OCaml baby steps

practice.ml

(* random code samples *)


Char.code 'a' (* => 97 *)
Char.uppercase 'a'
Char.chr 33 (* => ! *)


(* STRING CONCATENATION *)

"hello" ^ "world"

"hello".[1]

String.length "hello"
String.sub "asdf" 1 2 (* substring => sd *)

if true then 1 else 2

let x = 1
let y = 2

let z = x + y

(* OR *)

(* Sequential definition *)
let x = 100;;
let z =
  let x = 1 in 
  let y = 2 in 
  x + y;;
  
(* Simultaneous definition *)
let x = 1 and y = x in y + x;; (* 100 + 1*)


let foo x y = x + y ;;
(* val foo : int -> int -> int = <fun> *)
foo 2 4;;
(* - : int = 6 *)
let foo x y = x * y in foo 2 4;;
(* - : int = 8 *)
foo 2 4;;
(* - : int = 6 *)

(* Shadowing and binding *)
let a   = 10;;
let f x = x + a ;;
f 1 ;; (* 11*)
let a = 100 ;; (* New binding which shadows the old one - f "closes" a due to lexical scoping*)
f 1 ;; (* 11 *)


let sq = fun x -> x * x

(* Simplified form *)

let sq x = x * x 

let sum = fun i -> (fun j -> i + j)

let sum i j = i + j


let sum i = 
  let sum2 j = 
    i + j in
  sum2


(* sum of first n *)

let rec sum_of_n n acc = 
  if n = 0 then acc
  else (sum_of_n (n - 1) (acc + n)) ;;


(* Not tail-recursive *)
let rec sum_of_n n = 
  if n = 0 then n
  else n + sum_of_n (n - 1) ;;


(* keyword args *)
let foobar ~x:i ~y:j = i - j;;

let foobar ~x ~y = x + y ;;


(********** PATTERN MATCHING pg 31/41***********)

match coll with
    [] -> true
  | x :: xs -> false ;;

let is_empty coll = 
  match coll with
      [] -> true
    | x :: xs -> false ;;

(* Simplified form *)

let is_empty =
  function 
    | [] -> true
    | x :: xs -> false ;;

(* "as" keywords *)
let is_char_a = 
  function
    | ('a'|'A') as c -> true (* TUPLE PATTERN *)
    | _ -> false ;;

(* "when" keyword *)
let rec last = 
  function
    | []                   -> raise Empty_list
    | x :: xs when xs = [] -> x
    | _ :: xs              -> last xs ;;


(* SPECIFYING PARAMETER TYPES *)

let sum (x : int) (y : int) = x + y ;;

(* SPECIFYING FUNCTION RETURN TYPE *)

let sum x y : int = x + y ;;


let tuple = (1,2,4,4) ;;

let tuple = ('a',"asdf",4,4.0) ;;


(* CONS'ing *)

let lst = "a" :: [] ;; (* => ["a"] *)

let lst = ["a", "s", "d"] ;;

(* EQUIVALENT TO *) 
let lst = "a" :: "b" :: "c" :: [];;


(* UNIONS *)

type number = 
  | Zero
  | Integer of int 
  | Float of float ;;

(* Balanced Binary Tree *)

type 'a tree = 
    Leaf
  | Node of 'a * 'a tree * 'a tree ;;

let rec cardinality = 
  function
    | Leaf -> 0
    | Node (_, left, right) -> 1 + cardinality left + cardinality right;;

let rec cardinality' (t : 'a tree) = 
  match t with 
    | Leaf -> 0
    | Node (_, left, right) -> 1 + cardinality' left + cardinality' right;;

let rec cardinality' t = 
  match t with 
    | Leaf -> 0
    | Node (_, left, right) -> 1 + cardinality' left + cardinality' right;;

let rec card t = 
  function
    | Leaf -> 0
    | Node (_, left, right) -> 1 + card left + card right;;


(*  The above expression throws the following error. WHY ??????!
    Error: This expression has type 'a tree -> int
       but an expression was expected of type int
*)


(* Open Union types *)

let s_to_n s = 
  match s with 
      `Integer x -> 1
    | _ -> raise (Invalid_argument "unknown number");;


(* References *)

let flag = ref false;;

let flag := true;;

when you say

let rec card t = 
  function
    | Leaf -> 0
    | Node (_, left, right) -> 1 + card left + card right;;

note that card t is returning the function, and is therefore of type 'a tree -> int. this works:

let rec card t = 
  (function
    | Leaf -> 0
    | Node (_, left, right) -> 1 + card left + card right
  )  t;;

or more clearly

let rec card t = 
  let f = function
    | Leaf -> 0
    | Node (_, left, right) -> 1 + card left + card right
  in
  f t;;

Understood! Wah! We thanks it marteen sar!

It was trying to add 1 + ('a tree). Hence the error. Don't know how I missed it :)

no, it was trying to add 1 + (function Leaf -> 0 | Node -> 1 + card left + card right). the function wasn't being applied to anything, it was the return value of card.

oh yes. It was trying to add 1 + the function (which is of type 'a tree -> int)