diff --git a/.prettierrc.json b/.prettierrc.json
index 47174e4..f313bb2 100644
--- a/.prettierrc.json
+++ b/.prettierrc.json
@@ -1,6 +1,6 @@
 {
-  "semi": true,
-  "singleQuote": true,
+  "semi": false,
+  "singleQuote": false,
   "trailingComma": "all",
   "printWidth": 100
 }
diff --git a/eslint.config.js b/eslint.config.js
index 0162f79..e50a55f 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -1,16 +1,16 @@
-import js from '@eslint/js';
-import prettierConfig from 'eslint-config-prettier';
-import reactHooks from 'eslint-plugin-react-hooks';
-import reactRefresh from 'eslint-plugin-react-refresh';
-import globals from 'globals';
-import tseslint from 'typescript-eslint';
+import js from "@eslint/js"
+import prettierConfig from "eslint-config-prettier"
+import reactHooks from "eslint-plugin-react-hooks"
+import reactRefresh from "eslint-plugin-react-refresh"
+import globals from "globals"
+import tseslint from "typescript-eslint"
 
 export default tseslint.config(
   {
-    ignores: ['dist', 'node_modules'],
+    ignores: ["dist", "node_modules"],
   },
   {
-    files: ['src/**/*.{ts,tsx}'],
+    files: ["src/**/*.{ts,tsx}"],
     extends: [
       js.configs.recommended,
       ...tseslint.configs.strictTypeChecked,
@@ -25,32 +25,32 @@ export default tseslint.config(
       },
     },
     plugins: {
-      'react-hooks': reactHooks,
-      'react-refresh': reactRefresh,
+      "react-hooks": reactHooks,
+      "react-refresh": reactRefresh,
     },
     rules: {
       ...reactHooks.configs.recommended.rules,
-      '@typescript-eslint/consistent-type-imports': [
-        'error',
-        { prefer: 'type-imports', fixStyle: 'inline-type-imports' },
+      "@typescript-eslint/consistent-type-imports": [
+        "error",
+        { prefer: "type-imports", fixStyle: "inline-type-imports" },
       ],
-      '@typescript-eslint/no-unused-vars': [
-        'error',
+      "@typescript-eslint/no-unused-vars": [
+        "error",
         {
-          argsIgnorePattern: '^_',
-          varsIgnorePattern: '^_',
-          caughtErrorsIgnorePattern: '^_',
+          argsIgnorePattern: "^_",
+          varsIgnorePattern: "^_",
+          caughtErrorsIgnorePattern: "^_",
         },
       ],
-      '@typescript-eslint/switch-exhaustiveness-check': 'error',
-      '@typescript-eslint/no-floating-promises': 'error',
-      '@typescript-eslint/no-misused-promises': 'error',
-      '@typescript-eslint/no-unnecessary-condition': 'error',
-      '@typescript-eslint/prefer-nullish-coalescing': 'error',
-      '@typescript-eslint/prefer-optional-chain': 'error',
-      'default-case': 'off',
-      'react-refresh/only-export-components': ['warn', { allowConstantExport: true }],
+      "@typescript-eslint/switch-exhaustiveness-check": "error",
+      "@typescript-eslint/no-floating-promises": "error",
+      "@typescript-eslint/no-misused-promises": "error",
+      "@typescript-eslint/no-unnecessary-condition": "error",
+      "@typescript-eslint/prefer-nullish-coalescing": "error",
+      "@typescript-eslint/prefer-optional-chain": "error",
+      "default-case": "off",
+      "react-refresh/only-export-components": ["warn", { allowConstantExport: true }],
     },
   },
   prettierConfig,
-);
+)
diff --git a/src/App.tsx b/src/App.tsx
index 0e818e0..a030ba7 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -1,31 +1,61 @@
-import { z } from 'zod'
+import { z } from "zod"
+import { useReducer, useEffect, createContext, useContext } from "react"
+
 // TODO: Use Material UI later.
 // TODO: Improve error-handling. Introduce some proper server response.
 
