This pattern seems comprehensive, but I still get warnings

Question

I learn sml and wrote the following simple function:

(* Return a list with every other element of the input list *) fun everyOther [] = [] | everyOther [x] = [x] | everyOther x = let val head::head2::tail = x in head::everyOther(tail) end; 

Which causes the following warning:

 ! Toplevel input: ! val head::head2::tail = x ! ^^^^^^^^^^^^^^^^^ ! Warning: pattern matching is not exhaustive 

I believe that a function can never fail, since val head::head2::tail will always work for lists with two or more elements, and the case of one element and zero elements is considered. As far as I can tell, this function works as expected. I think the problem may be related to using [] , but I really don't know.

My question is actually three times:

• Why does sml think this is not exhaustive (how do I misinterpret it)?
• Is there a case where this function will not be executed?
• Am I doing something stupid by writing a function this way?

Answer 1

• SML gives you this warning because it does not know that x has at least two elements. All he knows is that x is a list, he does not remember the fact that x had to not match the first two patterns in order to go into the third case.

• No, the code cannot fail.

• There is no reason to match the pattern in let-statement. You can simply put the template in the fun statement, which will reduce the amount of code and remove the warning:

   fun everyOther [] = [] | everyOther [x] = [x] | everyOther (head::head2::tail) = head :: everyOther tail;