Metadata-Version: 2.1
Name: Expression
Version: 0.31.0
Summary: Practical functional programming for Python 3.8+
Home-page: https://github.com/dbrattli/expression
Author: Dag Brattli
Author-email: dag@brattli.net
License: MIT License
Download-URL: https://github.com/dbrattli/expression
Description: # Expression
        
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        > Pragmatic functional programming
        
        Expression aims to be a solid, type-safe, pragmatic, and high
        performance library for practical functional programming in Python 3.8+.
        By pragmatic we mean that the goal of the library is to use simple
        abstractions to enable you to do practical and productive functional
        programming in Python (instead of being a [Monad
        tutorial](https://github.com/dbrattli/OSlash)).
        
        Python is a multi-paradigm programming language that also supports
        functional programming constructs such as functions, higher-order
        functions, lambdas, and in many ways favors composition over inheritance.
        
        > Better Python with F#
        
        Expression tries to make a better Python by providing several functional
        features inspired by [F#](https://fsharp.org) into Python. This serves
        several purposes:
        
        - Enable functional programming in a Pythonic way. I.e make sure we are
          not over-abstracting things. Expressions will not be anywhere close to e.g
          Haskell.
        - Everything you learn with Expression can also be used with F#. Learn
          F# by starting in a programming language they already know. Perhaps
          get inspired to also [try out F#](https://aka.ms/fsharphome) by
          itself.
        - Make it easier for F# developers to use Python when needed, and re-use
          many of the concepts and abstractions they already know and love.
        
        Expression will enable you to work with Python using many of the same
        programming concepts and abstractions. This enables concepts such as
        [Railway oriented programming](https://fsharpforfunandprofit.com/rop/)
        (ROP) for better and predictable error handling. Pipelining for
        workflows, computational expressions, etc.
        
        F# is a functional programming language for .NET that is succinct
        (concise, readable, and type-safe) and kind of
        [Pythonic](https://docs.python.org/3/glossary.html). F# is in many ways
        very similar to Python, but F# can also do a lot of things better than
        Python:
        
        *Expressions evaluates to a value. Statements do something.*
        
        - Strongly typed, if it compiles it usually works making refactoring
          much safer. You can trust the type system. With mypy or Pylance you often wonder who is right and who is wrong.
        - Type inference, the compiler deduces types during compilation
        - Expression based language
        
        ## Getting Started
        
        You can install the latest `expression` from PyPI by running `pip` (or
        `pip3`). Note that `expression` only works for Python 3.8+.
        
        ```sh
        $ pip3 install expression
        ```
        
        ## Goals
        
        - Industrial strength library for functional programming in Python.
        - The resulting code should look and feel like Python
          ([PEP-8](https://www.python.org/dev/peps/pep-0008/)). We want to make
          a better Python, not some obscure DSL or academic Monad tutorial.
        - Provide pipelining and pipe friendly methods. Compose all the things!
        - Dot-chaining on objects as an alternative syntax to pipes.
        - Lower the cognitive load on the programmer by:
          - Avoid currying, not supported in Python by default and not a well
            known concept by Python programmers.
          - Avoid operator (`|`, `>>`, etc) overloading, this usually confuses
            more than it helps.
          - Avoid recursion. Recursion is not normally used in Python and any
            use of it should be hidden within the SDK.
        - Provide [type-hints](https://docs.python.org/3/library/typing.html)
          for all functions and methods.
        - Code must pass strict static type checking by
          [mypy](http://mypy-lang.org/) and
          [pylance](https://devblogs.microsoft.com/python/announcing-pylance-fast-feature-rich-language-support-for-python-in-visual-studio-code/).
          Pylance is awesome, use it!
        
        ## Supported features
        
        Expression will never provide you with all the features of F# and .NET.
        We are providing a few of the features we think are useful, and will add
        more on-demand as we go along.
        
        - **Pipelining** - for creating workflows.
        - **Composition** - for composing and creating new operators
        - **Pattern Matching** - an alternative flow control to
          `if-elif-else`.
        
        - **Option** - for optional stuff and better `None` handling.
        - **Result** - for better error handling and enables railway-oriented
          programming in Python.
        - **Collections** - immutable collections.
          - **Sequence** - a better
            [itertools](https://docs.python.org/3/library/itertools.html) and
            fully compatible with Python iterables.
          - **FrozenList** - a frozen and immutable list type.
          - **Map** - a frozen and immutable dictionary type.
          - **AsyncSeq** - Asynchronous iterables.
        - **Effects**: - lightweight computational expressions for Python. This
          is amazing stuff.
          - **option** - an optional world for working with optional values.
          - **result** - an error handling world for working with result values.
        - **Mailbox Processor**: for lock free programming using the [Actor
          model](https://en.wikipedia.org/wiki/Actor_model).
        - **Cancellation Token**: for cancellation of asynchronous (and
          synchronous) workflows.
        - **Disposable**: For resource management.
        
