Deparse RCall source when re-throwing R errors in JS

NEWS.md

@@ -16,6 +16,8 @@
 
 * The `RCall` and `RFunction` classes now have a `capture(options, ...args)` method that captures output during evaluation.
 
+* Errors re-thrown by `evalR` now include information about the source R call, helping to identify the original location of the error.
+
 * JavaScript objects of type `TypedArray`, `ArrayBuffer`, and `ArrayBufferView` (e.g. `Uint8Array`) may now be used with the `RRaw` R object constructor. The generic `RObject` constructor now converts objects of this type to R raw atomic vectors by default.
 
 * Constructing new R objects using `await new RObject(...)` now supports input objects of the form: `{a: [...], b: [...]}` or D3-style data arrays of the form: `[{a: ..., b: ...}, {a: ..., b: ...}, {a: ..., b: ...}, ... ]`. Where possible, lists are constructed of class `data.frame`. Direct construction with `RList()` does not create a `data.frame`.

src/webR/robj-worker.ts

@@ -569,6 +569,24 @@ export class RCall extends RObject {
   capture(options: EvalROptions = {}) {
     return Module.webr.captureR(this, options);
   }
+
+  deparse(): string {
+    const prot = { n: 0 };
+    try {
+      const call = Module._Rf_lang2(
+        new RSymbol('deparse1').ptr,
+        Module._Rf_lang2(new RSymbol('quote').ptr, this.ptr)
+      );
+      protectInc(call, prot);
+
+      const val = RCharacter.wrap(safeEval(call, objs.baseEnv));
+      protectInc(val, prot);
+
+      return val.toString();
+    } finally {
+      unprotect(prot.n);
+    }
+  }
 }
 
 export class RList extends RObject {

src/webR/webr-worker.ts

@@ -7,7 +7,7 @@ import { EmPtr, Module } from './emscripten';
 import { IN_NODE } from './compat';
 import { replaceInObject, throwUnreachable } from './utils';
 import { WebRPayloadRaw, WebRPayloadPtr, WebRPayloadWorker, isWebRPayloadPtr } from './payload';
-import { RObject, isRObject, REnvironment, RList, getRWorkerClass } from './robj-worker';
+import { RObject, isRObject, REnvironment, RList, RCall, getRWorkerClass } from './robj-worker';
 import { RCharacter, RString, keep, destroy, purge, shelters } from './robj-worker';
 import { RLogical, RInteger, RDouble, initPersistentObjects, objs } from './robj-worker';
 import { RPtr, RType, RTypeMap, WebRData, WebRDataRaw } from './robj';
@@ -711,9 +711,10 @@ function captureR(expr: string | RObject, options: EvalROptions = {}): {
         (out) => out.get('type').toString() === 'error'
       );
       if (error) {
-        throw new Error(
-          error.pluck('data', 'message')?.toString() || 'An error occurred evaluating R code.'
-        );
+        const call = error.pluck('data','call') as RCall;
+        const source = call && call.type() === 'call' ? call.deparse() : 'Unknown source';
+        const message = error.pluck('data', 'message')?.toString() || 'An error occurred evaluating R code.';
+        throw new Error(`Error in ${source}: ${message}`);
       }
     }