[TOC]

this week: structure code with types: tuples, records, arrays.

# 1. USER-DEFINED TYPES

### primary use of types: document your code

- use
`type type_identifier = some_type`

to define a new type (`type_identifier`

is synonym/abbrevation of`some_type`

) `type_identifier`

*must start with lowercase letter*- already known types:
`int, bool, string, char`

... - use
`:`

to add type annotation to identifiers`let x : some_type = some_expr`

- annotation param/return type of a function:
`let f(x: some_type): return_type = some_expr`

or`let f x : return_type = some_expr`

example: colors

```
# type color = int;;
type color = int
# let red: color = 0;;
val red : color = 0
# let white: color = 1;;
val white : color = 1
# let blue: color = 2;;
val blue : color = 2
```

example: positive integers

```
# type positive = int;;
type positive = int
# let abs(x:int) = (if x<0 then -x else x: positive);;
val abs : int -> positive = <fun>
# let abs' (x:int): positive = if x<0 then -x else x;;
val abs' : int -> positive = <fun>
```

### pitfalls

- in the REPL, careful with unintended hiding of type identifiers:

```
# type t = int;;
type t = int
# let x: t = 0;;
val x : t = 0
# type t = bool;;
type t = bool
# let f (x: t) = not x;;
val f : t -> bool = <fun>
# let z = f x;;
Error: This expression has type t/1027 = int
but an expression was expected of type t/1029 = bool
```

- limitations of type synonyms

⇒ but ocaml has ways to define more precise types to avoid such error statically

# 2. TUPLES: positioned components

Some objects are naturally made of several components, example: 2d point.

```
# let origin = (0,0);;
val origin : int * int = (0, 0)
# let positive_range = (max_int, 0);;
val positive_range : int * int = (4611686018427387903, 0)
# let negative_range = (min_int, 0);;
val negative_range : int * int = (-4611686018427387904, 0)
```

name tuples with `type`

: use `*`

to construct tuple types.

```
# type point2D = int * int ;;
type point2D = int * int
# let origin : point2D = (0,0);;
val origin : point2D = (0, 0)
```

### pattern matching: observing components of tuple

**pattern**: describe how values are observed by the program.- appear in let-bindings or in func arguments.
- simplest form of pattern: identifiers

example: `let x = 6*3 in x`

can be read as "*I observe the value of 6*3 by naming it as x*"

- ignore some component using wildcard
`_`

, example:`let _ = 6*3 in 1;;`

example for tuple: `let (x, _) = (5, 2) in x;;`

can read as: "*I observe the first component of tuple (5,2) by naming it as x, and I ignore the 2nd component*"

```
# let a = (3,4);;
val a : int * int = (3, 4)
# let (x,_) = a;;
val x : int = 3
# let x_coord (x, _) = x;;
val x_coord : 'a * 'b -> 'a = <fun>
# let y_coord (_, y) = y;;
val y_coord : 'a * 'b -> 'b = <fun>
```

### tuples in the machine

tuple is a block of memory, program holds *pointer* to the block,

pointers can be *shared*:

### structrual equality VS physical equaliity

in ocaml there are 2 types of equalities:

`=`

implies structrual eq — i.e. they have*the same content*.`==`

implies physical eq — i.e. they are*stored in the same memory location*.

```
# let x = (1,2)
let y = (1,2)
let z = x;;
val x : int * int = (1, 2)
val y : int * int = (1, 2)
val z : int * int = (1, 2)
# x = y;;
- : bool = true
# x == y;;
- : bool = false
# x == z;;
- : bool = true
```

### pitfalls

- mismatching number of components : can be caught by compiler
- semantic errors:

example: `# let x_coord (x, y) = y;;`

can't be caught by compiler → using **records**.

# 3. RECORDS: naming components

name each components of a tuple → record.

example:

```
# type point2D = {x:int ; y:int};;
type point2D = { x : int; y : int; }
# let origin = {x = 0; y = 0};;
val origin : point2D = {x = 0; y = 0}
# let from_tuple (x,y) = {x ; y};;
val from_tuple : int * int -> point2D = <fun>
# let a = from_tuple (4,2) ;;
val a : point2D = {x = 4; y = 2}
# let b = from_tuple (10, 5);;
val b : point2D = {x = 10; y = 5}
# type box = {left_upper: point2D; right_lower: point2D;};;
type box = { left_upper : point2D; right_lower : point2D; }
# let abox = {left_upper=a; right_lower=b};;
val abox : box = {left_upper = {x = 4; y = 2}; right_lower = {x = 10; y = 5}}
# let get_min_xcoord {left_upper = {x}} = x;; (*pattern matching here for the func*)
val get_min_xcoord : box -> int = <fun>
```

decalre a record type:

`type som_type_identifier = {field_name: some_type; ...; field_name: some_type}`

construct a record:

`{field_name = some_expr; ...; field_name = som_expr}`

observe a field:

`some_expr.field_name`

observe several fields: use record *patterns*:

`{field_name = some_pattern; ...; field_name = some_pattern}`

(don't have to write all fields here)

in the machine: like a tuple, a block of memory

### pitfalls: shadowing a field name

if 2 records share some identical field names:

```
# type a = {x:int; b:int};;
type a = { x : int; b : int; }
# type b = {y:int; c:int;};;
type b = { y : int; c : int; }
# {x=0; b=2};;
- : a = {x = 0; b = 2}
# type t = {x:bool};;
type t = { x : bool; }
# {x=true};;
- : t = {x = true}
# type u = {x:int};;
type u = { x : int; }
# {x=true};;
Error: This expression has type bool but an expression was expected of type int
```

advice: NOT share field names between records!

