Common Lisp: first returns first, but last returns a list of last -- huh?

Question

I'm not getting this first/last thing in Common-Lisp. Yes, I see how it works, but I don't get WHY it works that way. Basically, to get the first item in a list, I can use `(first mylist)`. However, if I want the last item, `(last mylist)` doesn't give me that; instead, it gives me a list containing the last item in my list! (I'm using Clozure-CL, which has a few other oddities that seem like bugs to me but, since I'm a Lisp-n00b, I'm trying not to fall for the old "the interpreter is broken!" trick :) ) So, for example: ``` ? (setq x '((1 2) (a b))) => ((1 2) (A B)) ? (first x) => (1 2) ; as expected ? (last x) => ((A B)) ; why a list with my answer in it?! ? (first (last x)) => '(A B) ; This is the answer I'd expect from plain-old (last x) ``` Can someone help me understand why last does this? Am I using these items incorrectly? Is `first` really the odd-ball?! Thanks!

Accepted Answer

In Common Lisp `last` is supposed to return a list, from the documentation:

```
last list &optional n => tail
list---a list, which might be a dotted list but must not be a circular list.
n---a non-negative integer. The default is 1.
tail---an object. 
```

last returns the last n conses (not the last n elements) of list. If list is (), last returns ().

For example:

```
(setq x (list 'a 'b 'c 'd))
(last x) =>  (d)
```

And yes, this is counterintuitive. In other flavors of Lisp it works as the name suggests, for example in Racket (a Scheme dialect):

```
(define x '((1 2) (a b)))
(first x) => '(1 2)
(last x) => '(a b)

(define x (list 'a 'b 'c 'd))
(last x) =>  'd
```

Common Lisp: first returns first, but last returns a list of last -- huh?

Solution

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