1 files changed, 22 insertions, 1 deletions
diff --git a/docs/bqn.js b/docs/bqn.js
index db3085c0..cbb2802a 100644
--- a/docs/bqn.js
+++ b/docs/bqn.js
@@ -233,7 +233,28 @@ let fmt1 = run(
 )(list([type, decompose, glyph, fmtnum]));
 let fmt = x => fmt1(x).map(c=>c===0?" ":c).join("");
 
+let fmtErr = (s,e) => {
+  let r=e.src, w=e.message, loc=[];
+  while (w&&w.loc||(r!=='!'&&w.sh&&w.sh[0]===2)) {
+    let is; [is,w]=w;
+    let n=is.sh?is.sh[0]:0, i=n?is[0]:is;
+    let pair=n&&is.sh.length>1; if (pair) n*=2;
+    let to=i=>s.slice(0,i).join('').split('\n').map(l=>Array.from(l));
+    let ll=to(i), l=ll.length-1, j=ll[l].length, m=to()[l];
+    let k=1,o=i-j,cl=j; while (k<n&&(cl=is[k]-o)<m.length) k++;
+    let ol=k<n; if (pair) { if (k%2) cl=m.length; else { k--; cl++; } }
+    let c=Array(cl).fill(0); c[j]=1;
+    for (let h=1;h<k;h++) c[is[h]-o+(pair?h%2:0)]=1;
+    if (pair) for (let h=1;h<cl;h++) c[h]^=c[h-1];
+    let add = ['',m.join(''),c.map(t=>t?'^':' ').join('')];
+    loc = add.concat(ol?['(and other lines)']:[], loc);
+  }
+  if (r==='!') w=w?fmt(w).replace(/^/gm,'! '):'! Error';
+  else w=w.sh?w.join(''):w;
+  return [w].concat(loc).join('\n');
+}
+
 if (typeof module!=='undefined') {
-  bqn.fmt=fmt; bqn.compile=compile; bqn.run=run;
+  bqn.fmt=fmt; bqn.fmtErr=fmtErr; bqn.compile=compile; bqn.run=run;
   module.exports=bqn;
 }