Add formatted error pages for console and browser

Errors now show app frames highlighted with relative paths and library
frames collapsed, both in ANSI on the console and as a styled HTML page
in the browser. Process-level uncaughtException/unhandledRejection
handlers also use the formatter.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app.ts

@@ -1,171 +1,30 @@
-import express, {
+import{cli}from'./diachron/cli'
-    type Request as ExpressRequest,
+import { formatError } from './diachron/errors';
-    type Response as ExpressResponse,
+
-} from "express";
+process.on('uncaughtException', (err) => {
-import { match } from "path-to-regexp";
+    console.error(formatError(err));
-import { Session } from "./diachron/auth";
+    process.exit(1);
-import { cli } from "./diachron/cli";
+});
-import { contentTypes } from "./diachron/content-types";
+
-import { runWithContext } from "./diachron/context";
+process.on('unhandledRejection', (reason) => {
+    console.error(formatError(reason));
+});
+
 import { core } from "./diachron/core";
-import { httpCodes } from "./diachron/http-codes";
+
-import { request } from "./diachron/request";
-// import { URLPattern } from 'node:url';
-import {
-    AuthenticationRequired,
-    AuthorizationDenied,
-    type Call,
-    type InternalHandler,
-    isRedirect,
-    type Method,
-    massageMethod,
-    methodParser,
-    type ProcessedRoute,
-    type Result,
-    type Route,
-} from "./diachron/types";
 import { routes } from "./routes";
+import {makeApp}from'./diachron/app'
 
-const app = express();
 
-// Parse request bodies
+const app = makeApp({routes});
-app.use(express.json());
+
-app.use(express.urlencoded({ extended: true }));
 
 core.logging.log({ source: "logging", text: ["1"] });
-const processedRoutes: { [K in Method]: ProcessedRoute[] } = {
-    GET: [],
-    POST: [],
-    PUT: [],
-    PATCH: [],
-    DELETE: [],
-};
 
-function _isPromise<T>(value: T | Promise<T>): value is Promise<T> {
-    return typeof (value as any)?.then === "function";
-}
 
-routes.forEach((route: Route, _idx: number, _allRoutes: Route[]) => {
-    // const pattern /*: URLPattern */ = new URLPattern({ pathname: route.path });
-    const matcher = match<Record<string, string>>(route.path);
-    const methodList = route.methods;
 
-    const handler: InternalHandler = async (
-        expressRequest: ExpressRequest,
-    ): Promise<Result> => {
-        const method = massageMethod(expressRequest.method);
 
-        console.log("method", method);
 
-        if (!methodList.includes(method)) {
-            // XXX: Worth asserting this?
-        }
-
-        console.log("request.originalUrl", expressRequest.originalUrl);
-
-        // Authenticate the request
-        const auth = await request.auth.validateRequest(expressRequest);
-
-        const req: Call = {
-            pattern: route.path,
-            path: expressRequest.originalUrl,
-            method,
-            parameters: { one: 1, two: 2 },
-            request: expressRequest,
-            user: auth.user,
-            session: new Session(auth.session, auth.user),
-        };
-
-        try {
-            const retval = await runWithContext({ user: auth.user }, () =>
-                route.handler(req),
-            );
-            return retval;
-        } catch (error) {
-            // Handle authentication errors
-            if (error instanceof AuthenticationRequired) {
-                return {
-                    code: httpCodes.clientErrors.Unauthorized,
-                    contentType: contentTypes.application.json,
-                    result: JSON.stringify({
-                        error: "Authentication required",
-                    }),
-                };
-            }
-            if (error instanceof AuthorizationDenied) {
-                return {
-                    code: httpCodes.clientErrors.Forbidden,
-                    contentType: contentTypes.application.json,
-                    result: JSON.stringify({ error: "Access denied" }),
-                };
-            }
-            throw error;
-        }
-    };
-
-    for (const [_idx, method] of methodList.entries()) {
-        const pr: ProcessedRoute = { matcher, method, handler };
-
-        processedRoutes[method].push(pr);
-    }
-});
-
-async function handler(
-    req: ExpressRequest,
-    _res: ExpressResponse,
-): Promise<Result> {
-    const method = await methodParser.parseAsync(req.method);
-
-    const byMethod = processedRoutes[method];
-    console.log(
-        "DEBUG: req.path =",
-        JSON.stringify(req.path),
-        "method =",
-        method,
-    );
-    for (const [_idx, pr] of byMethod.entries()) {
-        const match = pr.matcher(req.path);
-        console.log("DEBUG: trying pattern, match result =", match);
-        if (match) {
-            console.log("match", match);
-            const resp = await pr.handler(req);
-
-            return resp;
-        }
-    }
-
-    const retval: Result = {
-        code: httpCodes.clientErrors.NotFound,
-        contentType: contentTypes.text.plain,
-        result: "not found!",
-    };
-
-    return retval;
-}
-
-app.use(async (req: ExpressRequest, res: ExpressResponse) => {
-    const result0 = await handler(req, res);
-
-    const code = result0.code.code;
-    const result = result0.result;
-
-    console.log(result);
-
-    // Set any cookies from the result
-    if (result0.cookies) {
-        for (const cookie of result0.cookies) {
-            res.cookie(cookie.name, cookie.value, cookie.options ?? {});
-        }
-    }
-
-    if (isRedirect(result0)) {
-        res.redirect(code, result0.redirect);
-    } else {
-        res.status(code).send(result);
-    }
-});
-
-process.title = `diachron:${cli.listen.port}`;
 
