inspired by: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

arrow_function_expression.swift

// poor arrow function expression implementation like javascript's using infix operator and closure

precedencegroup ArrowFunctionExpressionPrecedence {
  associativity: left
  higherThan: BitwiseShiftPrecedence
}

infix operator =>: ArrowFunctionExpressionPrecedence
infix operator ==>: ArrowFunctionExpressionPrecedence

func =><T, U>(lhs: T, rhs: (T) -> U) -> U {
  rhs(lhs)
}

func =><T: AnyObject>(lhs: T, rhs: (T) -> Void) -> T {
  rhs(lhs)
  return lhs
}

// to avoid "ambiguous use of operator" error
func ==><T>(lhs: T, rhs: (inout T) -> Void) -> T {
  var t = lhs
  rhs(&t)
  return t
}

// usage
2 => { $0 / 2 } => { $0 + 1 } // 2

let value = 1 ==> { $0 += 2 } // 3

// configure instance right after initialization
let label = UILabel() => { $0.text = "hello" }
label.text // hello

// keyPath
let materials = [
  "Hydrogen",
  "Helium",
  "Lithium",
  "Beryllium"
]
materials.map { $0 => \.count }
// is equivalent to
materials.map(\.count)