Improve error message for non-dwimmy hyper ops

Patch courtesy by Timothy Bollman
With some wording improvements by moritz.

This recursively tracks the indexing path in hyperops, and prints that path
on error. Also elaborates on the nature of the different shapes, either
different length or types (Hash vs Scalar/List or so)

src/core/metaops.pm

@@ -45,7 +45,7 @@ our multi reduce(&op, *@list) {
     @list.reduce(&op)
 }
 
-our multi sub hyper(&op, @lhs, @rhs, :$dwim-left, :$dwim-right) {
+our multi sub hyper(&op, @lhs, @rhs, :$dwim-left, :$dwim-right, :@path) {
     my sub repeating-array(@a) {
         gather loop {
             my $prev-a;
@@ -61,7 +61,13 @@ our multi sub hyper(&op, @lhs, @rhs, :$dwim-left, :$dwim-right) {
     my $length;
     if !$dwim-left && !$dwim-right {
         if +@lhs != +@rhs {
-            die "Sorry, sides are of uneven length and not dwimmy.";
+            my $msg = "Sorry, lists on both sides of non-dwimmy hyperop are not of same length:\n"
+                ~ "    left:  @lhs.elems() elements\n"
+                ~ "    right: @lhs.elems() elements\n";
+            if (+@path) {
+                $msg ~= "At path [@path.join(', ')]";
+            }
+            die $msg;
         }
         $length = +@lhs;
     } elsif !$dwim-left {
@@ -84,21 +90,21 @@ our multi sub hyper(&op, @lhs, @rhs, :$dwim-left, :$dwim-right) {
     my @result;
     for ^$length -> $i {
         if Associative.ACCEPTS(@lhs[$i]) || Associative.ACCEPTS(@rhs[$i]) {
-            @result.push(hyper(&op, @lhs[$i], @rhs[$i], :$dwim-left, :$dwim-right).item);
+            @result.push(hyper(&op, @lhs[$i], @rhs[$i], :$dwim-left, :$dwim-right, path => (@path, $i)).item);
         } elsif Iterable.ACCEPTS(@lhs[$i]) || Iterable.ACCEPTS(@rhs[$i]) {
-            @result.push([hyper(&op, @lhs[$i].list, @rhs[$i].list, :$dwim-left, :$dwim-right)]);
+            @result.push([hyper(&op, @lhs[$i].list, @rhs[$i].list, :$dwim-left, :$dwim-right, path => (@path, $i))]);
         } else {
             @result.push(op(@lhs[$i], @rhs[$i]));
         }
     }
     @result
 }
 
-our multi sub hyper(&op, $lhs, $rhs, :$dwim-left, :$dwim-right) {
-    hyper(&op, $lhs.list, $rhs.list, :$dwim-left, :$dwim-right);
+our multi sub hyper(&op, $lhs, $rhs, :$dwim-left, :$dwim-right, :@path) {
+    hyper(&op, $lhs.list, $rhs.list, :$dwim-left, :$dwim-right, :@path);
 }
 
-our multi sub hyper(&op, %lhs, %rhs, :$dwim-left, :$dwim-right) {
+our multi sub hyper(&op, %lhs, %rhs, :$dwim-left, :$dwim-right, :@path) {
     my %result;
     my @keys;
     if $dwim-left && $dwim-right {
@@ -114,7 +120,13 @@ our multi sub hyper(&op, %lhs, %rhs, :$dwim-left, :$dwim-right) {
     }
 
     for @keys -> $key {
-        %result{$key} = &op(%lhs{$key}, %rhs{$key});
+        if Associative.ACCEPTS(%lhs{$key}) || Associative.ACCEPTS(%rhs{$key}) {
+            %result{$key} = hyper(&op, %lhs{$key}, %rhs{$key}, :$dwim-left, :$dwim-right, path => (@path, $key)).item;
+        } elsif Iterable.ACCEPTS(%lhs{$key}) || Iterable.ACCEPTS(%rhs{$key}) {
+            %result{$key} = hyper(&op, %lhs{$key}.list, %rhs{$key}.list, :$dwim-left, :$dwim-right, path => (@path, $key));
+        } else {
+            %result{$key} = op(%lhs{$key}, %rhs{$key});
+        }
     }
     %result;
 }
@@ -127,17 +139,33 @@ our multi sub hyper(&op, %arg) {
     %result;
 }
 
-our multi sub hyper(&op, %lhs, $rhs, :$dwim-left, :$dwim-right) {
-    die "Sorry, right side is too short and not dwimmy." unless $dwim-right;
+our multi sub hyper(&op, %lhs, $rhs, :$dwim-left, :$dwim-right, :@path) {
+    unless ($dwim-right) {
+        my $msg = "Sorry, structures on both sides of non-dwimmy hyperop are not of same shape:\n"
+            ~ "    left:  Hash\n"
+            ~ "    right: $rhs.WHAT.perl()\n";
+        if (+@path) {
+            $msg ~= "At path [@path.join(', ')]";
+        }
+        die $msg;
+    }
     my %result;
     for %lhs.keys -> $key {
         %result{$key} = &op(%lhs{$key}, $rhs);
     }
     %result;
 }
 
-our multi sub hyper(&op, $lhs, %rhs, :$dwim-left, :$dwim-right) {
-    die "Sorry, left side is too short and not dwimmy." unless $dwim-left;
+our multi sub hyper(&op, $lhs, %rhs, :$dwim-left, :$dwim-right, :@path) {
+    unless ($dwim-left) {
+        my $msg = "Sorry, structures on both sides of non-dwimmy hyperop are not of same shape:\n"
+            ~ "    left:  $lhs.WHAT.perl()\n"
+            ~ "    right: Hash\n";
+        if (+@path) {
+            $msg ~= "At path [@path.join(', ')]";
+        }
+        die $msg;
+    }
     my %result;
     for %rhs.keys -> $key {
         %result{$key} = &op($lhs, %rhs{$key});