# 4. ARRAYS

tuple/record: sizes are statically bounded.

array: *dynamically* change sequence size,

but all array elements must have *same type*.

```
# let p = [|1;2;3;|];;
val p : int array = [|1; 2; 3|]
# let square x = x * x;;
val square : int -> int = <fun>
# let squares n = Array.init n square;;
val squares : int -> int array = <fun>
# let s1 = squares 5;;
val s1 : int array = [|0; 1; 4; 9; 16|]
```

Std module `Array`

: provides functions over arrays

`some_array = [|some_expr; ...; some_expr|]`

`Array.make sz val`

: takes an int (size of array) and a value to initialize each component of the array (like Array.fill).`Array.init sz f`

: initial size of array`sz`

and a function`f`

to initilize each component of array, f takes the*index of*the component and returns a value.`Array.length arr`

: returns array size.- array indexing:
`arr.(some_expr:int)`

, index: 0 to sz-1 - array patterns: observe several components of array
`[|some_expr; ...; some_expr|]`

(not very useful)

```
# let swap a = [|a.(1); a.(0)|];;
val swap : 'a array -> 'a array = <fun>
# let b = swap [|0;1|];;
val b : int array = [|1; 0|]
# let swap [|x; y|] = [|y; x|];;
Characters 9-28:
Warning 8: this pattern-matching is not exhaustive.
Here is an example of a value that is not matched: [| |]
val swap : 'a array -> 'a array = <fun>
# let t = swap [|2;1|];;
val t : int array = [|1; 2|]
# let t = swap [|2;1;0|];;
Exception: Match_failure ("//toplevel//", 1, 9).
```

in the machine: a memory block, like a record

# 5. CASE STUDY: A SMALL TYPED DATABASE

a toy db for contacts,

3 queries: `insert, delete, search`

.

These functions have type: `database -> query_contact -> status * database * contact`

define the phone number/contact/database types:

```
type phone_number = int * int * int * int;;
type contact = {
name : string;
phone_number : phone_number
};;
let nobody = { name = ""; phone_number = (0, 0, 0, 0) };;
type database = {
number_of_contacts : int;
contacts : contact array;
};;
```

implement `make`

(to create a database with parameter=max capacity):

```
let make max_number_of_contacts =
{
number_of_contacts = 0;
contacts = Array.make max_number_of_contacts nobody
};;
(* Queries are represented by a code and a contact.
- If the code is 0 then the contact must be inserted.
- If the code is 1 then the contact must be deleted.
- If the code is 2 then we are looking for a contact
with the same name in the database. *)
type query = {
code : int;
contact : contact;
}
```

implement `search`

:

```
let search db contact =
let rec aux idx =
if idx >= db.number_of_contacts then
(false, db, nobody)
else if db.contacts.(idx).name = contact.name then
(true, db, db.contacts.(idx))
else
aux (idx + 1)
in
aux 0;;
```

implement `insert`

:

```
let insert db contact =
if db.number_of_contacts >= Array.length db.contacts then
(false, db, nobody)
else
let (status, db, _) = search db contact in
if status then (false, db, contact) else
let cells i =
if i = db.number_of_contacts then contact else db.contacts.(i)
in
let db' = {
number_of_contacts = db.number_of_contacts + 1;
contacts = Array.init (Array.length db.contacts) cells
}
in
(true, db', contact);;
```

implement `delete`

:

```
let delete db contact =
let (status, db, contact) = search db contact in
if not status then (false, db, contact)
else
let cells i =
let last = db.contacts.(db.number_of_contacts-1) in
if db.contacts.(i).name = contact.name then last
else if i==(db.number_of_contacts-1) then nobody
else db.contacts.(i) in
let db' = {
number_of_contacts = db.number_of_contacts - 1;
contacts = Array.init (Array.length db.contacts) cells
}
in (true, db', contact);;
```

implement an engine function to process all kinds of queries:

```
(* Engine parses and interprets the query. *)
let engine db { code ; contact } =
if code = 0 then insert db contact
else if code = 1 then delete db contact
else if code = 2 then search db contact
else (false, db, nobody);;
```

Remarks:

This is purely functional, a new db is created each time a query is processed.

#### Part 3 of series «Introduction to Functional Programming in OCaml»：

- [OCaml MOOC] week0: intro and overview
- [OCaml MOOC] week1: BASIC TYPES, DEFINITIONS AND FUNCTIONS
- [OCaml MOOC] week2: BASIC DATA STRUCTURES
- [OCaml MOOC] week3: MORE ADVANCED DATA STRUCTURES
- [OCaml MOOC] week4: HIGHER ORDER FUNCTIONS
- [OCaml MOOC] week5: EXCEPTIONS, INPUT OUTPUT AND IMPERATIVE CONSTRUCTS
- [OCaml MOOC] week6: MODULES AND DATA ABSTRACTION

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