undercat 0.1.1

Library implementing the covariant Hom (AKA "Reader") functor.

Undercat

Undercat is a small Python library implementing a functional programming construct called the Reader functor. This pattern is particularly useful for dependency injection, composing functions, and operating on immutable, context-aware computations.

The Reader functor is ideal when you need to:

• Pass a shared dependency or context (e.g., configuration, environment) through multiple computations without explicitly threading it through function calls.
• Create pipelines of computation where each step depends on a shared state.
• Enhance code readability by abstracting common operations such as accessing attributes or combining results.

Features

• Function composition: Compose computations using a clean, declarative style.
• Attribute and item access: Retrieve nested attributes or indexed elements with ease.
• Operator overloading: Perform arithmetic, logical, and comparison operations directly on Reader objects.

Installation

pip install undercat

or (using uv)

uv pip install undercat

Quickstart

1. Basic Usage

Define a Reader that retrieves data from a context and operates on it:

import undercat as uc

class Context:
    """A context with a dict attribute."""
    def __init__(self, value):
        self.data = {'key': value}

# Define a Reader to get a nested value from a context.
get_value = uc.attrgetter('data').getitem('key')

context = Context(42)

# Execute the Reader with a context.
get_value(context)  # output: 42

2. Function Composition

Readers can be composed using .map to transform outputs:

get_value_and_increment = get_value.map(lambda x: x + 1)

# Execute the modified Reader.
get_value_and_increment(context)  # output: 43

3. Combining Readers

Combine multiple Readers into one using operators:

# Define a Reader that squares its input.
square = uc.Reader(lambda x: x * x)

# Define a Reader that leaves its input alone.
identity = uc.Reader(lambda x: x)

# Combine Readers arithmetically.
combined = square + identity

# Execute the Reader.
# Its constituent Readers act independently on the same input; the results are then added.
combined(3)  # output: 12 (3 * 3 + 3)
combined(5)  # output: 30 (5 * 5 + 5)

# Combine Readers to make a new Reader that produces a tuple.
combined = uc.make_tuple(square, identity)

combined(3)  # output: (9, 3)

# Create a Reader that produces a constant value for any input.
const10 = uc.const(10)

# Create a Reader that multiplies the output of its constituent Readers.
combined = uc.prod([const10, identity, square])

combined(3)  # Output: 270 (10 * 3 * 9)

API Overview

Core Methods

• Reader(func): Wrap a function into a Reader.
• uc.const(val): Create a Reader that always returns a constant value.
• uc.attrgetter(attr, [default]): Access an attribute (or nested attributes) from a context.
• uc.make_tuple(*readers): Combine multiple Readers into one that returns a tuple.

Operators

Readers support most logical/arithmetic operations:

• Arithmetic: +, -, *, **, /, //, %, @
• Boolean/bitwise logic: and, or, &, |, ^, ~
• Comparisons: <, <=, >=, >
  • For equality you cannot use == and !=; instead use reader.equals(other) and reader.not_equals(other).

You can also apply operations to sequences of Readers using uc.sum, uc.prod, uc.all, uc.any, uc.min, uc.max, and uc.reduce.

License

This library is open-source and licensed under the MIT License.

Contributions are welcome!