procedural abstraction

procedural-abstraction.ts

// data version

type DraftPost = {
  kind: 'DraftPost';
  content: string;
};

type ReviewingPost = {
  kind: 'ReviewingPost';
  content: string;
};

type RejectedPost = {
  kind: 'RejectedPost';
  content: string;
  reason: string;
};

type ApprovedPost = {
  kind: 'ApprovedPost';
  content: string;
  reason: string;
};

const createPost = (content: string = ''): DraftPost => {
  return {
    kind: 'DraftPost',
    content,
  };
};

const appendPostContent = (draftPost: DraftPost, content: string = '') => {
  return {
    ...draftPost,
    content: draftPost.content + content
  }
}

const reviewPost = (draft: DraftPost): ReviewingPost => {
  return {
    kind: 'ReviewingPost',
    content: draft.content,
  };
};

const approvePost = (
  reviewingPost: ReviewingPost,
  reason: string = ''
): ApprovedPost => {
  return {
    kind: 'ApprovedPost',
    content: reviewingPost.content,
    reason,
  };
};

const rejectPost = (
  reviewingPost: ReviewingPost,
  reason: string = ''
): RejectedPost => {
  return {
    kind: 'RejectedPost',
    content: reviewingPost.content,
    reason,
  };
};

const rejectToDraftPost = (rejectedPost: RejectedPost): DraftPost => {
  return {
    kind: 'DraftPost',
    content: rejectedPost.content,
  };
};


const test = () => {
  let draftPost = createPost('hello')
  let newDraftPost = appendPostContent(draftPost, ' world')
  let reviewingPost = reviewPost(newDraftPost)
  let rejectedPost = rejectPost(reviewingPost, 'too bad')
  let approvedPost = approvePost(reviewingPost, 'so good')


  // uncomment to see the type error
  // appendPostContent(reviewingPost, 'adsfasdf')
  // reviewPost(approvedPost)
  // approvePost(rejectedPost)

}

// codata version
type DraftPost = {
  content: string;
  review: () => ReviewingPost;
  append: (content: string) => DraftPost;
};

type ReviewingPost = {
  content: string;
  reject: (reason: string) => RejectedPost;
  approve: (reason: string) => ApprovedPost;
};

type RejectedPost = {
  content: string;
  reason: string;
  toDraft: () => DraftPost;
};

type ApprovedPost = {
  content: string;
  reason: string;
};

const createDraftPost = (content: string = ''): DraftPost => {
  return {
    content,
    review: () => createReviewingPost(content),
    append: (newContent = '') => createDraftPost(content + newContent),
  };
};

const createReviewingPost = (content: string = ''): ReviewingPost => {
  return {
    content,
    reject: (reason = '') => createRejectedPost(content, reason),
    approve: (reason = '') => createApprovedPost(content, reason),
  };
};

const createRejectedPost = (
  content: string = '',
  reason: string = ''
): RejectedPost => {
  return {
    content,
    reason,
    toDraft: () => createDraftPost(content),
  };
};

const createApprovedPost = (
  content: string = '',
  reason: string = ''
): ApprovedPost => {
  return {
    content,
    reason,
  };
};

const createPost = createDraftPost;

const test = () => {
  createPost('hello')
    .append(' ')
    .append('world')
    .review()
    .reject('too bad')
    .toDraft()
    .append(' some content')
    .review()
    .approve('so good');
};