wongjiahau

// This language is based on the idea of json-like objects
// This is a comment

// Tagged union
// Like OCaml, the tag is not associated with the type name, in this case `nat`
// Essentially, `nat` is just a alias for the tagged union
type Nat = {#zero} | {#succ, of: nat}

// Variable & object initialization
people = {name='John', hobby='type theory'}

wongjiahau

const getArea = (shape: Shape) => {
  switch(shape.label) {
    case 'circle': return Math.PI * Math.pow(shape.radius, 2)
    case 'triangle': return 0.5 * shape.base * shape.height
    case 'rectangle': return shape.height * shape.width
  }
}

wongjiahau

const newShape2: Shape = {
  label: 'circle',
  width: 30 // You'll get a compiler error saying `width` does not exists when label is `circle`
}

wongjiahau

type Shape = {
  kind: 'circle'
  radius: number
} | {
  kind: 'rectangle'
  width: number
  height: number
} | {
  kind: 'triangle'
  base: number

wongjiahau

interface Shape {}
class Circle extends Shape {
  constructor(public radius: number) {}
}
class Rectangle extends Shape {
  construtor(public height: number, public width: number) {}
}
class Triangle extends Shape {
  constructor(public height: number, public base: number) {}
}

wongjiahau

interface ShapeV2 {
  label: 'rectangle' | 'circle' | 'triangle'
  radius ?: number // only for circle
  height ?: number // only for triangle and rectangle
  width  ?: number // only for rectangle
  base   ?: number // only for triangle
}

wongjiahau

const newShape1: Shape = { width: 30, base: 40 } // invalid combination of properties

const newShape2: Shape = { height: 40 } // missing base or width

wongjiahau

const getArea = (shape: Shape) => {
  if (shape.radius) {
    return Math.PI * Math.pow(shape.radius, 2)
  } 
  else if (shape.height && shape.width) {
    return shape.height * shape.width
  } 
  else if (shape.base && shape.height) {
    return 0.5 * shape.base * shape.height
  } 

wongjiahau

interface Shape {
  radius ?: number
  height ?: number
  width  ?: number
  base   ?: number
}

wongjiahau

\documentclass{article}

\begin{document}

\begin{table}[h!]
  \begin{center}
    \begin{tabular}{cl} 
      \\
      \(\frac
         {x \ : \ \sigma \ \in \ \Gamma}