 app.listen(cli.listen.port, cli.listen.host, () => {
     console.log(`Listening on ${cli.listen.host}:${cli.listen.port}`);

backend/diachron/app.ts

@@ -0,0 +1,190 @@
+// FIXME: rename this to make-app.ts and adjust imports accordingly
+
+import{contentTypes} from './content-types'
+import{httpCodes}from'./http-codes'
+import express, {
+    type NextFunction,
+    type Request as ExpressRequest,
+    type Response as ExpressResponse,
+} from "express";
+import { formatError, formatErrorHtml } from "./errors";
+import {isRedirect,     InternalHandler,    AuthenticationRequired,
+    AuthorizationDenied, Call,type Method, type ProcessedRoute,methodParser, type Result, type Route,massageMethod } from "./types";
+import { runWithContext } from "./context";
+import { Session } from "./auth";import { request } from "./request";
+import { match } from "path-to-regexp";
+
+import { cli } from "./cli";
+
+type ProcessedRoutes= {[K in Method]: ProcessedRoute[] }
+const processRoutes=(routes:Route[]) :ProcessedRoutes => {
+    const    retval:ProcessedRoutes= {
+        GET: [],
+        POST: [],
+        PUT: [],
+        PATCH: [],
+        DELETE: [],
+    };
+
+    routes.forEach((route: Route, _idx: number, _allRoutes: Route[]) => {
+        // const pattern /*: URLPattern */ = new URLPattern({ pathname: route.path });
+        const matcher = match<Record<string, string>>(route.path);
+        const methodList = route.methods;
+        
+        const handler: InternalHandler = async (
+            expressRequest: ExpressRequest,
+        ): Promise<Result> => {
+            const method = massageMethod(expressRequest.method);
+            
+            console.log("method", method);
+            
+            if (!methodList.includes(method)) {
+                // XXX: Worth asserting this?
+            }
+            
+            console.log("request.originalUrl", expressRequest.originalUrl);
+            
+            // Authenticate the request
+            const auth = await request.auth.validateRequest(expressRequest);
+            
+            const req: Call = {
+                pattern: route.path,
+                path: expressRequest.originalUrl,
+                method,
+                parameters: { one: 1, two: 2 },
+                request: expressRequest,
+                user: auth.user,
+                session: new Session(auth.session, auth.user),
+            };
+            
+            try {
+                const retval = await runWithContext({ user: auth.user }, () =>
+                    route.handler(req),
+                );
+                return retval;
+            } catch (error) {
+                // Handle authentication errors
+                if (error instanceof AuthenticationRequired) {
+                    return {
+                        code: httpCodes.clientErrors.Unauthorized,
+                        contentType: contentTypes.application.json,
+                        result: JSON.stringify({
+                            error: "Authentication required",
+                        }),
+                    };
+                }
+                if (error instanceof AuthorizationDenied) {
+                    return {
+                        code: httpCodes.clientErrors.Forbidden,
+                        contentType: contentTypes.application.json,
+                        result: JSON.stringify({ error: "Access denied" }),
+                    };
+                }
+                throw error;
+            }
+        };
+        
+        for (const [_idx, method] of methodList.entries()) {
+            const pr: ProcessedRoute = { matcher, method, handler };
+            
+          retval[method].push(pr);
+        }
+    });
+    
+    return retval;
+}
+
+type MakeAppArgs={routes:Route[],
+processTitle?: string,
+}
+ 
+const makeApp = ({routes, processTitle}: MakeAppArgs) => {
+    if (process.title) {
+        process.title = `diachron:${cli.listen.port}`;
+    }
+
+const processedRoutes = processRoutes(routes)
+
+async function handler(
+    req: ExpressRequest,
+    _res: ExpressResponse,
+): Promise<Result> {
+    const method = await methodParser.parseAsync(req.method);
+
+    const byMethod = processedRoutes[method];
+    console.log(
+        "DEBUG: req.path =",
+        JSON.stringify(req.path),
+        "method =",
+        method,
+    );
+    for (const [_idx, pr] of byMethod.entries()) {
+        const match = pr.matcher(req.path);
+        console.log("DEBUG: trying pattern, match result =", match);
+        if (match) {
+            console.log("match", match);
+            const resp = await pr.handler(req);
+
+            return resp;
+        }
+    }
+
+    const retval: Result = {
+        code: httpCodes.clientErrors.NotFound,
+        contentType: contentTypes.text.plain,
+        result: "not found!",
+    };
+
+    return retval;
+}
+
+
+    const app = express();
+
+    app.use(express.json())
+
+    app.use(express.urlencoded({ extended: true }));
+
+    app.use(async (req: ExpressRequest, res: ExpressResponse) => {
+        const result0 = await handler(req, res);
+        
+        const code = result0.code.code;
+    const result = result0.result;
+
+    console.log(result);
+
+    // Set any cookies from the result
+    if (result0.cookies) {
+        for (const cookie of result0.cookies) {
+            res.cookie(cookie.name, cookie.value, cookie.options ?? {});
+        }
+    }
+
+    if (isRedirect(result0)) {
+        res.redirect(code, result0.redirect);
+    } else {
+        res.status(code).send(result);
+    }
+    });
+
+    app.use(
+        (
+            err: Error,
+            _req: ExpressRequest,
+            res: ExpressResponse,
+            _next: NextFunction,
+        ) => {
+            console.error(formatError(err));
+            res.status(500).type("html").send(formatErrorHtml(err));
+        },
+    );
+
+return app;
+}
+
+export{makeApp};
+
+function _isPromise<T>(value: T | Promise<T>): value is Promise<T> {
+    return typeof (value as any)?.then === "function";
+}
+

