fun p( x::xl ) = if x::xl = [] [] else [0]; it received warning: match non exhaustive.
x::xl => ...
what want is:
p( [] ) = []
when this, gives uncaught exception match [nonexhaustive match failure]
what test, x::xl = [], never true. lists algebraic types , defined as
datatype 'a list = :: of 'a * 'a list | [] meaning value list either empty list or element put in front of list.
so once initial pattern matching of x::xl has succeeded, know not empty. (this should clear, though, since assign x if x::xl empty; first element of empty list?)
you seem mixing 2 styles here, 1 being pattern matching , other being if-then-else.
fun p [] = [] | p (_::_) = [0] fun p xs = if list.null xs [] else [0] 