thp-web/md/learn/ideas/idea_1.md

Idea 1

var x = 20 val y = 30

type Something = ...

Something s1 = ... Something s2 = s1

// Passes some by reference, but it's immutable. Cannot call mutable methods // or use it in mutable operations fun do_something(Something some) -> Bool {} do_something(s1)

// Passes some by reference, and it's mutable. Can call mutable methods // or use it in mutable operations fun do_something(&Something some) -> Bool {} do_something(&s1)

var arr1 = Array(10, 20, 30) var arr2 = &arr1

        Owned/Reference     Mutable

Type Owned n &Type Reference n mut Type Owned y &mut Type Reference y

        Copy/Reference      Mutable     Equivalent

Some Copy n 1 (technically) references the other data &Some Reference n 1 References the other data mut Some Copy y 2 Creates a mutable copy &mut Some Reference y 3 References the other data, mutable

Array[A]::map

fun map[B](this, (A) -> B callback) -> Array[B]

Applies callback to all the elements of this array, and returns those new values in a new array.

Example

val numbers = Array(1, 2, 3, 4, 5)

val numbers_squared = numbers.map {it ** 2}

print(numbers_squared)  // Array(1, 4, 9, 16, 25)

numbers.map(fun(v) {
    v - 2
})

Array[A]::reduce

fun reduce[B](
    this,
    B initial,
    (A previous, B current) -> B callback,
) -> B

Iteratively reduce the array to a single value using callback.

Example

val numbers = Array(1, 2, 3, 4, 5)

val sum = numbers.reduce(0, \+)
val sum = numbers.reduce(0) {$1 + $2}
val sum = numbers.reduce(0, fun(prev, curr) {prev + curr})

print(sum)  // 15

val numbers = Array(1, 2, 3, 4, 5)

val sum = numbers.reduce("", fun(prev, curr) {prev + curr})

val sum = numbers.reduce("") {prev, curr -> prev + curr}

print(sum)  // "12345"

// Functor

fun fmap(
    (A) -> B,
    f[A],
) -> f[B]

fun (<$)(
    A,
    f[B],
) -> f[A]


// Applicative

fun pure(A) -> f[A]

fun (<*>)(
    f[A -> B],
    f[A],
) -> f[B]

fun (*>)(
    f[_],
    f[B],
) -> f[B]

fun (<*)(
    f[A],
    f[_],
) -> f[A]


// Monad

fun (>>=)[m, A, B](
    m[A],
    (A) -> m[B],
) -> m[B]


(Array[Int], Int -> Array[String]) -> Array[String]

val result = Array(1, 2, 3, 4, 5) >>= {Array($.into[String]())}

print(result)   // Array("1", "2", "3", "4", "5")

Option[Int] result = "322".try_into()
Option[Int] result_halved = result >>= {Some($ / 2)}

print(result_halved)    // Some(161)


Option[Int] result = "abc".try_into()
Option[Int] result_halved = result >>= {Some($ / 2)}

print(result_halved)    // None

fun (<$>)[m, A, B](
    (A) -> B,
    m[A],
) -> m[B]


fun half(Int x) -> Int {
    x / 2
}


Option[Int] result = "322".try_into()
Option[Int] result_halved = result <$> half

print(result_halved)    // Some(161)


Option[Int] result = "abc".try_into()
Option[Int] result_halved = result <$> half

print(result_halved)    // None

fun (>>)[A, B, C](
    (A) -> B,
    (B) -> C,
) -> (A) -> C

val f1 = add1 >> times2

f1(5)   // 12