+// For better type-error messages when you have inexhaustive switch cases.
+function assertNever(value: never): never {
+  throw new Error(`Unexpected value: ${value}`)
+}
+
+// === Image ===
 type ImageId = string
 type ImageRef = string
- 
+
 type Image = {
-  id: ImageId,
-  dimension: Dimension,
-  source: ImageRef, 
+  id: ImageId
+  dimension: Dimension
+  source: ImageRef
 }
 
+// === Dimension ===
+
 type Dimension = {
-  width: number,
-  height: number,
+  width: number
+  height: number
+}
+const Dimension = {
+  medium: { width: 200, height: 200 } as Dimension,
+  big: { width: 300, height: 500 } as Dimension,
 }
 
-// WARNING: page starts at 1
-type Pagination = {
-  page: number,
-  limit: number, // page size
+// === Page ===
+type Page = {
+  page: number
+  limit: number // page size
 }
 
-const Pagination = {
-  init(): Pagination { return { page: 1, limit: 10 } },
-}
+const LIMIT = 10
+const FIRST_PAGE = 1
 
+const Page = {
+  init(): Page {
+    return { page: FIRST_PAGE, limit: LIMIT }
+  },
+  next(page: Page): Page {
+    return { ...page, page: page.page + 1 }
+  },
+  previous(page: Page): Page {
+    // Could do
+    // if (page.page == FIRST_PAGE) { return page }
+    // this preserves identity of object, so is nicer for `useEffect`, but it's waaaay to subtle. So I'm not relying on that.
+    return { ...page, page: Math.max(FIRST_PAGE, page.page - 1) }
+  },
+  eq(page0: Page, page1: Page): boolean {
+    return page0.page === page1.page && page0.limit === page1.limit
+  },
+}
 
 // === api ===
 const picsumApiImageSchema = z.object({
@@ -38,7 +68,7 @@ const picsumApiImageSchema = z.object({
 })
 type PicsumApiImage = z.infer<typeof picsumApiImageSchema>
 
-async function getPicsumImages({ page, limit }: Pagination): Promise<PicsumApiImage[]> {
+async function getPicsumImages({ page, limit }: Page): Promise<PicsumApiImage[]> {
   const response = await fetch(`https://picsum.photos/v2/list?limit=${limit}&page=${page}`)
   if (!response.ok) {
     throw new Error(`Failed to fetch images: ${response.status}`)
@@ -50,29 +80,170 @@ async function getPicsumImages({ page, limit }: Pagination): Promise<PicsumApiIm
   return data
 }
 
-async function getImages(pagination: Pagination): Promise<Image[]> {
-  const data = await getPicsumImages(pagination)
-  return data.map(({ id, download_url, width, height }) => ({ id: id as ImageId, dimension: { width, height }, source: download_url as ImageRef }))
+// TODO: We don't really need this.
+async function getImages(page: Page): Promise<Image[]> {
+  const data = await getPicsumImages(page)
+  return data.map(({ id, download_url, width, height }) => ({
+    id: id as ImageId,
+    dimension: { width, height },
+    source: download_url as ImageRef,
+  }))
 }
 
-async function getImageIds(pagination: Pagination): Promise<ImageId[]> {
-  const data = await getPicsumImages(pagination)
+async function getImageIds(page: Page): Promise<ImageId[]> {
+  const data = await getPicsumImages(page)
   return data.map(({ id }) => id)
 }
 
 // Use this for `<img src=... />`
 function getImageSource(id: ImageId, dimension: Dimension): ImageRef {
-  return `https://picsum.photos/id/${id}/${dimension.width}/${dimension.height}`;
+  return `https://picsum.photos/id/${id}/${dimension.width}/${dimension.height}`
 }
 
-// test
-// (async () => {
-//   const imgs = await getImages({ page: 2, limit: 10 })
-//   console.log(imgs)
-// })()
+// === Generic Remote Data ===
+type Remote<A, E> = { tag: "loading" } | { tag: "error"; error: E } | { tag: "loaded"; value: A }
 
