ML : match non-exhaustive

I want to make function named headcol that works like:

headcol[[1,2],[3,4]] = [1,3]; 

So i made function like this:

fun headcol [] = [] 
  | headcol [x::xs',y::ys'] = [x,y]

but when I call it, I get a match nonexhaustive.

IIUC, headcol will extract all heads of lists in the parameter, whose type is 'a-list-list. What you mathc is only [] and [x::xs', y::ys'], not anything else. So, if your argument has more than 2 sublists, execption will raise:

- headcol[[1,2],[3,4], [4, 5, 9]]; 

uncaught exception Match [nonexhaustive match failure]
  raised at: a.sml:9.34
- 

If you just want to deal with two-elems list, pair is a better choice. Otherwise, you should match more cases:

fun headcol list =
    case list of
        [] => []
      | x::xs' =>
        case x of
            [] => headcol xs'
         |  h::t => h::(headcol xs')

output:

- headcol[[1,2],[3,4], [4, 5, 9]]; 
- val it = [1,3,4] : int list