HaskPy - Haskell types and functions in Python

Hask is the category of types and functions in Haskell. This package provides classes and functions inspired by Hask.

Documentation is available at jluttine.github.io/haskpy.

Overview

Features

• Typeclasses: Functor, Applicative, Monad, Semigroup, Monoid, Commutative, Foldable, Contravariant, Profunctor, Cartesian, Cocartesian

  • TODO: Traversable, Bifunctor, Monoidal, Ord, Show, Read

• Types and type constructors: Identity, Maybe, Either, List, Function, Compose, LinkedList

  • TODO: Constant, Validation, Dictionary, State, Reader, Writer, IO

• Monad transformers: MaybeT, IdentityT

  • TODO: StateT, ReaderT, WriterT, ListT

• Simple monoids: Sum, All, Any, String, Endo

  • TODO: Product

• Profunctor optics: adapter, lens, prism

  • TODO: traversal, grate, affine, setter

• Operators for common tasks: ** for function composition or functorial mapping, @ for applicative application, % for monadic binding and >> for applicative/monadic sequencing.

• Property-based testing of typeclass laws

HaskPy has implemented typeclass laws as property-based tests. Thus, one can easily test that an implementation satisfies all the laws it should. Just add something like this to your test module and run with pytest:

from haskpy.utils import make_test_class
from mystuff import MyClass
TestMyClass = make_test_class(MyClass)

This will automatically verify that MyClass satisfies all the typeclass laws of those typeclasses that it inherits. It makes use of great Hypothesis package.

Examples

Functors

A minimal example of functorial mapping:

>>> from haskpy import map, List
>>> map(lambda x: x**2, List(1, 2, 3, 4, 5))
List(1, 4, 9, 16, 25)

Lift over two layers of functorial structure:

>>> from haskpy import map, List, Just, Nothing
>>> map(map(lambda x: x**2))(List(Just(1), Nothing, Just(3), Just(4), Nothing))
List(Just(1), Nothing, Just(9), Just(16), Nothing)

Note that haskpy.map works for all Functor instances. That is, you don't need to use a different function to lift over different functors. You can even create function that performs some operation to values contained in any two-layer functorial structure. In the following example, square squares the values inside a two-layer functor:

>>> square = map(map(lambda x: x**2))
>>> square(List(Just(1), Nothing, Just(3)))
List(Just(1), Nothing, Just(9))
>>> square(List(List(1, 2, 3), List(4, 5)))
List(List(1, 4, 9), List(16, 25))

Even functions are functors if they have been decorated with function:

>>> @function
... def f(x):
...     return List(x, 2*x, 3*x)
>>> square(f)(3)
List(9, 36, 81)

Functions

HaskPy provides type Function for functions so that they have, for instance, Functor and Monad implementations. These functions are curried and they should only take a fixed number of arguments (no optional arguments and at least one argument). Decorator function just converts a normal Python function into a curried Function object:

>>> from haskpy import curry
>>> @function
... def concat(x, y, z):
...    return x + y + z
>>> concat("a")("b")("c")
"abc"
>>> concat("a")("b", "c")
"abc"

Note that it doesn't convert the function into nested one-argument functions (as currying strictly speaking should do) but it gives more flexibility by accepting any number of arguments (even no arguments at all). Keyword arguments aren't supported because of consistency.

Almost all functions in HaskPy have been decorated with function.

Copyright

Copyright (C) 2019-2021 Jaakko Luttinen

HaskPy is licensed under the MIT License. See LICENSE file for a text of the license or visit http://opensource.org/licenses/MIT.