python-scala-functional

name	python-scala-functional
description	Guide for translating Python code to functional Scala style. Use when converting Python code involving numeric formatting, higher-order functions, decorators, closures, generators, or when aiming for idiomatic functional Scala with pattern matching, Option handling, and monadic operations.

Python to Scala Functional Programming Translation

Numeric Formatting

Use this translation when Python code formats floating-point or decimal values with fixed precision. This Scala version preserves the same intent: rounding to the specified number of decimal places and returning a plain decimal string without scientific notation.

Python:

f"{value:.{precision}f}"

Scala:

val strValue =
  BigDecimal.decimal(value)
    .setScale(precision, BigDecimal.RoundingMode.HALF_UP)
    .bigDecimal
    .toPlainString

Higher-Order Functions

# Python
def apply_twice(f, x):
    return f(f(x))

def make_multiplier(n):
    return lambda x: x * n

double = make_multiplier(2)
result = apply_twice(double, 5)  # 20

// Scala
def applyTwice[A](f: A => A, x: A): A = f(f(x))

def makeMultiplier(n: Int): Int => Int = x => x * n

val double = makeMultiplier(2)
val result = applyTwice(double, 5)  // 20

Decorators → Function Composition

# Python
def log_calls(func):
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}")
        result = func(*args, **kwargs)
        print(f"Finished {func.__name__}")
        return result
    return wrapper

@log_calls
def add(a, b):
    return a + b

// Scala - function composition
def logCalls[A, B](f: A => B, name: String): A => B = { a =>
  println(s"Calling $name")
  val result = f(a)
  println(s"Finished $name")
  result
}

val add = (a: Int, b: Int) => a + b
val loggedAdd = logCalls(add.tupled, "add")

// Alternative: using by-name parameters
def withLogging[A](name: String)(block: => A): A = {
  println(s"Calling $name")
  val result = block
  println(s"Finished $name")
  result
}

Pattern Matching

# Python (3.10+)
def describe(value):
    match value:
        case 0:
            return "zero"
        case int(x) if x > 0:
            return "positive int"
        case int(x):
            return "negative int"
        case [x, y]:
            return f"pair: {x}, {y}"
        case {"name": name, "age": age}:
            return f"{name} is {age}"
        case _:
            return "unknown"

// Scala - pattern matching is more powerful
def describe(value: Any): String = value match {
  case 0 => "zero"
  case x: Int if x > 0 => "positive int"
  case _: Int => "negative int"
  case (x, y) => s"pair: $x, $y"
  case List(x, y) => s"list of two: $x, $y"
  case m: Map[_, _] if m.contains("name") =>
    s"${m("name")} is ${m("age")}"
  case _ => "unknown"
}

// Case class pattern matching (preferred)
sealed trait Result
case class Success(value: Int) extends Result
case class Error(message: String) extends Result

def handle(result: Result): String = result match {
  case Success(v) if v > 100 => s"Big success: $v"
  case Success(v) => s"Success: $v"
  case Error(msg) => s"Failed: $msg"
}

Option Handling (None/null Safety)

# Python
def find_user(user_id: int) -> Optional[User]:
    user = db.get(user_id)
    return user if user else None

def get_user_email(user_id: int) -> Optional[str]:
    user = find_user(user_id)
    if user is None:
        return None
    return user.email

# Chained operations
def get_user_city(user_id: int) -> Optional[str]:
    user = find_user(user_id)
    if user is None:
        return None
    address = user.address
    if address is None:
        return None
    return address.city

// Scala - Option monad
def findUser(userId: Int): Option[User] = db.get(userId)

def getUserEmail(userId: Int): Option[String] =
  findUser(userId).map(_.email)

// Chained operations with flatMap
def getUserCity(userId: Int): Option[String] =
  findUser(userId)
    .flatMap(_.address)
    .map(_.city)

// For-comprehension (cleaner for multiple operations)
def getUserCity(userId: Int): Option[String] = for {
  user <- findUser(userId)
  address <- user.address
  city <- Option(address.city)
} yield city

// Getting values out
val email = getUserEmail(1).getOrElse("[e-mail]")
val emailOrThrow = getUserEmail(1).get  // Throws if None

