List.SelectPositions.pq

let func =  
 (SelectionList as list, ListOfPositions as list, optional StartWithOne as number, optional FillWithNulls as number) =>
let

selectionList = List.Buffer(SelectionList),
listOfPositions = ListOfPositions,
StartPosition = if StartWithOne = 1 then -1 else 0,

Result = {List.Transform(listOfPositions, each selectionList{_ + StartPosition}), // null
          List.Transform(listOfPositions, each selectionList{_+StartPosition}?), // 0
          Table.RemoveRowsWithErrors(Table.FromColumns({List.Transform(listOfPositions, each selectionList{_+StartPosition})})) //1
         }{List.PositionOf({null,0,1}, FillWithNulls)}
in
    Result
, documentation = [
Documentation.Name =  " List.SelectPositions
", Documentation.Description = " Selects all items from  <code>SelectionList</code> that are on positions in <code>ListOfPositions</code>.
" , Documentation.LongDescription = " Selects all items from  <code>SelectionList</code> that are on positions in <code>ListOfPositions</code>. For the behaviour of the optional parameters see: https://wp.me/p6lgsG-N8 .
", Documentation.Category = " List 
", Documentation.Source = "  https://wp.me/p6lgsG-N8 .
", Documentation.Author = " Imke Feldmann: www.TheBIccountant.com .
", Documentation.Examples = {[Description =  " For details see: https://wp.me/p6lgsG-N8 .
" , Code = " List.SelectPositions({1..5}, {0, 3, 4})
 ", Result = " {1,4,5}
"]}] 
 in 
  Value.ReplaceType(func, Value.ReplaceMetadata(Value.Type(func), documentation)) 

Hey Imke, Great packaging of the function!
btw what do you think if you use
"each SelectionList{_+StartPosition}?"
It will result in "cleaner" way of try X otherwise null

Hey Igor @Hugoberry, absolutely! thanks for the hint :)