Functional programming


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Functional programming for OO programmers:
1.) Immutability
2.) Memoization
3.) Recursion, Tail recursion
4.) Higher-order functions
5.) Currying and Partial application
6.) Pattern matching

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Functional programming

  1. 1. Functional Programming for OO programmers 1 July 2013 Martin Škurla
  2. 2. Agenda •Functional programming (what and why) •The basic concepts of functional programming: • Immutability • Memoization • Recursion, Tail recursion • Higher-order functions • Currying and Partial application • Pattern matching 2 | Functional Programming | 1 July 2013
  3. 3. Functional programming (what and why) •What: • no strict definition as opposed to OOP • programming without mutable variables, assignments, conditions, loops and other imperative control structures • functions should have no side-effects • “Functions as primary building blocks” (first-class functions) •Why: • simpler reasoning principles (data abstraction, concurrency) • better modularity • much easier to exploit parallelism for multicore and cloud computing 3 | Functional Programming | 1 July 2013
  4. 4. Immutability •“No notion of mutation and variables” • variable as memory cell • rather variable bindings •Consequences: • different programming style with collections (Cons Nil lists) • usually more complicated collection implementation • knowledge of implementation details of the underlying collection is essential • sometimes even necessary to refactor the code/algorithm when switching to different collection implementation •Datomic • immutable internal database model 4 | Functional Programming | 1 July 2013
  5. 5. Memoization •“Function scope caching” •Purely mathematical definition of a function •Implementation details: • hashmap-like data structure (Scala) • not changing the value, but rather a reference (Racket, Clojure) •Consequences: • can lead to exponentially faster programs for recursive functions • memory tradeoff 5 | Functional Programming | 1 July 2013
  6. 6. Recursion, tail recursion •“When a function calls even indirectly itself” •Call stacks and stack frames • java.lang.StackOverflowError •“If a function calls itself as its last action, the function’s stack frame can be reused” •Consequences: • JVM does not directly support tail-call optimizations • Scala & @tailrec annotation, Clojure & recur form • algorithms to convert recursive calls to tail recursive calls (e.g. accumulation) •Scala example 6 | Functional Programming | 1 July 2013
  7. 7. Higher-order functions •“Passing or returning code/function as argument” •Functions as first-class values •OOP comparison: • pretty much every OOP developer is doing “higher-order” programming • design patterns: Strategy, Observer, ... • practical reasons because of JVM design •Scala example 7 | Functional Programming | 1 July 2013
  8. 8. Currying and Partial application •“The act of transforming a function of several arguments into a chain of functions of one argument that will yield the same result when called in sequence with the same arguments” •f(x,y,z) = g(x)(y)(z) •Consequences: • functions because of currying can be partially applied • language runtimes can be easier (Haskell) •Scala example 8 | Functional Programming | 1 July 2013
  9. 9. Pattern matching •“Decomposing data structures and ADTs” • Abstract Data Types (tuples) • data structures (Cons Nil lists) •Nothing to do with switch statement • constant-time runtime lookup over constants •Consequences: • no missing cases • no exceptions for wrong variants •SML example 9 | Functional Programming | 1 July 2013
  10. 10. Wrap up •Functional programming is not only, but also about: • Immutability • Memoization • Recursion, Tail recursion • Higher-order functions • Currying and Partial application • Pattern matching 10 | Functional Programming | 1 July 2013
  11. 11. Functional programming is much more stuff… •Influential people: • Alonso Church • Haskell Curry • John McCarthy •More advanced concepts: • composition, functors, monads, ... •Tons of languages: • Clojure, F#, Lisp, OCaml, Python, Racket, Scala, Scheme, SML 11 | Functional Programming | 1 July 2013
  12. 12. Thank you for attention Functional programming rocks
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