Generators → Iterators/LazyList

# Python
def fibonacci():
    a, b = 0, 1
    while True:
        yield a
        a, b = b, a + b

# Take first 10
fibs = list(itertools.islice(fibonacci(), 10))

// Scala - LazyList (was Stream in Scala 2.12)
def fibonacci: LazyList[BigInt] = {
  def loop(a: BigInt, b: BigInt): LazyList[BigInt] =
    a #:: loop(b, a + b)
  loop(0, 1)
}

val fibs = fibonacci.take(10).toList

// Alternative: Iterator
def fibonacciIterator: Iterator[BigInt] = new Iterator[BigInt] {
  private var (a, b) = (BigInt(0), BigInt(1))
  def hasNext: Boolean = true
  def next(): BigInt = {
    val result = a
    val newB = a + b
    a = b
    b = newB
    result
  }
}

Try/Either for Error Handling

# Python - exceptions
def parse_int(s: str) -> int:
    try:
        return int(s)
    except ValueError:
        return 0

# Python - Optional for errors
def safe_parse_int(s: str) -> Optional[int]:
    try:
        return int(s)
    except ValueError:
        return None

// Scala - Try monad
import scala.util.{Try, Success, Failure}

def parseInt(s: String): Try[Int] = Try(s.toInt)

val result = parseInt("123") match {
  case Success(n) => s"Got: $n"
  case Failure(e) => s"Error: ${e.getMessage}"
}

// Chaining Try operations
val doubled = parseInt("123").map(_ * 2)

// Either for custom error types
def parsePositive(s: String): Either[String, Int] = {
  Try(s.toInt).toEither
    .left.map(_ => "Not a number")
    .flatMap { n =>
      if (n > 0) Right(n)
      else Left("Must be positive")
    }
}

Function Composition

# Python
def compose(f, g):
    return lambda x: f(g(x))

def pipe(*functions):
    def inner(x):
        result = x
        for f in functions:
            result = f(result)
        return result
    return inner

# Usage
add_one = lambda x: x + 1
double = lambda x: x * 2
pipeline = pipe(add_one, double, add_one)  # (x + 1) * 2 + 1

// Scala - built-in composition
val addOne: Int => Int = _ + 1
val double: Int => Int = _ * 2

// compose: f.compose(g) = f(g(x))
val composed = addOne.compose(double)  // addOne(double(x))

// andThen: f.andThen(g) = g(f(x))
val pipeline = addOne.andThen(double).andThen(addOne)  // (x + 1) * 2 + 1

Currying and Partial Application

# Python
from functools import partial

def add(a, b, c):
    return a + b + c

add_5 = partial(add, 5)
result = add_5(3, 2)  # 10

// Scala - curried functions
def add(a: Int)(b: Int)(c: Int): Int = a + b + c

val add5 = add(5) _  // Partially applied
val result = add5(3)(2)  // 10

// Converting between curried and uncurried
val uncurried = Function.uncurried(add _)
val curried = (uncurried _).curried

// Multiple parameter lists
def fold[A, B](init: B)(list: List[A])(f: (B, A) => B): B =
  list.foldLeft(init)(f)

val sum = fold(0)(List(1, 2, 3))(_ + _)

Tail Recursion

# Python - no tail call optimization
def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

# Workaround: iterative
def factorial_iter(n):
    result = 1
    for i in range(2, n + 1):
        result *= i
    return result

// Scala - tail recursion with annotation
import scala.annotation.tailrec

def factorial(n: Int): BigInt = {
  @tailrec
  def loop(n: Int, acc: BigInt): BigInt = {
    if (n <= 1) acc
    else loop(n - 1, n * acc)
  }
  loop(n, 1)
}

Implicit Conversions and Type Classes

# Python - no direct equivalent
# Duck typing provides flexibility

// Scala - type classes via implicits (Scala 2) or given/using (Scala 3)

// Scala 3
trait Show[A]:
  def show(a: A): String

given Show[Int] with
  def show(a: Int): String = s"Int: $a"

def display[A](a: A)(using s: Show[A]): String = s.show(a)

// Scala 2
trait Show[A] {
  def show(a: A): String
}

implicit val intShow: Show[Int] = new Show[Int] {
  def show(a: Int): String = s"Int: $a"
}

def display[A](a: A)(implicit s: Show[A]): String = s.show(a)