Introduce result type and generic safe fetch functions. Do proper error handling/display.

src/App.tsx

@@ -1,5 +1,5 @@
 import { z } from "zod"
-import { useReducer, useRef, useEffect, createContext, useContext } from "react"
+import { useReducer, useRef, useEffect, createContext, useContext, useState } from "react"
 import {
   CssBaseline,
   Box,
@@ -12,12 +12,51 @@ import {
 
 // TODO: Improve error-handling. Introduce some proper server response.
 
+type Result<A, E> = { tag: "ok"; value: A } | { tag: "error"; error: E }
+
+const Result = {
+  ok<A>(value: A): Result<A, never> {
+    return { tag: "ok", value }
+  },
+  err<E>(error: E): Result<never, E> {
+    return { tag: "error", error }
+  },
+  map<A, B, E>(result: Result<A, E>, f: (x: A) => B): Result<B, E> {
+    switch (result.tag) {
+      case "ok":
+        return { tag: "ok", value: f(result.value) }
+      case "error":
+        return { tag: "error", error: result.error }
+    }
+  },
+}
+
+type RequestError =
+  | { tag: "networkError"; error: unknown }
+  | { tag: "httpError"; status: number; statusText: string }
+  | { tag: "jsonParseError"; error: unknown }
+  | { tag: "invalidResponse"; error: z.ZodError }
+
+const RequestError = {
+  toString(error: RequestError): string {
+    switch (error.tag) {
+      case "networkError":
+        return "Network error. Check your connection and try again."
+      case "httpError":
+        return `Server returned ${String(error.status)} ${error.statusText}.`
+      case "jsonParseError":
+        return "The server returned an invalid response."
+      case "invalidResponse":
+        return "The image data had an unexpected format."
+    }
+  },
+}
+
 // === Image ===
 type ImageId = string
 type ImageRef = string
 
 // === Dimension ===
-
 type Dimension = {
   width: number
   height: number
@@ -67,6 +106,69 @@ const Page = {
   },
 }
 
+// === Generic safe fetch functions ===
+async function fetchJsonSafe(url: string): Promise<Result<unknown, RequestError>> {
+  let response: Response
+  try {
+    response = await fetch(url)
+  } catch (error: unknown) {
+    return Result.err({ tag: "networkError", error })
+  }
+
+  if (!response.ok) {
+    return Result.err({
+      tag: "httpError",
+      status: response.status,
+      statusText: response.statusText,
+    })
+  } else {
+    try {
+      const json: unknown = await response.json()
+      return Result.ok(json)
+    } catch (error: unknown) {
+      return Result.err({ tag: "jsonParseError", error })
+    }
+  }
+}
+
+// Assumes `f` is some sort of a zod parsing function that may throw `z.ZodError`
+async function fetchJsonSafeWith<A>(
+  url: string,
+  f: (json: unknown) => A,
+): Promise<Result<A, RequestError>> {
+  const result = await fetchJsonSafe(url)
+  switch (result.tag) {
+    case "error":
+      return Result.err(result.error)
+    case "ok":
+      // TODO
+      try {
+        return Result.ok(f(result.value))
+      } catch (error: unknown) {
+        if (error instanceof z.ZodError) {
+          return Result.err({ tag: "invalidResponse", error })
+        } else {
+          throw error
+        }
+      }
+  }
+}
+
+// === Generic Remote Data ===
+type Remote<A, E> = { tag: "loading" } | Result<A, E>
+
+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: "ok", value }
+  },
+}
+
 // === api ===
 const picsumApiImageSchema = z.object({
   id: z.string(),
@@ -78,23 +180,19 @@ const picsumApiImageSchema = z.object({
 })
 type PicsumApiImage = z.infer<typeof picsumApiImageSchema>
 
-async function getPicsumImages({ page, limit }: Page): Promise<PicsumApiImage[]> {
+async function getPicsumImages({
-  const response = await fetch(
+  page,
+  limit,
+}: Page): Promise<Result<PicsumApiImage[], RequestError>> {
+  return fetchJsonSafeWith(
     `https://picsum.photos/v2/list?limit=${String(limit)}&page=${String(page)}`,
+    (json) => z.array(picsumApiImageSchema).parse(json),
   )
-  if (!response.ok) {
-    throw new Error(`Failed to fetch images: ${String(response.status)}`)
-  }
-
-  const json: unknown = await response.json()
-  const data = z.array(picsumApiImageSchema).parse(json)
-
-  return data
 }
 
-async function getImageIds(page: Page): Promise<ImageId[]> {
+async function getImageIds(page: Page): Promise<Result<ImageId[], RequestError>> {
-  const data = await getPicsumImages(page)
+  const result = await getPicsumImages(page)
-  return data.map(({ id }) => id)
+  return Result.map(result, (data) => data.map(({ id }) => id))
 }
 
