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OCaml The PL for the discerning hacker.

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Hello. I’m Zach, one of Sorin’s students. ztatlock@cs.ucsd.edu

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ML Anatomy 101 ML Program = One Giant, Complex Expression Controlling complexity is the essence of computer programming. B. Kerninghan A complex system that works is invariably found to have evolved from a simple system that worked. J. Gall ML Program = ? ? ?

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Building ML Programs ML provides tools to control complexity Build complex exprs from simple exprs Build complex types from simple types PREV NOW

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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basic Who cares about types anyway? Every good programmer! (not just old timers) Types provide: 1.Documentation 2.Early bug warning system 3.Performance!

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basic Who cares about types anyway? Even programmers without a type system! I think it may just be a property of large systems in dynamic languages, that eventually you end up rewriting your own type system, and you sort of do it badly. -- Twitter

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basic ExpressionType Kind of Type 5intbasic “hello”stringbasic (5, “hello”)int * stringtuple [1; 2; 3; 4]int listlist [ (1, 1) ; (2, 2) ; (3, 3) ](int * int) listtuple+list

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more on tuples ExpressionType (5, (10, 15))int * (int * int) ((5, 10), 15)(int * int) * int

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more on tuples ExpressionType (5, (10, 15))int * (int * int) ((5, 10), 15)(int * int) * int ( [ (1, 2); (3, 4); (5, 6) ], (“hello”, “india”), (5, “int”, (1, 2, 3)) )

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more on tuples ExpressionType (5, (10, 15))int * (int * int) ((5, 10), 15)(int * int) * int ( [ (1, 2); (3, 4); (5, 6) ], (“hello”, “india”), (5, “int”, (1, 2, 3)) ) (int * int) list * (string * string) * (int * string * (int * int * int))

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basic Don’t know how it works ? Try it in the toplevel !

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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function A -> B Type of a function which: expects parameter of type A produces a value of type B Contract: caller satisfies A callee satisfies B precondition postcondition

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function let f x = x + 5 f: int -> int let f x = “hello “ ^ x f: string -> string let f x = “number “ ^ (string_of_int x) f: int -> string let f x y = x * x + y * y f: int -> int -> int

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function let f x y = 1 :: x :: y f: int -> int list -> int list let f x y z = [1] @ x @ [y + z] f: int list -> int -> int -> int list let rec f = f f ERROR

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polymorphic functions Some functions work on many types: let id x = x id: ‘a -> ‘a Takes a value of any type, call it ‘a Returns a value of that type ‘a

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polymorphic functions let f a b = a f: ‘a -> ‘b -> ‘a let f a b = b f: ‘a -> ‘b -> ‘b let pipe x f = f x f: ‘a -> (‘a -> ‘b) -> ‘b let (|>) = pipe “hello” |> id |> print_string (print_string (id (“hello”))) binary infix operator

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polymorphic functions let f g x = g x f: (‘a -> ‘b) -> ‘a -> ‘b let f g h x = g (h x) f: (‘a -> ‘b) -> (‘c -> ‘a) -> ‘c -> ‘b

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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record type RT = { NM1 : T1 ;... ; NMN : TN } Like a tuple, but refer to members by name.

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record type person = { name : string ; age : int ; hair : string ; job : string }

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making a record value let pres = { name = “Obama” ; age = 49 ; hair = “black” ; job = “president” }

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updating a record value let pres = { name = “Obama” ; age = 49 ; hair = “black” ; job = “president” } let pres’ = {pres with hair = “gray” }

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updating a record value let year_older p = if p.age > 45 then { p with age = p.age + 1, hair = “gray” } else { p with age = p.age + 1 } year_older: person -> person

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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variant type VT = | C1 of T1 |... | CN of TN C1 to CN are “constructors” Ci like function from Ti to VT value of type VT can only be constructed with one of these

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variant type pet = | Dog of string | Cat of string | Fish of string Value of type pet constructed with one of: Dog, Cat, Fish Each takes a string and returns a pet

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variant values type pet = | Dog of string | Cat of string | Fish of string let d = Dog “spot” let c = Cat “whiskers” let f = Fish “nemo”

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matching variant values let name pet = match pet with | Dog nm -> nm | Cat nm -> nm | Fish nm -> nm let d = Dog “sparky” in name d

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matching variant values let says pet = match pet with | Dog _ -> “woof” | Cat _ -> “meow” | Fish _ -> “bubble bubble” let c = Cat “walter” in says c

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variant type fuel_level = | Empty | Middle | Full type vehicle = | Car of int * fuel_level | Tank of int * fuel_level | Boat of int * fuel_level

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matching variant values let miles v = match v with | Car (m, _) -> m | Tank (m, _) -> m | Boat (m, -) -> m

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updating variant values let reduce f = match f with | Empty -> Empty | Middle -> Empty | Full -> Middle let drive v = match v with | Car (m, f) -> Car (m, reduce f) | Tank (m, f) -> Tank (m, reduce f) | Boat (m, f) -> Boat (m, reduce f)

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updating variant values let refill v = match v with | Car (m, f) -> Car (m, Full) | Tank (m, f) -> Tank (m, Full) | Boat (m, f) -> Boat (m, Full)

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recursive variant type expr = | Val of int | Add of expr * expr | Sub of expr * expr | Mul of expr * expr let e1 = Val 5 let e2 = Add (e1, e1) let e3 = Mul (e2, e1)

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recursive variant let rec eval e =

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recursive variant let rec eval e = match e with | Val i -> i | Add (l, r) -> (eval l) + (eval r) | Sub (l, r) -> (eval l) - (eval r) | Mul (l, r) -> (eval l) * (eval r)

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mutual recursion let rec expr_dot e = match e with | Val i -> string_of_int i | Add (l, r) -> (aux "add" l) ^ (aux "add" r) | Sub (l, r) -> (aux "sub" l) ^ (aux "sub" r) | Mul (l, r) -> (aux "mul" l) ^ (aux "mul" r) and aux p e = match e with | Val i -> p ^ " -> " ^ string_of_int i ^ ";\n" | Add _ -> p ^ " -> add;\n" ^ (expr_dot e) | Sub _ -> p ^ " -> sub;\n" ^ (expr_dot e) | Mul _ -> p ^ " -> mul;\n" ^ (expr_dot e)

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Pattern Matching: a PL Masterpiece match is one of ML’s very best features simultaneous test / extract / bind auto checks any missed cases leads to compact, readable code

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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demo Conway’s Game of Life

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demo Conway’s Game of Life code at: http://github.com/ztatlock/simple-life

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demo OCaml in the “real world” at JaneStreet http://ocaml.janestreet.com/ Check out their leader, Yaron Minsky http://ocaml.janestreet.com/?q=node/82

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Building Types 1.basic (recap) 2.function 3.record 4.variant 5.demo M.C. Escher’s Relativity in LEGO

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