Role::Cost->cost returns undef if cost not found. Closes interchange#19

Previous behaviour was to croak if a valid-looking argument was provided
but the cost was not found. This can cause a problem if for example no
shipping has been defined but a cart page tries to show the shipping.

lib/Interchange6/Role/Costs.pm

@@ -176,6 +176,8 @@ B<Example:> Return tax value by position
 
 Returns the cost that was first applied to subtotal. By increasing the number you can retrieve other costs applied.
 
+Returns undef if the cost cannot be found.
+
 =cut
 
 sub cost {
@@ -197,6 +199,9 @@ sub cost {
                 }
             }
         }
+        else {
+            croak "Bad argument to cost";
+        }
     }
     else {
         croak "Either position or name required as argument to cost";
@@ -207,7 +212,8 @@ sub cost {
         $self->total;
     }
     else {
-        croak "Bad argument to cost: " . $loc;
+        # not found
+        return undef;
     }
 
     return $cost->current_amount;

t/unit/costs_role.t

@@ -121,14 +121,16 @@ cmp_ok( $cost, "==", 2, "amount is good" );
 throws_ok { $obj->cost } qr/position or name required/,
   "fail call cost with no args";
 
-throws_ok { $obj->cost(2) } qr/Bad argument to cost/,
-  "fail call cost with index that doesn't exist";
+lives_ok { $cost = $obj->cost(2) }
+  "call cost with index that doesn't exist lives";
+ok !defined $cost, "returned undef";
 
 throws_ok { $obj->cost("") } qr/Bad argument to cost/,
   "fail call cost with empty name";
 
-throws_ok { $obj->cost("BadName") } qr/Bad argument to cost/,
-  "fail call cost with name that doesn't exist";
+lives_ok { $cost = $obj->cost("BadName") }
+  "call cost with name that doesn't exist lives";
+ok !defined $cost, "returned undef";
 
 lives_ok { $obj->clear_costs } "clear_costs";