#### Synopsis

`length(`?List`, `?Integer`)`

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.

#### Arguments

List
list of term
Integer
integer, non-negative

#### Description

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.

#### Backtracking

If Integer is unbound, then it is unified with all possible lengths for the list List.

#### Exceptions

`type_error`
Integer is not an integer
`domain_error`
Integer < 0

#### Examples

```
| ?- length([1,2], 2).

yes
| ?- length([1,2], 0).

no
| ?- length([1,2], X).

X = 2 ;

no
```

ref-lte-acl, `library(lists)`.