diff --git a/backend/src/api.ts b/backend/src/api.ts
index c0fcf50..9131634 100644
--- a/backend/src/api.ts
+++ b/backend/src/api.ts
@@ -35,6 +35,12 @@ export class Session {
     writer: rpc.StreamMessageWriter;
   } | null = null;
   daemon: { proc: ChildProcess } | null = null;
+  formatter: {
+    proc: ChildProcess;
+    live: boolean;
+    input: string;
+    output: string;
+  } | null = null;
 
   get homedir() {
     return `/tmp/riju/${this.uuid}`;
@@ -224,6 +230,13 @@ export class Session {
           }
           await this.runCode(msg.code);
           break;
+        case "formatCode":
+          if (typeof msg.code !== "string") {
+            this.logBadMessage(msg);
+            break;
+          }
+          await this.formatCode(msg.code);
+          break;
         case "lspInput":
           if (typeof msg.input !== "object" || !msg) {
             this.logBadMessage(msg);
@@ -257,12 +270,11 @@ export class Session {
         compile,
         run,
         template,
-        hacks,
       } = this.config;
       if (this.term) {
         const pid = this.term.pty.pid;
         const args = this.privilegedSpawn(
-          bash(`kill -SIGTERM ${pid}; sleep 3; kill -SIGKILL ${pid}`)
+          bash(`kill -SIGTERM ${pid}; sleep 1; kill -SIGKILL ${pid}`)
         );
         spawn(args[0], args.slice(1));
         // Signal to terminalOutput message generator using closure.
@@ -304,18 +316,6 @@ export class Session {
         ]),
         { input: code }
       );
-      if (hacks && hacks.includes("ghci-config") && run) {
-        if (code) {
-          await this.run(
-            this.privilegedSpawn(["sh", "-c", `cat > ${this.homedir}/.ghci`]),
-            { input: ":load Main\nmain\n" }
-          );
-        } else {
-          await this.run(
-            this.privilegedSpawn(["rm", "-f", `${this.homedir}/.ghci`])
-          );
-        }
-      }
       const termArgs = this.privilegedSpawn(bash(cmdline));
       const term = {
         pty: pty.spawn(termArgs[0], termArgs.slice(1), {
@@ -338,6 +338,66 @@ export class Session {
     }
   };
 
+  formatCode = async (code: string) => {
+    if (!this.config.format) {
+      this.log("formatCode ignored because format is null");
+      return;
+    }
+    if (this.formatter) {
+      const pid = this.formatter.proc.pid;
+      const args = this.privilegedSpawn(
+        bash(`kill -SIGTERM ${pid}; sleep 1; kill -SIGKILL ${pid}`)
+      );
+      spawn(args[0], args.slice(1));
+      this.formatter.live = false;
+      this.formatter = null;
+    }
+    const args = this.privilegedSpawn(bash(this.config.format));
+    const formatter = {
+      proc: spawn(args[0], args.slice(1)),
+      live: true,
+      input: code,
+      output: "",
+    };
+    formatter.proc.stdout!.on("data", (data) => {
+      if (formatter.live) {
+        formatter.output += data.toString("utf8");
+      }
+    });
+    formatter.proc.stderr!.on("data", (data) => {
+      if (formatter.live) {
+        this.send({
+          event: "serviceLog",
+          service: "formatter",
+          output: data.toString("utf8"),
+        });
+      }
+    });
+    formatter.proc.on("exit", (code, signal) => {
+      if (code === 0) {
+        this.send({
+          event: "formattedCode",
+          code: formatter.output,
+          originalCode: formatter.input,
+        });
+      } else {
+        this.send({
+          event: "serviceFailed",
+          service: "formatter",
+          error: `Exited with status ${signal || code}`,
+        });
+      }
+    });
+    formatter.proc.on("error", (err) =>
+      this.send({
+        event: "serviceFailed",
+        service: "formatter",
+        error: `${err}`,
+      })
+    );
+    this.formatter = formatter;
+  };
+
   teardown = async () => {
     try {
       if (this.tearingDown) {
diff --git a/backend/src/langs.ts b/backend/src/langs.ts
index 8d92728..e2a41c6 100644
--- a/backend/src/langs.ts
+++ b/backend/src/langs.ts
@@ -11,6 +11,7 @@ export interface LangConfig {
   createEmpty?: string;
   compile?: string;
   run: string;
+  format?: string;
   pkg?: {
     install: string;
     uninstall?: string;
@@ -24,7 +25,6 @@ export interface LangConfig {
   lspConfig?: any;
   lspLang?: string;
   template: string;
-  hacks?: "ghci-config"[];
   test?: {
     ensure?: string;
   };
@@ -764,9 +764,10 @@ function main(): void {
   haskell: {
     aliases: ["ghc", "ghci", "hs"],
     name: "Haskell",
-    repl: "ghci",
+    repl: "rm -f .ghci && ghci",
     main: "Main.hs",
-    run: "ghci",
+    run: "(echo ':load Main' && echo 'main') > .ghci && ghci",
+    format: "brittany Main.hs",
     lspSetup: "cp /opt/haskell/hie.yaml hie.yaml",
     lsp: "HIE_HOOGLE_DATABASE=/opt/haskell/hoogle.hoo hie --lsp",
     lspInit: {
@@ -1337,6 +1338,7 @@ main = do
     repl: "python3 -u",
     main: "main.py",
     run: "python3 -u -i main.py",
+    format: "cat main.py | black -",
     pkg: {
       install: "pip3 install --user NAME",
       uninstall: "pip3 uninstall NAME",
@@ -1350,7 +1352,7 @@ main = do
         },
       },
     },
-    template: `print("Hello, world!")
+    template: `print('Hello, world!')
 `,
   },
   قلب: {
diff --git a/frontend/pages/app.ejs b/frontend/pages/app.ejs
index bc2e09a..c6b046e 100644
--- a/frontend/pages/app.ejs
+++ b/frontend/pages/app.ejs
@@ -16,6 +16,7 @@
       <div id="editor" class="column"></div>
       <div id="terminal" class="column"></div>
       <button type="button" class="btn btn-success" id="runButton">Run</button>
+      <button type="button" class="btn btn-info" id="formatButton">Prettify</button>
       <a href="/" class="btn btn-secondary" id="backButton">Switch to a different language</a>
     </div>
     <script>
diff --git a/frontend/src/app.ts b/frontend/src/app.ts
index 6cb71bd..fca4d4a 100644
--- a/frontend/src/app.ts
+++ b/frontend/src/app.ts
@@ -25,6 +25,7 @@ interface RijuConfig {
   id: string;
   monacoLang?: string;
   main: string;
+  format?: string;
   lspDisableDynamicRegistration?: boolean;
   lspInit?: any;
   lspConfig?: any;
@@ -133,7 +134,7 @@ async function main() {
 
