ksaldana1

function premiumColor(role: USER_ROLE) {
  switch (role) {
    case "PREMIUM": {
      return "red";
    }
    case "WHALE": {
      return "blue";
    }
    case "FREE": {
      return "black";

ksaldana1

function assertNever(x: never): never {
  throw new Error("Unexpected object: " + x);
}

const Message: React.FC<MessageProps> = ({ message }) => {
  switch (message.author.__typename) {
    case "User": {
      return <div style={{color: premiumColor(message.author.role)}}>{`${message.author.name}: ${message.text}`}</div>;
    }
    case "Guest": {

ksaldana1

query AppQuery {
    messages {
      id
      text
      author {
        __typename
        ... on User {
          id
          name
          role

ksaldana1

type Query {
  messages: [Message!]!
}

type Message {
  id: ID!
  text: String!
  author: MessageAuthor!
}

ksaldana1

const Message: React.FC<MessageProps> = ({ message }) => {
  switch (message.author.__typename) {
    case "User": {
      return <div>{`${message.author.name}: ${message.text}`}</div>;
    }
    case "Guest": {
      return <div>{`${message.author.placeholder}: ${message.text}`}</div>;
    }
    case "%other": {
      return null;

ksaldana1

import React from "react";
import { useLazyLoadQuery } from "react-relay/hooks";
import { AppQuery as TAppQuery } from "./__generated__/AppQuery.graphql";
import { graphql } from "babel-plugin-relay/macro";

const query = graphql`
  query AppQuery {
    messages {
      id
      text

ksaldana1

type Query {
  messages: [Message!]!
}

type Message {
  id: ID!
  text: String!
  author: MessageAuthor!
}

ksaldana1

function assertNever(x: never): never {
    throw new Error("Unexpected object: " + x);
}
function area(s: Shape) {
    switch (s.kind) {
        case "square": return s.size * s.size;
        case "rectangle": return s.height * s.width;
        case "circle": return Math.PI * s.radius ** 2;
        default: return assertNever(s); // error here if there are missing cases
    }

ksaldana1

interface Square {
    kind: "square";
    size: number;
}

interface Rectangle {
    kind: "rectangle";
    width: number;
    height: number;
}

ksaldana1

// 1) Types that have a common, singleton type property — the discriminant.
// In this example the "kind" property is the discriminant.
interface Square {
    kind: "square";
    size: number;
}

interface Rectangle {
    kind: "rectangle";
    width: number;