        ### Pipelining
        
        Expression provides a `pipe` function similar to `|>` in F#. We don't
        want to overload any Python operators e.g `|` so `pipe` is a plain old
        function taking N-arguments, and will let you pipe a value through any
        number of functions.
        
        ```py
        from expression.core import pipe
        
        gn = lambda g: g * y
        fn = lambda x: x + z
        value = pipe(
            x,
            fn,
            gn
        )
        
        assert value == gn(fn(x))
        ```
        
        Expression objects also have a pipe method so you can dot chain
        pipelines directly on the object:
        
        ```py
        from expression.core import pipe
        
        gn = lambda g: g * y
        fn = lambda x: x + z
        value = x.pipe(
            fn,
            gn
        )
        
        assert value == gn(fn(x))
        ```
        
        So for example with sequences you may create sequence transforming
        pipelines:
        
        ```py
        ys = xs.pipe(
            seq.map(lambda x: x * 10),
            seq.filter(lambda x: x > 100),
            seq.fold(lambda s, x: s + x, 0)
        )
        ```
        
        ### Composition
        
        Functions may even be composed directly into custom operators:
        
        ```py
        from expression.core import compose
        
        custom = compose(
            seq.map(lambda x: x * 10),
            seq.filter(lambda x: x > 100),
            seq.fold(lambda s, x: s + x, 0)
        )
        
        ys = custom(xs)
        ```
        
        ### Options
        
        The option type is used when a function or method cannot produce a
        meaningful output for a given input.
        
        An option value may have a value of a given type i.e `Some(value)`, or
        it might not have any meaningful value, i.e `Nothing`.
        
        ```py
        from expression.core import Some, Nothing, Option
        
        def keep_positive(a: int) -> Option[int]:
            if a > 0:
                return Some(a)
        
            return Nothing
        ```
        
        ```py
        def exists(x : Option[int]) -> bool:
            for value in x.match(Ok):
                return True
        
            return False
        ```
        
        ## Options as effects.
        
        Effects in Expression is implemented as specially decorated coroutines
        ([enhanced generators](https://www.python.org/dev/peps/pep-0342/)) using
        `yield`, `yield from` and `return` to consume or generate optional
        values:
        
        ```py
        from expression import effect
        from expression.core import Some
        
        @effect.option
        def fn():
            x = yield 42
            y = yield from Some(43)
        
            return x + y
        
        xs = fn()
        ```
        
        This enables ["railway oriented
        programming"](https://fsharpforfunandprofit.com/rop/) e.g if one part of
        the function yields from `Nothing` then the function is side-tracked
        (short-circuit) and the following statements will never be executed. The
        end result of the expression will be `Nothing`. Thus results from such
        an option decorated function can either be `Ok(value)` or
        `Error(error_value)`.
        
        ```py
        from expression import effect
        from expression.core import Some, Nothing
        
        @effect.option
        def fn():
            x = yield from Nothing # or a function returning Nothing
        
            # -- The rest of the function will never be executed --
            y = yield from Some(43)
        
            return x + y
        
        xs = fn()
        assert xs is Nothing
        ```
        
        For more information about options:
        
        - [Tutorial](https://github.com/dbrattli/Expression/blob/maom/notebooks/Options.ipynb)
        - [API reference](https://dbrattli.github.io/Expression/main/core/option.html)
        
        ### Results
        
        The `Result[T, TError]` type lets you write error-tolerant code that can
        be composed. A Result works similar to `Option` but lets you define the
        value used for errors, e.g an exception type or similar. This is great
        when you want to know why some operation failed (not just `Nothing`).
        
        ```py
        from expression import effect
        from expression.core import Result, Ok, Error, pipe
        
        @effect.result
        def fn():
            x = yield from Ok(42)
            y = yield from OK(10)
            return x + y
        
        xs = fn()
        assert isinstance(xs, Some)
        ```
        
        ### Sequences
        
        Contains operations for working with iterables. Thus all the functions
        in this module will work on normal Python iterables. Iterables are
        already immutable by design, so they are already perfectly suited for
        using with functional programming.
        