backend/diachron/core/index.ts

@@ -1,5 +1,6 @@
 import nunjucks from "nunjucks";
 import { db, migrate, migrationStatus } from "../database";
+import { formatError, formatErrorHtml } from "../errors";
 import { getLogs, log } from "../logging";
 
 // FIXME: This doesn't belong here; move it somewhere else.
@@ -40,6 +41,7 @@ const misc = {
 const core = {
     conf,
     database,
+    errors: { formatError, formatErrorHtml },
     logging,
     misc,
     random,

backend/diachron/errors.ts

@@ -0,0 +1,229 @@
+// ANSI escape codes
+const bold = "\x1b[1m";
+const red = "\x1b[31m";
+const cyan = "\x1b[36m";
+const dim = "\x1b[2m";
+const reset = "\x1b[0m";
+
+interface ParsedFrame {
+    raw: string;
+    fn: string;
+    file: string;
+    line: string;
+    col: string;
+    isApp: boolean;
+}
+
+const frameRe = /^\s*at\s+(?:(.+?)\s+)?\(?((?:\/|[a-zA-Z]:\\).+?):(\d+):(\d+)\)?$/;
+
+function parseFrame(line: string): ParsedFrame | null {
+    const m = line.match(frameRe);
+    if (!m) return null;
+
+    const fn = m[1] ?? "<anonymous>";
+    const file = m[2];
+    const lineNum = m[3];
+    const col = m[4];
+
+    const isApp =
+        !file.includes("node_modules") && !file.startsWith("node:");
+
+    return { raw: line, fn, file, line: lineNum, col, isApp };
+}
+
+function relativePath(absPath: string): string {
+    const marker = "backend/";
+    const idx = absPath.lastIndexOf(marker);
+    if (idx !== -1) return absPath.slice(idx);
+    return absPath;
+}
+
+function libraryName(file: string): string {
+    const nmIdx = file.indexOf("node_modules/");
+    if (nmIdx === -1) return "node";
+    const after = file.slice(nmIdx + "node_modules/".length);
+    // Handle scoped packages like @scope/pkg
+    if (after.startsWith("@")) {
+        const parts = after.split("/");
+        return `${parts[0]}/${parts[1]}`;
+    }
+    return after.split("/")[0];
+}
+
+interface ParsedError {
+    message: string;
+    frames: ParsedFrame[];
+}
+
+function parseError(error: unknown): ParsedError {
+    if (!(error instanceof Error)) {
+        return { message: String(error), frames: [] };
+    }
+
+    const message = error.message ?? String(error);
+    const stack = error.stack ?? "";
+
+    const lines = stack.split("\n");
+    const frameLines: string[] = [];
+    for (const line of lines) {
+        if (line.trimStart().startsWith("at ")) {
+            frameLines.push(line);
+        }
+    }
+
+    const frames = frameLines
+        .map(parseFrame)
+        .filter((f): f is ParsedFrame => f !== null);
+
+    return { message, frames };
+}
+
+// Group consecutive library frames into collapsed runs
+type FrameGroup =
+    | { kind: "app"; frame: ParsedFrame }
+    | { kind: "lib"; count: number; names: string[] };
+
+function groupFrames(frames: ParsedFrame[]): FrameGroup[] {
+    const groups: FrameGroup[] = [];
+    let i = 0;
+    while (i < frames.length) {
+        if (frames[i].isApp) {
+            groups.push({ kind: "app", frame: frames[i] });
+            i++;
+        } else {
+            const libNames = new Set<string>();
+            let count = 0;
+            while (i < frames.length && !frames[i].isApp) {
+                libNames.add(libraryName(frames[i].file));
+                count++;
+                i++;
