recursion - Can't recursively call function in VB.NET -

As a little program to learn about language, I apply linq to The method of trying (usually known as a map outside of VB), although it seems that I have participated in a small problem, it will not let me call the function again because It assumes that the type does not match

here is the code:

  Iterator function map (A, B (As a joke, A, B), In the form of input (AK)) as IEnumerable (B) If there is nothing in input then there is no use when the fun (head) input map (Fun, Input)) 'Error Here End End Function   
 Note here that  head ()  is a function that is the first element of the input list , And  tail ()  is a method that gives everything, but the first element  
 I did a test first For what would a  function I (A, B)  was possible to have a function, such as I;  
 
 
 
  
 
  
  
 / Code> has the following signature:  
  In the form of the function head (tk) (i.e. as IEnumerable (tk)) as t ... and function Function TEL (TK) (as IEInumerable (T)) as IEnumerable (TK) ... end function   
 Then the query in the row generates the following error (signal Always include error messages in your SO question Data type of type parameter (A, B) (In the form of draft. Function (A, B) input, input as the system. Editor. Generic.list (ASK)) as a system. Colllections.Generic IEnumerable (BK) These arguments can not be estimated from. Specifying the data type can clearly correct this error.   
 Then let's ask for the error message:  
  Iterator function map ( Interesting as A, B) (Music, A, B), In the form of Input (AK) as IEnumerable (B) ... Map (A, B) (Interesting, Tail (Input)) ... function   
 and we get a more specific error:  
 
 Selection. Generic 'System.Collections.Generic.List (A)'.   
 It makes sense:  Tail  a  IEnumerable  returns  map  to a  The list requires . Since  map  does not use any list-specific properties, let's just make the  map  more generous in its input:  
  Iterator Function map, like A, B) (such as A, B), as IEnumerable (AK) as input) like IEnumerable (B) ... end function   
 now , Everything compiled properly In fact, we can now remove    (A, B)  again, because VB.NET is required to fix the required data type of the normal type parameter It can be assumed: Interesting as    < / Html>