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[CLJ-1060] 'list*' returns not a list Created: 03/Sep/12  Updated: 18/Jan/13

Status: Open
Project: Clojure
Component/s: None
Affects Version/s: Release 1.4
Fix Version/s: None

Type: Defect Priority: Trivial
Reporter: Andrei Zhlobich Assignee: Unassigned
Resolution: Unresolved Votes: 0
Labels: None

Attachments: File list-star-fix.diff    
Patch: Code and Test

 Description   

Function 'list?' returns sequence, but not a list.
It is a bit confusing.

user=> (list? (list* 1 '(2 3)))
false


 Comments   
Comment by Stuart Halloway [ 17/Sep/12 6:52 AM ]

should the docstring for list* change to say it returns a seq?

Comment by Timothy Baldridge [ 27/Nov/12 11:58 AM ]

Is there a reason why we can't have list* actually return a list? The cost of creating a list vs a cons is negligible.

Comment by Marek Srank [ 04/Jan/13 2:02 PM ]

The question is what to do with the one-argument case of list*, because in cases like: (list* {:a 1 :b 2}) it doesn't return IPersistentList as well. I propose just applying 'list'.

I added patch 'list-star-fix.diff' (dated 04/Jan/2013) with Cons implementing IPersistentList and doing (apply list args) in one-argument case of list*. To be able to use 'apply' in list* I had to declare it before the definition of list* in the source code. The apply itself also uses list*, but luckily not the one-argument version of list*, so it should be safe... The patch also contains simple unit tests.

Discussion is also here: https://groups.google.com/forum/#!topic/clojure/co8lcKymfi8

Comment by Michał Marczyk [ 04/Jan/13 4:11 PM ]

(apply list args) would make (list* (range)) hang, where currently it returns a partially-realized lazy seq. Also, even for non-lazy seqs – possibly lists themselves – it would always build a new list from scratch, right?

Also, if I'm reading the patch correctly, it would make 2+-ary list* overloads and cons return lists – that is, IPersistentList instances – always (Conses would now be lists), but repeatedly calling next on such a list might eventually produce a non-list. The only way around that would be to make all seqs into IPersistentLists – that doesn't seem desirable at first glance...?

On a side note, for the case where the final "args" argument is in fact a list, we already have a "prepend many items" function, namely conj. list* currently acts as the vararg variant of cons (in line with Lisp tradition); I'm actually quite happy with that.

Comment by Michał Marczyk [ 04/Jan/13 4:19 PM ]

I'm in favour of the "list" -> "seq" tweak to the docstring though, assuming impl remains unchanged.

Comment by Marek Srank [ 04/Jan/13 6:13 PM ]

Yep, these are all valid points, thanks! I see this as a question whether the list* function is a list constructor or not. If yes (and it would be possible to implement it in a satisfactory way), it should probably return a list.

We could avoid building a new list by sth like:

(if (list? args)
  args
  (apply list args))

(btw, 'vec' also creates a new vector even when the argument itself is a vector)

The contract of next seems to be to return a seq, not a list - its docstring reads: "Returns a seq of the items after the first. Calls seq on its argument. If there are no more items, returns nil."

Btw, in some Lisp/Scheme impls I checked, cons seems to be a list as well. E.g. in CLisp (and similar in Guile and Racket):

> (listp (cons 2 '()))
T
> (listp (list* 1 2 '()))
T
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