+            }
+            groups.push({ kind: "lib", count, names: [...libNames] });
+        }
+    }
+    return groups;
+}
+
+function libSummary(count: number, names: string[]): string {
+    const s = count === 1 ? "" : "s";
+    return `... ${count} internal frame${s} (${names.join(", ")})`;
+}
+
+// --- Console formatting (ANSI) ---
+
+function formatError(error: unknown): string {
+    const { message, frames } = parseError(error);
+    if (frames.length === 0) {
+        return `${bold}${red}ERROR${reset}  ${message}`;
+    }
+
+    const parts: string[] = [];
+    parts.push(`${bold}${red}ERROR${reset}  ${message}`);
+    parts.push("");
+
+    for (const group of groupFrames(frames)) {
+        if (group.kind === "app") {
+            const rel = relativePath(group.frame.file);
+            const loc = `${rel}:${group.frame.line}`;
+            parts.push(
+                `  ${bold}${cyan}${loc.padEnd(24)}${reset}at ${group.frame.fn}`,
+            );
+        } else {
+            parts.push(
+                `  ${dim}${libSummary(group.count, group.names)}${reset}`,
+            );
+        }
+    }
+
+    return parts.join("\n");
+}
+
+// --- HTML formatting (browser) ---
+
+function esc(s: string): string {
+    return s
+        .replace(/&/g, "&amp;")
+        .replace(/</g, "&lt;")
+        .replace(/>/g, "&gt;")
+        .replace(/"/g, "&quot;");
+}
+
+function formatErrorHtml(error: unknown): string {
+    const { message, frames } = parseError(error);
+    const groups = groupFrames(frames);
+
+    let frameRows = "";
+    for (const group of groups) {
+        if (group.kind === "app") {
+            const rel = relativePath(group.frame.file);
+            const loc = `${rel}:${group.frame.line}`;
+            frameRows += `<tr class="app">
+  <td class="loc">${esc(loc)}</td>
+  <td class="fn">at ${esc(group.frame.fn)}</td>
+</tr>\n`;
+        } else {
+            frameRows += `<tr class="lib">
+  <td colspan="2">${esc(libSummary(group.count, group.names))}</td>
+</tr>\n`;
+        }
+    }
+
+    return `<!doctype html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>Error</title>
+<style>
+  * { margin: 0; padding: 0; box-sizing: border-box; }
+  body {
+    font-family: "SF Mono", "Menlo", "Consolas", monospace;
+    font-size: 14px;
+    background: #1a1a2e;
+    color: #e0e0e0;
+    padding: 40px;
+  }
+  .error-label {
+    display: inline-block;
+    background: #e74c3c;
+    color: #fff;
+    font-weight: 700;
+    font-size: 12px;
+    padding: 2px 8px;
+    border-radius: 3px;
+    letter-spacing: 0.5px;
+  }
+  .message {
+    margin-top: 12px;
+    font-size: 18px;
+    font-weight: 600;
+    color: #f8f8f2;
+    line-height: 1.4;
+  }
+  table {
+    margin-top: 24px;
+    border-collapse: collapse;
+  }
+  tr.app td { padding: 4px 0; }
+  tr.app .loc {
+    color: #56d4dd;
+    font-weight: 600;
+    padding-right: 24px;
+    white-space: nowrap;
+  }
+  tr.app .fn { color: #ccc; }
+  tr.lib td {
+    color: #666;
+    padding: 4px 0;
+    font-style: italic;
+  }
+</style>
+</head>
+<body>
+  <span class="error-label">ERROR</span>
+  <div class="message">${esc(message)}</div>
+  <table>${frameRows}</table>
+</body>
+</html>`;
+}
+
+export { formatError, formatErrorHtml };