monadspy 2.1.1

Monads implementation in pure Python code

Option and Try Monads for Python

Functional programming utilities inspired by Scala.

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Option Monad

Option represents a value that may or may not exist.

• Some(value) — a present value
• Nothing — absence of a value

This eliminates manual None checks and enables functional pipelines.

Creating Options

Option.of(10)      # Some(10)
Option.of(None)    # Nothing

Properties

Property	Some	Nothing
is_defined	True	False
is_empty	False	True

Methods

map(fn)

Transforms the wrapped value if present.

Option.of(10).map(lambda x: x * 2)   # Some(20)
Option.of(None).map(lambda x: x * 2) # Nothing

flat_map(fn)

Chaining operations that return Option.

def safe_div(a, b):
    return Option.of(a / b if b != 0 else None)

Option.of(10).flat_map(lambda x: safe_div(x, 2)) # Some(5)

filter(predicate)

Keeps the value only if it satisfies the predicate.

Option.of(10).filter(lambda x: x > 5)  # Some(10)
Option.of(3).filter(lambda x: x > 5)   # Nothing

get_or_else(default)

Option.of(None).get_or_else(99)  # 99

fold(default, mapping_fn)

Useful for extraction.

Option.of(20).fold(0, lambda x: x * 2)  # 40

or_else(other_option)

Option.of(None).or_else(Option.of(5))  # Some(5)

flatten()

Option.of(Option.of(10)).flatten()  # Some(10)

Iteration

for value in Option.of(10):
    print(value)   # 10

Decorator: @to_option

Converts function output into Option.

@to_option
def compute(x):
    return x * 2

compute(10)  # Some(20)

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Try Monad

Try models computations that may throw exceptions.

• Success(value)
• Failure(exception)

It removes the need for try/except in functional code.

Creating Try Values

Try.of(lambda: 10 / 2)  # Success(5.0)
Try.of(lambda: 10 / 0)  # Failure(ZeroDivisionError)

Properties

Property	Success	Failure
is_success	True	False
is_failure	False	True

Methods

map(fn)

Only applied if successful.

Try.of(lambda: 5).map(lambda x: x + 1)   # Success(6)
Try.of(lambda: 1/0).map(lambda x: x + 1) # Failure(...)

flat_map(fn)

Chaining computations that return Try.

def parse_int(s: str):
    return Try.of(lambda: int(s))

Try.of(lambda: "123").flat_map(parse_int)  # Success(123)

filter(predicate)

Try.of(lambda: 10).filter(lambda x: x > 5)  # Success(10)
Try.of(lambda: 3).filter(lambda x: x > 5)   # Failure(ValueError)

get_or_else(default)

Try.of(lambda: 10/0).get_or_else(99)  # 99

or_else(default_try)

Try.of(lambda: 10/0).or_else(Success(5))  # Success(5)

fold(default_fn, mapping_fn)

Try.of(lambda: 10 / 0).fold(
    default=lambda ex: f"Error: {ex}",
    mapping_fn=lambda x: x * 2
)
# "Error: division by zero"

flatten()

Try.of(lambda: Success(10)).flatten()  # Success(10)

Converting to Option

Try.of(lambda: 10).to_option()      # Some(10)
Try.of(lambda: 1/0).to_option()     # Nothing

Iteration

for v in Try.of(lambda: 10):
    print(v)

Decorator: @to_try

Automatically wraps exceptions into a Try.

@to_try
def divide(a, b):
    return a / b

divide(10, 2)  # Success(5.0)
divide(10, 0)  # Failure(ZeroDivisionError)