Exercises

Each lesson's key functions with the bodies removed and the tests left in. Read the post, then make the tests pass.

The exercise copies live in clojure/ex/exercises/lessonNN and go/exercises/lessonNN; each blank is a TODO that throws, with a hint above it. At a REPL: clj -A:ex, then (require 'exercises.lessonNN.ns-test :reload) and (clojure.test/run-tests 'exercises.lessonNN.ns-test). The reference lessons are the answer key.

Part 1 · HTML over the wire

cd clojure && clojure -M:ex -d ex -n exercises.lesson01.<ns>-test
cd go && go test ./exercises/lesson01/

• sse-event Clojure clojure/ex/exercises/lesson01/hypermedia.clj
  Return the text of one SSE event: "event: <type>", then "data: <line>" for each data line, each line ended by a newline, and one empty line at the end.
• hello-response Clojure clojure/ex/exercises/lesson01/hypermedia.clj
  Return a Ring response map: status 200, Content-Type text/event-stream, Cache-Control no-cache, and a body that is one datastar-patch-elements event whose single data line is "elements " followed by the greeting HTML.

Part 2 · The event stream

cd clojure && clojure -M:ex -d ex -n exercises.lesson02.<ns>-test
cd go && go test ./exercises/lesson02/

• send-event Clojure clojure/ex/exercises/lesson02/event_stream.clj
  Return the event text in this order: "event: <type>", then "id: <id>" when id is given, then "retry: <ms>" when retry is given and is not 1000, then "data: <line>" for each data line, then one empty line. Every line ends with "\n".
• patch-elements Clojure clojure/ex/exercises/lesson02/event_stream.clj
  Build the data lines, leaving out defaults: "selector …" if given, "mode …" unless outer, "useViewTransition true" if true, "viewTransitionSelector …" if given with a view transition, "namespace …" unless html, then "elements <line>" for each line of the HTML (none when html is nil). Send them with send-event.
• patch-signals Clojure clojure/ex/exercises/lesson02/event_stream.clj
  Encode signals as JSON unless it is already a string. Send "onlyIfMissing true" first when that option is true, then "signals <line>" for each line of the JSON.
• execute-script Clojure clojure/ex/exercises/lesson02/event_stream.clj
  Make a <script> tag with each attribute as name="value", plus data-effect="el.remove()" when auto-remove is on. Send it as a patch-elements event that appends to the selector body.

Part 3 · Signals

cd clojure && clojure -M:ex -d ex -n exercises.lesson03.<ns>-test
cd go && go test ./exercises/lesson03/

• price-cents Clojure clojure/ex/exercises/lesson03/signals.clj
  Add the drink's base price, the size delta, and the milk delta (no milk adds nothing).
• read-signals Clojure clojure/ex/exercises/lesson03/signals.clj
  Take the JSON text from the "datastar" query parameter for GET and DELETE, and from the request body for other methods. Parse it with keyword keys. Return {} when there is nothing to read, when the text is not valid JSON, or when the JSON is not an object.
• quote-response Clojure clojure/ex/exercises/lesson03/signals.clj
  Make a draft from the signals. If draft-error finds a problem, render [:p#quote.error message]. Otherwise render [:p#quote "Server price: " [:strong total]] where total is price × qty in dollars. Return a one-shot SSE response with one patch-elements event.

Part 4 · When to use a signal

cd clojure && clojure -M:ex -d ex -n exercises.lesson04.<ns>-test
cd go && go test ./exercises/lesson04/

• drink-options-view Clojure clojure/ex/exercises/lesson04/where_state_lives.clj
  Return hiccup for div#drink-options. It holds select#size (data-bind:size) with one option per size of the drink, and select#milk (data-bind:milk) with one option per milk, or [:p.muted "No milk"] when the drink takes no milk. Mark the chosen size and milk as selected. A sold-out milk is disabled and its label ends with " (sold out)".
• drink-options-response Clojure clojure/ex/exercises/lesson04/where_state_lives.clj
  Read drink, size and milk from the signals. Find the valid choice. Send two events: first patch #drink-options with the view in replace mode, then patch only the signals that changed (size, milk). A missing milk is sent as "" so the bound select keeps a string.
• add-to-cart Clojure clojure/ex/exercises/lesson04/where_state_lives.clj
  Return nil when draft-error finds a problem (use the state's sold-out set). Otherwise add the line {:drink :size :milk :qty} to the end of the cart for sid and return the new state.

Part 5 · Shaping the backend

cd clojure && clojure -M:ex -d ex -n exercises.lesson05.<ns>-test
cd go && go test ./exercises/lesson05/

• handle-command Clojure clojure/ex/exercises/lesson05/routes.clj
  Return the next state for each command type: :add-to-cart (append line to sid's cart when line-error is nil), :remove-from-cart (drop the line at idx), :place-order (turn a non-empty cart into a :placed order for a non-blank customer, then empty the cart), :advance (move an order one step along next-status; an order that reaches :picked-up leaves :orders), :toggle-stock (add or remove milk from :sold-out).
• command-response Clojure clojure/ex/exercises/lesson05/routes.clj
  Return a one-shot SSE response that patches exactly the regions this command type changed (see the docstring), rendered from the new state.
• wrap-datastar-request Clojure clojure/ex/exercises/lesson05/routes.clj
  Return a handler that calls `handler` only when the request header datastar-request is "true", and answers 403 otherwise.

Part 6 · One stream, many viewers

cd clojure && clojure -M:ex -d ex -n exercises.lesson06.<ns>-test
cd go && go test ./exercises/lesson06/

• view-for Clojure clojure/ex/exercises/lesson06/live.clj
  Return the hiccup for <main id="app"> for this viewer: a customer tab sees its own draft and error, its browser's cart and only its browser's orders; the board sees every order and the milk stock. (customer-view and board-view are already written.)
• broadcast Clojure clojure/ex/exercises/lesson06/live.clj
  Lock the hub, then read the current state from !state. For every subscriber, render (view-for state viewer) to an HTML string. If it differs from the subscriber's :last, call its send-fn with a patch-elements event of that HTML and record it as :last, but only if the subscriber is still in the hub: its stream may close mid-broadcast. If one subscriber throws, print the error to *err* and skip it. Return the ids that were sent to.
• stream-lifecycle Clojure clojure/ex/exercises/lesson06/live.clj
  Make a fresh id for this stream. On open: for a viewer with a :tabid, run an :open-tab command (sid, tabid, this stream's id, the draft); subscribe send-fn with the viewer (without :draft); send it the whole current view right away (a reconnect needs it too). On close: unsubscribe, then run :close-tab for that tab and this stream.

Part 7 · Replacing the React Native app (optional)

cd clojure && clojure -M:ex -d ex -n exercises.lesson07.<ns>-test
cd go && go test ./exercises/lesson07/

• shell-request Clojure clojure/ex/exercises/lesson07/native_shell.clj
  The shell appends `BrewShell/1` to its User-Agent. Return true when the request's user-agent header contains it, false otherwise (including no header).
• bridge-script Clojure clojure/ex/exercises/lesson07/native_shell.clj
  Return an execute-script event whose script calls window.BrewShell.post with msg as a JSON object literal. The JSON must be safe inside a <script> tag: write every < as \u003c, so text such as "</script>" or "<!--<script>" in a customer's name cannot end or confuse the tag.
• ready-notifications Clojure clojure/ex/exercises/lesson07/native_shell.clj
  Return one {:sid … :message …} per order that is :ready in new but was not :ready in old, in order-id order. The message reads "Ada, your order #1 is ready."