none (idea)

In ML, NONE is one of the constructors of the 'a option datatype. Partial functions evaluate to NONE when applied to arguments not in their domain. For example, the function lookup : (int * string) list -> int -> string option may be implemented as follows:

fun lookup nil x = NONE
  | lookup (n, s)::t x = if x = n then SOME(s) else lookup t x