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- 1. HEY!There’s OCaml In My Rust! Kel Cecil @praisechaos
- 2. I ❤ Languages
- 3. Incredible Shrinking Operating System Steve Jones DevOps Days 2015
- 4. OCaml is pretty cool!
- 5. It's a functional programming language... # let double x = x * 2 in List.map double [ 1; 2; 3 ];; - : int list = [2; 4; 6] • OCaml is an almost pure functional programming language by default. • Functions are first-class citizens.
- 6. ... and an imperative programming language... let update_collectable_value order new_value = order.old_value <- order.current_value; order.current_value <- new_value; order • It can be convenient to be able to code in the imperative style. • OCaml includes imperative constructs like for and while loops.
- 7. ... and an object-oriented language! class ['a] queue init = object val mutable q: 'a list = init method enqueue item = q <- q @ item end;; • OCaml can also be object-oriented if needed.
- 8. OCaml powers some pretty sweet stuff. • MirageOS: Unikernel tech recently acquired by Docker • Flow: A static type checker for JavaScript • Hack: A programming languages for Facebook's HHVM • and...
- 9. Rust's first compiler! • rustboot was written in OCaml • Replaced for Rust 0.7
- 10. Rust is pretty excellent too! • Guaranteed Memory Safety • Threads without data races • Incredibly fast • Crazy awesome potential
- 11. Rust was inspired by OCaml in a few interesting ways!
- 12. Let's start with a simple, imperative OCaml program • Define a type named collectable_value that olds the previous value and current value of an item. • Provide a function to update the current value and update the old value. • Have a "main" function to create the record and update the value.
- 13. Starting with our type type collectable_value = { mutable old_value: ﬂoat; mutable current_value: ﬂoat; } • A record is a collection of values stored together as a data type. • Values are immutable by default but can be marked as mutable.
- 14. Adding our first function let update_collectable_value collectable new_value = collectable.old_value <- collectable.current_value; collectable.current_value <- new_value; collectable • Define a function with a let-binding. • Accepts a collectablevalue record (collectable) and a float (newvalue). • Updates and returns the record.
- 15. Adding a "main" let _ = let pinball_machine = { old_value = 0.0; current_value = 800.00; } in update_collectable_value pinball_machine 900.0 • Underscore discards the return value. • This let-binding allows us to execute code at the top level.
- 16. Anything seem a little Rust-y to you? type collectable_value = { mutable old_value: ﬂoat; mutable current_value: ﬂoat; } let update_collectable_value order new_value = order.old_value <- order.current_value; order.current_value <- new_value; order let _ = let pinball_machine = { old_value = 0.0; current_value = 800.00; } in update_collectable_value pinball_machine 900.0
- 17. Semicolon Separation in OCaml let update_collectable_value collectable new_value = collectable.old_value <- collectable.current_value; collectable.current_value <- new_value; collectable • Notice the semi-colons separating the expressions? • The last expression is the return value for the function. • Look familiar?
- 18. Statements in Rust fn greeting() -> String { let greeting = "Hello, there!"; String::from(greeting) // Return value from fn } • Rust code is mostly statements. • Most common statements are • declaring a variable • using an expression with a semi-colon • Last expression omitting the semi-colon is returned.
- 19. What is Type Inference? • Automatic deduction of the data type of an expression.
- 20. Rust's Type Inference // Rust knows that name is a String. let name = "Kel!" • Rust's type inference is loosely based on the Hindley-Milner algorithm4 . • Extensions were added to use accommodate subtyping. 4 https://github.com/rust-lang/rust/blob/master/src/librustc/infer/README.md
- 21. OCaml's Type Inference (* OCaml also knows this is a string. *) let name = "Kel!" • ML originally used the Hindley-Milner algorithm. • OCaml's implementation works through solving constraints. • Tons of research hours poured into their algorithm(s)5 5 https://www.cis.upenn.edu/~sweirich/icfp-plmw15/slides/pottier.pdf
- 22. Workshop: Binary Search Tree • Follow along in your IDEs! • Learn about binary search trees here: • https://en.wikipedia.org/wiki/Binary_search_tree • Check out the examples • https://github.com/kelcecil/rust-ocaml-workshop
- 23. Building our Tree Structure OCaml, then Rust.
- 24. Tree in OCaml type tree;; • Types in OCaml are defined with the type keyword.
- 25. Tree in OCaml type tree = Empty ;; • We need to represent a node that does not exist. • We'll represent this as Empty.