        ```py
        # Normal python way. Nested functions are hard to read since you need to
        # start reading from the end of the expression.
        xs = range(100)
        ys = functools.reduce(lambda s, x: s + x, filter(lambda x: x > 100, map(lambda x: x * 10, xs)), 0)
        
        # With Expression you pipe the result so it flows from one operator to the next:
        ys = pipe(
            xs,
            seq.map(lambda x: x * 10),
            seq.filter(lambda x: x > 100),
            seq.fold(lambda s, x: s + x, 0),
        )
        assert ys == zs
        ```
        
        ### Pattern Matching
        
        Pattern matching is tricky for a language like Python. We are
        waiting for [PEP 634](https://www.python.org/dev/peps/pep-0634/) and
        structural pattern matching for Python. But we need something that can
        by handled by static type checkers and will also unwrap inner e.g
        optional values and results.
        
        What we want to achieve with pattern matching:
        
        - Check multiple cases with default handling if no match is found.
        - Only one case will ever match. This reduces the cognitive load on the
          programmer.
        - Type safety. We need the code to pass static type checkers.
        - Decomposing of wrapped values, e.g options and results.
        - Case handling must be inline, i.e we want to avoid lambdas which would
          make things difficult for e.g async code.
        - Pythonic. Is it possible to use something that still looks like Python
          code?
        
        The solution we propose is based on loops, and singleton iterables. This
        lets us write our code inline, decompose and unwrap inner values, and
        also effectively skip the cases that do not match.
        
        ```py
        from expression.core import match
        
        with match("expression") as m:
            while m.case("rxpy"):  # will not match
                assert False
        
            for value in m.case(str):  # will match
                assert value == "expression"
        
            for value in m.case(float):  # will not match
                assert False
        
            while m.default():  # will run if any previous case does not match
                assert False
        ```
        
        Using `match` as a context manager will make sure that a case was
        actually found. You might need to have a default handler to avoid
        `MatchFailureError`.
        
        Test cases may be additionally be wrapped in a function to have a match
        expression that returns a value:
        
        ```py
        def matcher(value) -> Option[int]:
            with match(value) as m:
                for value in m.case(Some):
                    return Some(42)
        
                while m.default():
                    return Some(2)
        
            return Nothing
        
        result = matcher(42).
        ```
        
        Classes should also support `match` with pattern directly, i.e:
        `xs.match(pattern)` is effectively the same as
        `match(xs).case(pattern)`, except that the class can then provide
        overloads for correct typing of the unwrapped values without having to
        cast.
        
        ```py
            xs = Some(42)
            ys = xs.map(lambda x: x + 1)
        
            for value in ys.match(Some):
                assert value == 43
                break
            else:
                assert False
        ```
        
        Pattern matching can also be used with destructuring of e.g iterables:
        
        ```py
        xs: FrozenList[int] = empty.cons(42)
        for (head, *tail) in xs.match(FrozenList):
            assert head == 42
        ```
        
        Classes can support more advanced pattern matching and decompose inner
        values by subclassing or implementing the matching protocol:
        
        ```py
        class Matchable(Protocol[TSource]):
            """Pattern matching protocol."""
        
            @classmethod
            def case(cls, matcher: Matcher) -> Iterable[TSource]:
                """Helper to cast the match result to correct type."""
        
                return matcher.case(cls)
        
        
            @abstractmethod
            def __match__(s elf, pattern: Any) -> Iterable[TSource]:
                """Return a singleton iterable item (e.g `[value]`) if pattern
                matches, else an empty iterable (e.g. `[]`)."""
                raise NotImplementedError
        ```
        
        This significantly simplifies the decomposition and type handling
        compared to using `isinstance` checks. E.g code from aioreactive:
        
        ```
        if isinstance(msg, InnerObservableMsg):
            msg = cast(InnerObservableMsg[TSource], msg)
            xs: AsyncObservable[TSource] = msg.inner_observable
            ...
        ```
        
        Now becomes:
        
        ```py
        with match(msg) as m:
            for xs in InnerObservableMsg.case(m):
                ...
        ```
        
        ## Notable Differences
        
        In F# you modules are capitalized, in Python they are lowercase
        ([PEP-8](https://www.python.org/dev/peps/pep-0008/#package-and-module-names)).
        E.g in F# `Option` is both a module (`OptionModule` internally) and a
        type. In Python the module is `option` and the type is capitalized i.e
        `Option`.
        
        Thus in Expression you use `option` as the module to access module
        functions such as `option.map` and the name `Option` for the type
        itself.
        
