Functional programming (FP) is a paradigm that emphasizes immutability, pure functions, and higher-order functions. While Python, Java, and C# are all multi-paradigm languages (supporting both object-oriented and functional programming), they each have different levels of FP support. Here’s a comparison:

1. First-Class Functions

All three languages support first-class functions, meaning functions can be assigned to variables, passed as arguments, and returned from other functions.

  • Python: Full support for first-class functions using lambda, map(), filter(), reduce(), and list comprehensions.
  • Java: Added functional support in Java 8 with lambda expressions and the Stream API.
  • C#: Supports functional programming via delegates, lambda expressions, and LINQ (Language Integrated Query).

2. Higher-Order Functions

A function is considered higher-order if it takes another function as an argument or returns one.

  • Python: Natively supports higher-order functions with built-in functions like map(), filter(), and reduce() from functools.
  • Java: Provides functional interfaces like Function<T, R>, Predicate<T>, and Consumer<T> in java.util.function for higher-order functions.
  • C#: Uses Func<>, Action<>, and Predicate<> delegates for higher-order functions.

3. Immutability

Functional programming emphasizes immutability (no changing of data after it’s created).

  • Python: Supports immutability using tuple and frozenset, but built-in data structures like lists and dictionaries are mutable by default.
  • Java: Supports immutability with final variables and immutable classes (java.util.Collections.unmodifiableList()).
  • C#: Supports immutability with readonly fields and record types introduced in C# 9.

4. Pure Functions & Side Effects

Pure functions don’t modify state or have side effects.

  • Python: Functions can be pure, but there are no strict enforcement mechanisms.
  • Java: Encourages immutability with functional programming but does not enforce purity.
  • C#: Supports purity but allows mutation by default.

5. Lazy Evaluation

Lazy evaluation delays computation until it’s needed.

  • Python: Generators and iterators (yield keyword) allow lazy evaluation.
  • Java: Stream API (Stream<T>) enables lazy computations.
  • C#: LINQ supports deferred execution (IEnumerable<T>).

6. Pattern Matching

Pattern matching is a key feature in some functional programming languages.

  • Python: Introduced pattern matching in Python 3.10 (match statement).
  • Java: Added pattern matching in switch expressions (Java 17).
  • C#: Supports pattern matching with switch expressions and type patterns.

7. Functional Libraries

  • Python: functools, itertools, operator
  • Java: java.util.function, Stream API
  • C#: System.Linq, Func<>, Action<>

Conclusion

Feature Python Java C#
First-Class Functions ✅ Yes ✅ Yes (since Java 8) ✅ Yes
Higher-Order Functions ✅ Yes ✅ Yes ✅ Yes
Immutability ✅ Partial (tuple, frozenset) ✅ Partial (final, immutable classes) ✅ Stronger (readonly, record types)
Pure Functions ✅ Possible but not enforced ✅ Possible but not enforced ✅ Possible but not enforced
Lazy Evaluation ✅ Generators, iterators ✅ Stream API ✅ LINQ (deferred execution)
Pattern Matching ✅ match (Python 3.10) ✅ switch (Java 17) ✅ switch expressions
Functional Libraries functools, itertools java.util.function, Stream System.Linq, Func<>, Action<>

Overall, Python has a strong FP presence due to its built-in functions and expressiveness. Java improved significantly with Java 8’s functional interfaces and streams. C# has excellent FP support through LINQ, delegates, and records.