F SharpF#

// 불변 바인딩 (let)
let name = "홍길동"
let add x y = x + y

// 타입 추론 + 파이프라인 연산자
let result =
  [1..10]
  |> List.filter (fun x -> x % 2 = 0)
  |> List.map (fun x -> x * x)
  |> List.sum
// result = 220

// 판별 합집합 (Discriminated Union)
type Shape =
  | Circle of radius: float
  | Rectangle of width: float * height: float

let area = function
  | Circle r     -> System.Math.PI * r * r
  | Rectangle(w,h) -> w * h

// 레코드 타입
type Person = {
  Name: string
  Age: int
}
let alice = { Name = "Alice"; Age = 30 }
let older = { alice with Age = 31 }  // 불변 업데이트

open System.Net.Http

let fetchAsync (url: string) = async {
  use client = new HttpClient()
  let! response = client.GetStringAsync(url) |> Async.AwaitTask
  return response.Length
}

// 병렬 비동기 실행
let urls = ["https://example.com"; "https://google.com"]

let lengths =
  urls
  |> List.map fetchAsync
  |> Async.Parallel
  |> Async.RunSynchronously

// Result 모나드
type Result<'T, 'E> = Ok of 'T | Error of 'E

let result = result {
  let! n1 = parseInt "42"    // 실패 시 즉시 Error 반환
  let! n2 = parseInt "10"
  return n1 + n2
}

// 시퀀스 (지연 평가)
let fibs = seq {
  let mutable a, b = 0, 1
  while true do
    yield a
    let next = a + b
    a <- b
    b <- next
}
Seq.take 10 fibs |> Seq.toList
// [0; 1; 1; 2; 3; 5; 8; 13; 21; 34]