Closures & Higher-Order Functions

# Closure — function remembers its environment
def make_multiplier(factor):
    def multiply(x):
        return x * factor  # 'factor' is captured!
    return multiply

double = make_multiplier(2)
triple = make_multiplier(3)
print(double(5))   # 10
print(triple(5))   # 15

# Higher-order functions
def apply(func, items):
    return [func(x) for x in items]

numbers = [1, -2, 3, -4, 5]
print(apply(abs, numbers))    # [1, 2, 3, 4, 5]
print(apply(str, numbers))    # ['1', '-2', '3', '-4', '5']

# map, filter, reduce
from functools import reduce

nums = [1, 2, 3, 4, 5]
squared = list(map(lambda x: x**2, nums))
evens = list(filter(lambda x: x % 2 == 0, nums))
total = reduce(lambda a, b: a + b, nums)

print(f"Squared: {squared}")
print(f"Evens: {evens}")
print(f"Sum: {total}")

# Function factory
def make_validator(min_val, max_val):
    def validate(value):
        return min_val <= value <= max_val
    return validate

age_valid = make_validator(0, 150)
score_valid = make_validator(0, 100)
print(age_valid(25))    # True
print(score_valid(150)) # False