function construction - Returning all positions for all different occurences of elements in a list

Is there way to construct oneliner as pure function(s), so that I enter mylist only on one place - on the end of line. And that function return the same result as last line bellow but paired with mylist. So the result should look like this:

{{1, {8}}, {2, {2, 4, 6}}, {4, {1}}, {5, {5}}, {7, {3, 7}}}

mylist = {4, 2, 7, 2, 5, 2, 7, 1};    
alldiffelem = Sort@DeleteDuplicates@mylist    

(* {1, 2, 4, 5, 7} *)
(Flatten@Position[mylist, #]) & /@ alldiffelem    
(* {{8}, {2, 4, 6}, {1}, {5}, {3, 7}} *)

Answer

Here is an approach using Sow and Reap:

Last@Reap[MapThread[Sow, {Range[Length[mylist]], mylist}], _, List]

yielding

{{4, {1}}, {2, {2, 4, 6}}, {7, {3, 7}}, {5, {5}}, {1, {8}}}

if you wish to sort:

SortBy[Last@Reap[MapThread[Sow, {Range[Length[mylist]],mylist}], _, List], First]

yielding:

{{1, {8}}, {2, {2, 4, 6}}, {4, {1}}, {5, {5}}, {7, {3, 7}}}

Comments

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