-export default function App() {
+const Remote = {
+  loading(): Remote<never, never> {
+    return { tag: "loading" }
+  },
+  error<E>(error: E): Remote<never, E> {
+    return { tag: "error", error }
+  },
+  loaded<A>(value: A): Remote<A, never> {
+    return { tag: "loaded", value }
+  },
+}
+
+// === App ===
+// TODO: introduce proper error type
+type AppError = string
+
+type State = {
+  imageIds: Remote<ImageId[], AppError>
+  page: Page
+  selectedImage: ImageId | undefined
+}
+
+const State = {
+  init(): State {
+    return {
+      imageIds: Remote.loading(),
+      page: Page.init(),
+      selectedImage: undefined,
+    }
+  },
+}
+
+type Msg =
+  | { tag: "imagesReceived"; imageIds: ImageId[]; forPage: Page }
+  | { tag: "previousButtonClicked" }
+  | { tag: "nextButtonClicked" }
+  | { tag: "imageClicked"; imageId: ImageId }
+  | { tag: "modalCloseButtonClicked" }
+
+function useApp(): [State, Dispatch] {
+  const [state, dispatch] = useReducer(update, State.init())
+
+  // === initialization & reloading ===
+  useEffect(() => {
+    // TODO: error-handling
+    getImageIds(state.page).then((imageIds) => {
+      dispatch({ tag: "imagesReceived", forPage: state.page, imageIds })
+    })
+  }, [state.page])
+
+  function update(state: State, msg: Msg): State {
+    switch (msg.tag) {
+      case "imagesReceived":
+        if (Page.eq(state.page, msg.forPage)) {
+          return { ...state, imageIds: Remote.loaded(msg.imageIds) }
+        } else {
+          return state
+        }
+      case "previousButtonClicked": {
+        console.log("prev")
+        const newPage = Page.previous(state.page)
+        if (Page.eq(newPage, state.page)) {
+          return state // preserves identity
+        } else {
+          return { ...state, page: newPage, imageIds: Remote.loading() }
+        }
+      }
+      case "nextButtonClicked": {
+        console.log("next")
+        return { ...state, page: Page.next(state.page), imageIds: Remote.loading() }
+      }
+      case "imageClicked": {
+        console.log(msg.imageId)
+        return { ...state, selectedImage: msg.imageId }
+      }
+      case "modalCloseButtonClicked": {
+        console.log("modal closed")
+        return { ...state, selectedImage: undefined }
+      }
+      default:
+        return assertNever(msg)
+    }
+  }
+
+  return [state, dispatch]
+}
+
+type Dispatch = (msg: Msg) => void
+const DispatchContext = createContext<Dispatch | null>(null)
+
+function useDispatch() {
+  const dispatch = useContext(DispatchContext)
+
+  if (dispatch === null) {
+    throw new Error("useDispatch must be used inside DispatchContext.Provider")
+  }
+
+  return dispatch
+}
+
+// === Views ===
+function RemoteImages({ images }: { images: Remote<ImageId[], AppError> }) {
+  switch (images.tag) {
+    case "loading":
+      return <div>Loading...</div>
+    case "error":
+      return <div>Error: TODO</div>
+    case "loaded":
+      return <Images images={images.value} />
+  }
+}
+
+function Images({ images }: { images: ImageId[] }) {
+  const dispatch = useDispatch()
+  // TODO: Is there some basic `Col/Row` component?
   return (
-    <h1>hello world</h1>
+    <div style={{ display: "flex", flexDirection: "column" }}>
+      {images.map((imageId) => (
+        <img
+          style={{ width: Dimension.medium.width, height: Dimension.medium.height }}
+          key={imageId}
+          src={getImageSource(imageId, Dimension.medium)}
+          onClick={() => dispatch({ tag: "imageClicked", imageId })}
+          alt=""
+        />
+      ))}
+    </div>
+  )
+}
+
+export default function App() {
+  const [state, dispatch] = useApp()
+
+  return (
+    <DispatchContext.Provider value={dispatch}>
+      <main>
+        <h1>Picturarium</h1>
+        <RemoteImages images={state.imageIds} />
+        <button onClick={() => dispatch({ tag: "previousButtonClicked" })}>prev</button>
+        <button onClick={() => dispatch({ tag: "nextButtonClicked" })}>next</button>
+      </main>
+    </DispatchContext.Provider>
   )
 }
diff --git a/src/main.tsx b/src/main.tsx
index 5784f84..57d77e4 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -1,16 +1,16 @@
-import { StrictMode } from 'react';
-import { createRoot } from 'react-dom/client';
-import App from './App';
-import './index.css';
+import { StrictMode } from "react"
+import { createRoot } from "react-dom/client"
+import App from "./App"
+import "./index.css"
 
-const rootElement = document.getElementById('root');
+const rootElement = document.getElementById("root")
 
 if (rootElement === null) {
-  throw new Error('Root element #root was not found.');
+  throw new Error("Root element #root was not found.")
 }
 
 createRoot(rootElement).render(
   <StrictMode>
     <App />
   </StrictMode>,
-);
+)
diff --git a/vite.config.ts b/vite.config.ts
index fabde1a..322f82f 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -1,6 +1,6 @@
-import react from '@vitejs/plugin-react';
-import { defineConfig } from 'vite';
+import react from "@vitejs/plugin-react"
+import { defineConfig } from "vite"
 
 export default defineConfig({
   plugins: [react()],
-});
+})