?- length(List,4).
List = [_27940,_27946,_27952,_27958].

?- length(List,Length).
List = [], Length = 0 ;
List = [_24698], Length = 1 ;
List = [_24698,_25826], Length = 2
...

It raises errors if Length is bound to a non-integer or a negative integer or if List is neither a list nor a partial list. This error condition includes cyclic lists:^{138ISO demands failure here. We think an error is more appropriate.}

?- A=[1,2,3|A], length(A,L).
ERROR: Type error: `list' expected ...

Covering an edge case, the predicate fails if the tail of List is equivalent to Length:^{139This is logically correct. An exception would be more appropriate, but to our best knowledge, current practice in Prolog does not describe a suitable candidate exception term.}

?- List=[1,2,3|Length],length(List,Length).
false.

?- length(Length,Length).
false.