- 26. Tree in OCaml type tree = | Empty | Node ;; • We also want to represent a node that exists. • We'll represent this as node.
- 27. Tree in OCaml type 'a tree = | Empty | Node of 'a * 'a tree * 'a tree ;; • Our node must have a key, a left child, and a right child. • Node now contains generic 'a that is our key. • We also have two values of tree presenting our left and right leaves.
- 28. Algebraic Data Types • Composite type formed from other types • Common types of Algebraic Types • Product Types • Sum Types
- 29. Product Types • Compound types in a structure • Operands of the product are types • Structure is determined by fixed order of operands
- 30. (* Record *) type person = {name: string; age: int};; let kel = {name = "Kel"; age = 30};; (* Tuple *) let next_obsession = ("Dead Rising 4", 59.99);; Tuples and records are product types in OCaml
- 31. let my_love = ("Rust", 9001) • Tuples are product types in Rust • There's also the unit () type • Tuple with no operands.
- 32. Sum Types type 'a tree = | Empty | Node of 'a * 'a tree * 'a tree ;; • Multiple classes of values called variants. • Each class has it's own constructor. • Class represents it's own concept
- 33. Tree in Rust enum Tree { Empty } • Algebraic data types are enums in Rust. • We start with defining Empty.
- 34. enum Tree<T>{ Empty, Node(T, Box<Tree<T>>, Box<Tree<T>>) } • Node is a generic variable for the key, a left leaf, and a right leaf. • Recursive structures must be boxed1 . • We want the size of the Tree to be bounded. • Pointer using Box allocates memory on the heap. 1 https://doc.rust-lang.org/std/boxed/
- 35. Recap: Rust and OCaml Tree Structures6 type 'a tree = | Empty | Node of 'a * 'a tree * 'a tree ;; enum Tree<T>{ Empty, Node(T, Box<Tree<T>>, Box<Tree<T>>) } 6 https://github.com/kelcecil/rust-ocaml-workshop/tree/master/workshop/01-structure
- 36. Writing a depth function
- 37. depth (OCaml using Pattern Matching) let rec depth tree = match tree with | Empty -> 0 | Node(_, left, right) -> 1 + max (depth left) (depth right) ;; • let defines a variable or function in OCaml. • rec denotes a recursive function. • The match keyword matches by data type.
- 38. depth (OCaml using function) let rec depth = function | Empty -> 0 | Node(_, left, right) -> 1 + max (depth left) (depth right) ;; • OCaml functions can also be defined using the function keyword. • function has built in pattern matching for types. • This syntax is a bit more concise.
- 39. Unit Test for depth (OCaml) open OUnit (* tree type here *) (* depth function here *) let test_tree = Node(5, Node(3, Empty, Empty), Empty) let tree_depth _ = assert_equal 2 (depth test_tree) let suite = "Binary Tree Tests" >::: ["tree_depth" >:: tree_depth] let _ = run_test_tt_main suite
- 40. Running OCaml Tests ocamlﬁnd ocamlc -package oUnit -linkpkg -o <output_binary> <source_ﬁle>
- 41. depth (Rust) impl<T> Tree<T> where T: PartialOrd { fn depth(&self) -> usize { match *self { Tree::Empty => 0, Tree::Node(_, ref left, ref right) => 1 + max(left.depth(), right.depth()) } } } • Restricting our generic to PartialOrd lets us choose our key type similarly to OCaml.
- 42. Unit Test for depth (Rust) #[test] fn depth_test() { let tree = Tree::Node(5, Box::new( Tree::Node(4,Box::new(Tree::Empty), Box::new(Tree::Empty) )), Box::new(Tree::Empty)); assert!(tree.depth() == 2) }
- 43. Running Rust Unit Tests rustc --test <source_ﬁle>
- 44. tl;dr • OCaml's inspiration for Rust can been seen in: • Semicolon Statement Separation • Type Inference • Algebraic Data Types • Pattern Matching • Rust and OCaml are both totally awesome.
- 45. Time for you to try! • Adjust the our tree implementations to hold values. • Write insert functions for OCaml and Rust! • Write member functions for OCaml and Rust!
- 46. Stumped? No problem. • https://github.com/kelcecil/rust-ocaml-workshop • Clone the repository and experiment! • Check out the other random examples in the repository. • Ask for help!
- 47. Branching in OCaml OCaml has an if statement like so2 : if <boolean_condition> then <expression> else if <boolean_condition> then <expression> else <expression> 2 https://ocaml.org/learn/tutorials/ifstatementsloopsandrecursion.html

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