(add2(3)) # Also prints '5'\n\n1.2.5 Next Steps\n^^^^^^^^^^^^^^^^\n\nThe PyMonad documentation is a work in progress. For tutorials,\nhow-to, and more head over to the `PyMonad Documentation Project <https://jasondelaat.github.io/pymonad_docs/>`_.\nIf you'd like to contribute visit the documentation repository\n`here <https://github.com/jasondelaat/pymonad_docs>`_.\n\n1.3 Upgrading from PyMonad 1.3\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n\nIf you've used the 1.x versions of PyMonad you'll notice that\nthere are a few differences:\n\n1.3.1 Curried functions\n^^^^^^^^^^^^^^^^^^^^^^^\n\nCurrying functions in PyMonad version 1.x wrapped a function in\nan instance of the Reader monad. This is no longer the case and\ncurrying simply produces a new function as one might expect. \n\nThe signature of ``curry`` has changed slightly. The new ``curry``\ntakes two arguments: the number of arguments which need to be\ncurried and the function.\n\n.. code:: python\n\n from pymonad.tools import curry\n\n def add(x, y):\n return x + y\n\n curried_add = curry(2, add)\n # add = curry(2, add) # If you don't need access to the uncurried version.\n\n``curry`` is itself a curried function so it can be used more\nconcisely as a decorator.\n\n.. code:: python\n\n from pymonad.tools import curry\n\n @curry(2)\n def add(x, y):\n return x + y\n\n1.3.2 Operators\n^^^^^^^^^^^^^^^\n\nVersion 2 of PyMonad discourages the use of operators (>>, \\\\\\*, and\n&) used in version 1 so old code which uses them will\nbreak. Operators have been removed from the default monad\nimplementation but are still available for users that still wish\nto use them in the ``operators`` package. To use operators:\n\n.. code:: python\n\n # Instead of this:\n # import pymonad.maybe\n\n # Do this:\n import pymonad.operators.maybe\n\nWhile it's unlikely operators will be removed entirely, it is\nstrongly suggested that users write code that doesn't require\nthem.\n\n1.3.3 Renamed Methods\n^^^^^^^^^^^^^^^^^^^^^\n\nThe ``fmap`` method has been renamed to simply ``map`` and ``unit`` is now called ``insert``.\n\n.. code:: python\n\n from pymonad.maybe import Maybe\n\n def add2(x):\n return x + 2\n\n m = (Maybe.insert(1)\n .map(add2)\n )\n\n print(m) # Just 3\n\n1.3.4 Applicative Syntax\n^^^^^^^^^^^^^^^^^^^^^^^^\n\nPreviously applicative syntax used the ``&`` operator or the ``amap``\nmethod. ``amap`` still exists but there's now another way to use\napplicatives: ``apply().to_arguments()``\n\n.. code:: python\n\n from pymonad.tools import curry\n from pymonad.maybe import Maybe, Just\n\n @curry(2)\n def add(x, y):\n return x + y\n\n a = Just(1)\n b = Just(2)\n\n c = Maybe.apply(add).to_arguments(a, b)\n print(c) # Just 3\n\nIf the function passed to ``apply`` accepts multiple arguments then\nit *must* be a curried function.\n\n1.3.5 New ``then`` method\n^^^^^^^^^^^^^^^^^^^^^^^^^\n\nThe ``then`` method combines the functionality of both ``map`` and\n``bind``. It first tries to ``bind`` the function passed to it and,\nif that doesn't work, tries ``map`` instead. It will be slightly\nless efficient than using ``map`` and ``bind`` directly but frees\nusers from having to worry about specifically which functions are\nbeing used where.\n\n.. code:: python\n\n from pymonad.tools import curry\n from pymonad.maybe import Maybe, Just, Nothing\n\n @curry(2)\n def add(x, y):\n return x + y\n\n @curry(2)\n def div(y, x):\n if y == 0:\n \treturn Nothing\n else:\n \treturn Just(x / y)\n\n m = (Maybe.insert(2)\n .then(add(2)) # Uses map\n .then(div(4)) # Uses bind\n )\n\n print(m) # Just 1.0\n\n1.3.6 Getting values out of ``Maybe`` and ``Either``\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n\nPreviously, if you need to get a value out of a ``Maybe`` or an\n``Either`` after a series of calculations you would have to access\nthe ``.value`` property directly. By the very nature of these two\nmonads, ``.value`` may not contain valid data and checking whether\nthe data is valid or not is the problem these monads are supposed\nto solve. As of PyMonad 2.3.0 there are methods -- ``maybe`` and\n``either`` -- for properly extracting values from these\nmonads.\n\nGiven a ``Maybe`` value ``m``, the ``maybe`` method takes a default\nvalue, which will be returned if ``m`` is ``Nothing``, and a function\nwhich will be applied to the value inside of a ``Just``.\n\n.. code:: python\n\n from pymonad.maybe import Just, Nothing\n\n a = Just(2)\n b = Nothing\n\n print(a.maybe(0, lambda x: x)) # 2\n print(b.maybe(0, lambda x: x)) # 0\n\nThe ``either`` method works essentially the same way but takes two\nfunctions as arguments. The first is applied if the value is a\n``Left`` value and the second if it's a ``Right``.\n\n.. code:: python\n\n from pymonad.either import Left, Right\n\n a = Right(2)\n b = Left('Invalid')\n\n print(a.either(lambda x: f'Sorry, {x}', lambda x: x)) # 2\n print(b.either(lambda x: f'Sorry, {x}', lambda x: x)) # Sorry, Invalid\n\n1.4 Note on efficiency in versions <2.3.5\n~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\n\nIn pymonad versions 2.3.4 and earlier, an error in the\nimplementation of ``then`` meant that some monad\ntypes executed ``then`` with exponential complexity. As of version\n2.3.5 this has been corrected. All monad types now execute ``then``\nin linear time. 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