   function sendMessage(message: any) {
     if (DEBUG) {
-      console.log("SEND", message);
+      console.log("SEND:", message);
     }
     if (socket) {
       socket.send(JSON.stringify(message));
@@ -165,8 +166,7 @@ async function main() {
         DEBUG &&
         message &&
         message.event !== "lspOutput" &&
-        message.event !== "lspLog" &&
-        message.event !== "daemonLog"
+        message.event !== "serviceLog"
       ) {
         console.log("RECEIVE:", message);
       }
@@ -184,6 +184,18 @@ async function main() {
           }
           term.write(message.output);
           return;
+        case "formattedCode":
+          if (
+            typeof message.code !== "string" ||
+            typeof message.originalCode !== "string"
+          ) {
+            console.error("Unexpected message from server:", message);
+            return;
+          }
+          if (editor.getValue() === message.originalCode) {
+            editor.setValue(message.code);
+          }
+          return;
         case "lspStarted":
           if (typeof message.root !== "string") {
             console.error("Unexpected message from server:", message);
@@ -308,6 +320,12 @@ async function main() {
   document.getElementById("runButton")!.addEventListener("click", () => {
     sendMessage({ event: "runCode", code: editor.getValue() });
   });
+  if (config.format) {
+    document.getElementById("formatButton")!.classList.add("visible");
+    document.getElementById("formatButton")!.addEventListener("click", () => {
+      sendMessage({ event: "formatCode", code: editor.getValue() });
+    });
+  }
 }
 
 main().catch(console.error);
diff --git a/frontend/styles/app.css b/frontend/styles/app.css
index 6adf8e0..6d5efc5 100644
--- a/frontend/styles/app.css
+++ b/frontend/styles/app.css
@@ -27,6 +27,17 @@ body {
   right: calc(50% + 25px);
 }
 
+#formatButton {
+  position: absolute;
+  bottom: 25px;
+  right: calc(50% + 25px);
+  visibility: hidden;
+}
+
+#formatButton.visible {
+  visibility: visible;
+}
+
 #backButton {
   position: absolute;
   left: 25px;
diff --git a/scripts/docker-install-phase4.bash b/scripts/docker-install-phase4.bash
index 852b92e..acb1c8b 100755
--- a/scripts/docker-install-phase4.bash
+++ b/scripts/docker-install-phase4.bash
@@ -82,6 +82,9 @@ npm install -g pug-cli
 # PureScript
 npm install -g purescript spago
 
+# Python
+pip3 install black
+
 # ReasonML
 npm install -g bs-platform
 
diff --git a/scripts/docker-install-phase5.bash b/scripts/docker-install-phase5.bash
index 5baf9ad..05cf4a5 100755
--- a/scripts/docker-install-phase5.bash
+++ b/scripts/docker-install-phase5.bash
@@ -121,6 +121,9 @@ tar -xf linux-x86_64-static.tar.gz
 mv stack-*-linux-x86_64-static/stack /usr/bin/stack
 rm -rf stack-*-linux-x86_64-static linux-x86_64-static.tar.gz
 
+wget "https://drive.google.com/uc?export=download&id=1MpozlNLmWeUaQuT-5t6gyE3Yv56gUbea" -O /usr/local/bin/brittany
+chmod +x /usr/local/bin/brittany
+
 mkdir -p /opt/haskell
 gdown "https://drive.google.com/uc?export=download&id=1GPoR_ja4ns16KCamRgwB-JVag4HK0igz" /usr/bin/hie
 gdown "https://drive.google.com/uc?export=download&id=1qSxj8JjAeetAmNjUGayX0RBARgr5R4Ij" /opt/haskell/hoogle.hoo