        ```py
        >>> from expression.core import Option, option
        >>> Option
        <class 'expression.core.option.Option'>
        >>> option
        <module 'expression.core.option' from '/Users/dbrattli/Developer/Github/Expression/expression/core/option.py'>
        ```
        
        F# pattern matching is awesome and the alternative we present here
        cannot be compared. But it helps us match and decompose without having
        to type-cast every time.
        
        ## Why
        
        - I love F#, and know F# quite well. I'm the creator of projects such as
          [Oryx](https://github.com/cognitedata/oryx),
          [Fable.Reaction](https://github.com/dbrattli/Fable.Reaction) and
          [Feliz.ViewEngine](https://github.com/dbrattli/Feliz.ViewEngine)
        - I love Python, and know Python really well. I'm the creator of both
          [RxPY](https://github.com/ReactiveX/RxPY) and
          [OSlash](https://github.com/dbrattli/OSlash), two functional style
          libraries for Python.
        
        For a long time I'm been wanting to make a "bridge" between these two
        languages and got inspired to write this library after watching "[F# as
        a Better Python](https://www.youtube.com/watch?v=_QnbV6CAWXc)" - Phillip
        Carter - NDC Oslo 2020. Doing a transpiler like
        [Fable](https://fable.io) for Python is one option, but a Python library
        may give a lower barrier and a better introduction to existing Python
        programmers.
        
        Expression is an F# inspired version of my previously written
        [OSlash](https://github.com/dbrattli/OSlash) monad tutorial where I
        ported several Haskell abstractions to Python. I never felt that
        OSlash was practically usable in Python, but F# is much closer to
        Python than Haskell, so it makes more sense to try and make a functional
        library inspired by F# instead.
        
        ## Common Gotchas and Pitfalls
        
        A list of common problems and how you may solve it:
        
        ### Expression is missing the function/operator I need
        
        Remember that everything is a function, so you can easily implement the
        function yourself and use it with Expression. If you think the function
        is also usable for others, then please open a PR to include it with
        Expression.
        
        ## Resources and References
        
        A collection of resources that were used as reference and inspiration
        for creating this library.
        
        - F# (http://fsharp.org)
        - Get Started with F# (https://aka.ms/fsharphome)
        - F# as a Better Python - Phillip Carter - NDC Oslo 2020
          (https://www.youtube.com/watch?v=_QnbV6CAWXc)
        - OSlash (https://github.com/dbrattli/OSlash)
        - RxPY (https://github.com/ReactiveX/RxPY)
        - PEP 8 -- Style Guide for Python Code (https://www.python.org/dev/peps/pep-0008/)
        - PEP 342 -- Coroutines via Enhanced Generators
          (https://www.python.org/dev/peps/pep-0342/)
        - PEP 380 -- Syntax for Delegating to a Subgenerator
          (https://www.python.org/dev/peps/pep-0380)
        - PEP 479 -- Change StopIteration handling inside generators (https://www.python.org/dev/peps/pep-0479/)
        - PEP 634 -- Structural Pattern Matching (https://www.python.org/dev/peps/pep-0634/)
        - Thunks, Trampolines and Continuation Passing
          (https://jtauber.com/blog/2008/03/30/thunks,_trampolines_and_continuation_passing/)
        - Tail Recursion Elimination
          (http://neopythonic.blogspot.com/2009/04/tail-recursion-elimination.html)
        - Final Words on Tail Calls
          (http://neopythonic.blogspot.com/2009/04/final-words-on-tail-calls.html)
        - Python is the Haskell You Never Knew You Had: Tail Call Optimization
          (https://sagnibak.github.io/blog/python-is-haskell-tail-recursion/)
        
        ## How-to Contribute
        
        You are very welcome to contribute with PRs :heart_eyes: It is nice if
        you can try to align the code with F# modules, functions and
        documentation. But submit a PR even if you should feel unsure.
        
        Code, doc-strings, and comments should also follow the [Google Python
        Style Guide](https://google.github.io/styleguide/pyguide.html). Code is
        formatted using [Black](https://github.com/psf/black).
        
        ## License
        
        MIT, see [LICENSE](https://github.com/dbrattli/Expression/blob/main/LICENSE).
Platform: UNKNOWN
Classifier: Development Status :: 3 - Alpha
Classifier: Environment :: Other Environment
Classifier: Intended Audience :: Developers
Classifier: License :: OSI Approved :: MIT License
Classifier: Operating System :: OS Independent
Classifier: Programming Language :: Python :: 3.8
Classifier: Topic :: Software Development :: Libraries :: Python Modules
Requires-Python: >=3.8
Description-Content-Type: text/markdown