 // Use this for `<img src=... />`
@@ -102,29 +200,15 @@ function getImageSource(id: ImageId, dimension: Dimension): ImageRef {
   return `https://picsum.photos/id/${id}/${String(dimension.width)}/${String(dimension.height)}`
 }
 
-// === Generic Remote Data ===
-type Remote<A, E> = { tag: "loading" } | { tag: "error"; error: E } | { tag: "loaded"; value: A }
-
-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 AppError = RequestError
 
 type State = {
   imageIds: Remote<ImageId[], AppError>
   page: Page
   selectedImage: ImageId | undefined
+  refreshSignal: boolean // This is here only because of the refresh button. It is an artificial way to trigger a reload without the current page changing. The value is meaningless. It just has to change.
 }
 
 const State = {
@@ -133,12 +217,14 @@ const State = {
       imageIds: Remote.loading(),
       page: Page.init(),
       selectedImage: undefined,
+      refreshSignal: false,
     }
   },
 }
 
 type Msg =
-  | { tag: "imagesReceived"; imageIds: ImageId[]; forPage: Page }
+  | { tag: "imagesReceived"; result: Result<ImageId[], AppError>; forPage: Page }
+  | { tag: "retryButtonClicked" }
   | { tag: "previousButtonClicked" }
   | { tag: "nextButtonClicked" }
   | { tag: "imageClicked"; imageId: ImageId }
@@ -151,36 +237,42 @@ function useApp(): [State, Dispatch] {
   // Could also cache `Promise<ImageId[]>` in case two requests are made really fast one after another (not really the case in this app so whatever)
   const cacheRef = useRef<Map<PageKey, ImageId[]>>(new Map())
 
-  async function getImageIdsCached(page: Page): Promise<ImageId[]> {
+  async function getImageIdsCached(page: Page): Promise<Result<ImageId[], AppError>> {
     const pageKey = Page.key(page)
     const maybeImages = cacheRef.current.get(pageKey)
     if (maybeImages === undefined) {
-      console.log("CACHE-MISS")
+      // console.log("CACHE-MISS")
-      const images = await getImageIds(page)
+      const result = await getImageIds(page)
-      cacheRef.current.set(pageKey, images)
+      if (result.tag === "ok") {
-      return images
+        cacheRef.current.set(pageKey, result.value)
+      }
+      return result
     } else {
-      console.log("CACHE-HIT")
+      // console.log("CACHE-HIT")
-      return maybeImages
+      return Result.ok(maybeImages)
     }
   }
 
   // === initialization & reloading ===
   useEffect(() => {
     // TODO: error-handling
-    getImageIdsCached(state.page).then((imageIds) => {
+    void getImageIdsCached(state.page).then((result) => {
-      dispatch({ tag: "imagesReceived", forPage: state.page, imageIds })
+      dispatch({ tag: "imagesReceived", forPage: state.page, result })
     })
-  }, [state.page]) // Would have been amazing if we could put `Page.key(state.page)` inside of this. Then the trouble with identity would be gone. But we can't, because React compiler and linter would complain /facepalm
+  }, [state.page, state.refreshSignal]) // Would have been amazing if we could put `Page.key(state.page)` inside of this. Then the trouble with identity would be gone. But we can't, because React compiler and linter would complain /facepalm
 
   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) }
+          return { ...state, imageIds: msg.result }
         } else {
           return state
         }
+      case "retryButtonClicked":
+        console.log("TODO")
+        // TODO: This actually needs to trigger the reload somehow /facepalm. But we can refresh only on page change.
+        return { ...state, imageIds: Remote.loading(), refreshSignal: !state.refreshSignal }
       case "previousButtonClicked": {
         const newPage = Page.previous(state.page)
         if (Page.eq(newPage, state.page)) {
@@ -193,11 +285,9 @@ function useApp(): [State, Dispatch] {
         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:
@@ -223,16 +313,51 @@ function useDispatch() {
 
 // === Views ===
 function RemoteImages({ images }: { images: Remote<ImageId[], AppError> }) {
+  const dispatch = useDispatch()
   switch (images.tag) {
     case "loading":
       return <ImagesSkeleton />
     case "error":
-      return <div>Error: TODO</div>
+      return (
-    case "loaded":
+        <Box sx={{ position: "relative" }}>
+          <ImagesSkeleton visible={false} />
+          <Box sx={{ position: "absolute", inset: 0, display: "grid", placeItems: "center" }}>
+            <ErrorView
+              error={images.error}
+              onRetry={() => {
+                dispatch({ tag: "retryButtonClicked" })
+              }}
+            />
+          </Box>
+        </Box>
+      )
+    case "ok":
       return <Images images={images.value} />
   }
 }
 
