Improve transform error handling with compiled function annotations

deps.ts

@@ -1,4 +1,4 @@
-export * as path from "jsr:@std/[email protected]";
+export * as path from "https://deno.land/[email protected]/path/mod.ts";
 export * as html from "jsr:@std/[email protected]";
 
 export * as astring from "jsr:@davidbonnet/[email protected]";

src/environment.ts

@@ -1,7 +1,7 @@
 import tokenize, { Token } from "./tokenizer.ts";
 
 import type { Loader } from "./loader.ts";
-import { transformTemplateCode } from "./transformer.ts";
+import { TransformError, transformTemplateCode } from "./transformer.ts";
 
 export interface TemplateResult {
   content: string;
@@ -139,21 +139,8 @@ export class Environment {
       try {
         code = transformTemplateCode(code, dataVarname);
       } catch (error) {
-        const end = (error as ParseError).start;
-        const lastPos = code.slice(0, end).lastIndexOf("__pos = ");
-        const lastPosEnd = code.slice(lastPos).indexOf(";");
-
-        if (lastPos > -1 && lastPosEnd > lastPos) {
-          const pos = parseInt(
-            code.slice(lastPos + 8, lastPos + lastPosEnd),
-            10,
-          );
-          throw this.createError(
-            path || "",
-            source,
-            pos,
-            new Error((error as ParseError).message),
-          );
+        if (error instanceof TransformError) {
+          throw this.createError(path, source, error.pos, error);
         }
 
         throw error;
@@ -192,7 +179,7 @@ export class Environment {
     const { position, error } = result;
 
     if (error) {
-      throw this.createError(path || "unknown", source, position, error);
+      throw this.createError(path, source, position, error);
     }
 
     for (const tokenPreprocessor of this.tokenPreprocessors) {
@@ -325,19 +312,17 @@ export class Environment {
   }
 
   createError(
-    path: string,
-    source: string,
-    position: number,
+    path: string = "unknown",
+    source: string = "<empty file>",
+    position: number = 0,
     cause: Error,
   ): Error {
-    if (!source) {
-      return cause;
-    }
-
     const [line, column, code] = errorLine(source, position);
 
     return new Error(
-      `Error in the template ${path}:${line}:${column}\n\n${code.trim()}\n\n> ${cause.message}\n`,
+      `Error in the template ${path}:${line}:${column}\n\n${code.trim()}\n\n${
+        cause.message.replaceAll(/^/gm, "> ")
+      }\n`,
       { cause },
     );
   }
@@ -386,9 +371,3 @@ export function errorLine(
 function checkAsync(fn: () => unknown): boolean {
   return fn.constructor?.name === "AsyncFunction";
 }
-
-interface ParseError extends Error {
-  start: number;
-  end: number;
-  range: [number, number];
-}

src/transformer.ts

@@ -1,5 +1,27 @@
 import { astring, ESTree, meriyah, walker } from "../deps.ts";
 
+// Declare types
+interface ParseError extends Error {
+  start: number;
+  end: number;
+  range: [number, number];
+  loc: Record<"start" | "end", Record<"line" | "column", number>>;
+  description: string;
+}
+
+interface TransformErrorOptions extends ErrorOptions {
+  pos?: number;
+}
+
+export class TransformError extends Error {
+  pos?: number;
+  constructor(message: string, options?: TransformErrorOptions) {
+    super(message);
+    this.name = "TransformError";
+    this.pos = options?.pos;
+  }
+}
+
 // List of identifiers that are in globalThis
 // but should be accessed as templateState.identifier
 const INCLUDE_GLOBAL = [
@@ -119,7 +141,29 @@ export function transformTemplateCode(
     return code;
   }
 
-  const parsed = meriyah.parseScript(code, { module: true }) as ESTree.Program;
+  let parsed;
+  try {
+    parsed = meriyah.parseScript(code, { module: true }) as ESTree.Program;
+  } catch (error) {
+    const { message, start, loc } = error as ParseError;
+
+    // Use information from `meriyah` to annotate the part of
+    // the compiled template function that triggered the ParseError
+    const annotation = code.split("\n")[loc.start.line - 1] + "\n" +
+      " ".repeat(loc.start.column) + "\x1b[31m^\x1b[0m";
+
+    // Grab the last instance of Vento's `__pos` variable before the
+    // error was thrown. Pass this back to Vento's createError to
+    // tie this error with problmatic template code
+    const matches = [...code.slice(0, start).matchAll(/__pos = (\d+);/g)];
+    const pos = Number(matches.at(-1)?.[1]);
+
+    throw new TransformError(
+      `[meriyah] ${message}\nthrown while parsing compiled template function:\n\n${annotation}`,
+      { pos },
+    );
+  }
+
   const tracker = new ScopeTracker();
 
   const exclude = [
@@ -128,11 +172,11 @@ export function transformTemplateCode(
   ];
 
   if (parsed.type !== "Program") {
-    throw new Error("Expected a program");
+    throw new TransformError("[meriyah] Expected a program");
   }
 
   if (parsed.body.length === 0) {
-    throw new Error("Empty program");
+    throw new TransformError("[meriyah] Empty program");
   }
 
   // Transforms an identifier to a MemberExpression
@@ -166,7 +210,7 @@ export function transformTemplateCode(
   }
 
   walker.walk(parsed, {
-    enter(node) {
+    enter(node: ESTree.Node) {
       switch (node.type) {
         // Track variable declarations
         case "VariableDeclaration":