# chickadee » srfi-116 » ilist=

##### Identifier search
ilist= elt= ilist1 ...procedure

Determines ilist equality, given an element-equality procedure. Proper ilist A equals proper ilist B if they are of the same length, and their corresponding elements are equal, as determined by elt=. If the element-comparison procedure's first argument is from ilisti, then its second argument is from ilisti+1, i.e. it is always called as (elt= a b) for a an element of ilist A, and b an element of ilist B.

In the n-ary case, every ilisti is compared to ilisti+1 (as opposed, for example, to comparing ilist1 to ilisti, for i>1). If there are no ilist arguments at all, ilist= simply returns true.

It is an error to apply ilist= to anything except proper ilists. It cannot reasonably be extended to dotted ilists, as it provides no way to specify an equality procedure for comparing the ilist terminators.

Note that the dynamic order in which the elt= procedure is applied to pairs of elements is not specified. For example, if ilist= is applied to three ilists, A, B, and C, it may first completely compare A to B, then compare B to C, or it may compare the first elements of A and B, then the first elements of B and C, then the second elements of A and B, and so forth.

The equality procedure must be consistent with eq?. That is, it must be the case that

`(eq? x y) ;=> (elt= x y).`

Note that this implies that two ilists which are eq? are always ilist=, as well; implementations may exploit this fact to "short-cut" the element-by-element comparisons.

```(ilist= eq?)        ;=> #t ;Trivial cases
(ilist= eq? (iq a)) ;=> #t```