+function ErrorView({ error, onRetry }: { error: AppError; onRetry: () => void }) {
+  return (
+    <Box
+      sx={{
+        display: "flex",
+        flexDirection: "column",
+        alignItems: "center",
+        gap: 1,
+        textAlign: "center",
+      }}
+    >
+      <Typography variant="h6">Could not load images</Typography>
+      <Typography variant="body2" sx={{ color: "oklch(50% 0.02 var(--base-hue))" }}>
+        {RequestError.toString(error)}
+      </Typography>
+      <Button variant="outlined" onClick={onRetry}>
+        Retry
+      </Button>
+    </Box>
+  )
+}
+
 function imageGridStyle(dimension: Dimension) {
   return {
     display: "grid",
@@ -246,18 +371,29 @@ function imageGridStyle(dimension: Dimension) {
 }
 
 // 2x5 on mobile, 5x2 on desktop (assuming 10 images per page)
-function ImagesSkeleton() {
+function ImagesSkeleton({ visible = true }: { visible?: boolean }) {
   const images = Array.from({ length: 10 })
   return (
     <Box sx={imageGridStyle(Dimension.medium)}>
-      {images.map((_, index) => (
+      {images.map((_, index) =>
-        <Skeleton
+        visible ? (
-          key={index}
+          <Skeleton
-          variant="rectangular"
+            key={index}
-          width={Dimension.medium.width}
+            animation="pulse"
-          height={Dimension.medium.height}
+            variant="rectangular"
-        ></Skeleton>
+            width={Dimension.medium.width}
-      ))}
+            height={Dimension.medium.height}
+          />
+        ) : (
+          <Box
+            key={index}
+            sx={{
+              width: Dimension.medium.width,
+              height: Dimension.medium.height,
+            }}
+          />
+        ),
+      )}
     </Box>
   )
 }
@@ -267,26 +403,68 @@ function Images({ images }: { images: ImageId[] }) {
   return (
     <Box sx={imageGridStyle(Dimension.medium)}>
       {images.map((imageId) => (
-        <Box
+        <Image
-          component="img"
+          imageId={imageId}
-          src={getImageSource(imageId, Dimension.medium)}
+          dimension={Dimension.medium}
           onClick={() => {
             dispatch({ tag: "imageClicked", imageId })
           }}
-          sx={{
-            width: Dimension.medium.width,
-            height: Dimension.medium.height,
-            objectFit: "cover",
-            cursor: "pointer",
-          }}
           key={imageId}
-          alt=""
         />
       ))}
     </Box>
   )
 }
 
+function Image({
+  imageId,
+  dimension,
+  onClick,
+}: {
+  imageId: ImageId
+  dimension: Dimension
+  onClick?: () => void
+}) {
+  const [loaded, setLoaded] = useState(false)
+
+  return (
+    <Box
+      sx={{
+        position: "relative",
+        width: dimension.width,
+        height: dimension.height,
+      }}
+    >
+      {!loaded && (
+        <Skeleton
+          animation="pulse"
+          variant="rectangular"
+          width={dimension.width}
+          height={dimension.height}
+        />
+      )}
+      <Box
+        component="img"
+        src={getImageSource(imageId, dimension)}
+        onLoad={() => {
+          setLoaded(true)
+        }}
+        onClick={onClick}
+        sx={{
+          position: "absolute",
+          inset: 0,
+          width: dimension.width,
+          height: dimension.height,
+          objectFit: "cover",
+          cursor: onClick === undefined ? "default" : "pointer",
+          opacity: loaded ? 1 : 0,
+        }}
+        alt=""
+      />
+    </Box>
+  )
+}
+
 function ImageModal({ selectedImage }: { selectedImage: ImageId | undefined }) {
   const dispatch = useDispatch()
   return (
@@ -298,19 +476,7 @@ function ImageModal({ selectedImage }: { selectedImage: ImageId | undefined }) {
       maxWidth={false}
     >
       <DialogContent sx={{ p: 0, overflow: "hidden" }}>
-        {selectedImage !== undefined && (
+        {selectedImage !== undefined && <Image imageId={selectedImage} dimension={Dimension.big} />}
-          <Box
-            component="img"
-            src={getImageSource(selectedImage, Dimension.big)}
-            sx={{
-              display: "block",
-              maxWidth: "90vw",
-              maxHeight: "80vh",
-              objectFit: "contain",
-            }}
-            alt=""
-          />
-        )}
       </DialogContent>
     </Dialog>
   )