Functional Programming in PHP

Transcript

1. Functional Programming in PHP
2. Lochemem Bruno Michael PHP enthusiast from Kampala, Uganda
3. Functional Programming, huh? • A declarative programming paradigm • Mandates that pure functions be written • Saliently features the use of expressions • Emphasizes the importance of composition
4. What are the benefits? • Functional Programming helps address complexity ◦ Complexity often deters project advancement ◦ Functions exist only as callable units and do not affect histories • FP techniques truncate the cognitive load of developing software ◦ Short-term memory is a finite resource ◦ FP conditions one to parameterize constructs ◦ The paradigm sturdies one’s intellection when writing programs
5. 4-7 Quanta in working memory models
6. All of this in PHP?
7. Expressions
8. First-class functions
9. Do not forget everything you know about programming!
10. Functions everywhere!
11. User-defined functions
12. Anonymous functions
13. Class methods
14. Keep the functions pure, please! Side effects Side causes
15. Not so pure! • Mutable global variable (side-cause) • Value mutated after function is called (side-effect)
16. An improvement? • Use a closure • Mutate only within the function
17. Parameterize the things!
18. Another fix? • Parameterize the accumulator • Return one integer for every $x-$acc parameter combination
19. Every argument input should yield the same output.
20. What about classes?
21. Also not pure! • Setter mutates internal state whenever invoked • Each setter call increases temporal coupling
22. The cardinal rule of purity still applies!
23. Either • Remove the setter • Single $empName value for each object
24. Or • Make the setter a pure function • Create new copy of Employee
25. Pure Functions • Do not create side-effects • Yield the same output given the same input • Are referentially transparent
26. Compose the functions! f(x) = x + 1 g(x) = 2x f ∘ g = f(g(x))
27. “If you’re chaining, you’re composing” - From Eric Elliott in Composing Software: An Introduction
28. Traditionally • Invoke functions as function arguments • Cumbersome to deal with long chains
29. Approaching helper library territory!
30. Point-free? • No explicit parameter mentions • Seemingly more compact
31. const for functions?
32. Some FP sauce to add to your program • Partial application • Currying • Map, Filter, Fold/Reduce • Pattern Matching • Persistent Data Structures
33. You might have used some of these techniques before.
34. Currying • Decompose a function into a series of sub-functions • Each sub-function takes one argument
35. Partial Application • Similar to currying • Also useful for deferred function calls • Each sub-function has variable arity
36. Map, Filter, Fold • Higher-order functions for array transformations • Highly composable • Single iteration for every transformation
37. Pattern Matching • Native support in pure FP languages • Useful whenever data has discernible patterns • Especially good for flow-control • Composable, as a function
38. Immutable Lists • Persistent data structure • Each transformation outputs a new copy • Single iteration for each transformation
39. Real world IO is messy!
40. Messy, huh? Why? • Many operations are inherently impure • Exceptions - used commonly - are also inherently impure • IO can get complicated
41. Impure IO? • Filesystem interaction • Writing to and reading from standard output and standard input • Opening windows • Database interactions
42. Exceptions • Inherently impure • Not good for function chains
43. How you can cope with this • Use functors and monads • Parameterize (might lead to callback hell) • Favor composition with forward function chaining • Use persistent data structures
44. Functor • Something you can map over • Typically a type class • A type of category • Supports forward chaining
45. Monad • A special type of functor • Convenient because of inherent ad-hoc value • Especially good for flow-control
46. Real world monads • Either and Maybe union types • IO monad • State monad • Reader monad • Writer monad
47. Either and Maybe Types • Great for error handling • Emphasize writing two-track functions (Railway pattern)
48. IO Monad • Great for IO-related tasks • Encapsulated within the monad is an executable function
49. State Monad • Stores an initial state • Transforms copies of initial state in separate container
50. Reader Monad • Useful for sharing data amongst functions • Akin to Dependency Injection (DI) in OO
51. Writer Monad • Great for logging data • Can, in some contexts, be used for event sourcing
52. How do I know I’m doing it right? The functions you write should have the following qualities: • Purity • Referential Transparency • Composability • Conciseness
53. My Book Functional Programming in PHP
54. Thank you!