Integer is the length of List. If List is instantiated to a proper list of term, or Integer to an integer, the predicate is determinate.
If List is a list of indefinite length (that is, either a variable
or of the form
[...|X]) and if Integer is bound to an integer,
then List is made to be a list of length Integer with unique
variables used to “pad” the list. If List cannot be
made into a list of length Integer, the call fails.
| ?- List = [a,b|X], length(List, 4). List = [a,b,_A,_B], X = [_A,_B] ; | ?-
If List is bound, and is not a list,
length/2 simply fails.
If Integer is unbound, then it is unified with all possible lengths for the list List.
| ?- length([1,2], 2). yes | ?- length([1,2], 0). no | ?- length([1,2], X). X = 2 ; no