smlnj - SOME([]) interpreted as 'z -> 'z option rather then 'z list option -

The Hi IM is trying to write a simple function using the standard library

The following logic Should take

  try = fn: 'a - & gt; 'B list option a_list =' a list  

and of course, the following is:

  Try all_answers a_list - Enjoy ACC (some ), Some (b)) = some (b @ a). List (FORUM (X, Y) => ACC (Try (X), Y) AOC (_, _) = None ([]) a_list End  

And I get the following error:

  hw3provided.sml: 70.3-70.60 Error: operator and operation do not agree [Tycoon mismatch] Operator Domain: 'Z List Option Operand:' Y - & gt; Y option in expression: (List Fondal (FN ( gt ;, & lt; pat & gt;) => ACC  XP; gt;); some C: \ program files (x 86) \ SMLNJ \ bin. \ Run.x86-win32.exe: Fatal error - Uncaught exception with error raised on 0 ../compiler/TopLevel/interact/evalloop.sml:66.19-66.27  

ivcom decomposing Trying

I'm completely in disadvantage

Just think of using something else ([]), function and error, if I do nothing, then its Verify the correct

Detected the solution but not really the problem < / P>

Some ([] surrounds with brackets (some ([]))

Just comes to the same issue in someone else