flatten that does not drop nil

[Antony]

Hi

I needed a flatten that keeps the nils
All the ones I found seem to drop or ignore nils
So, I slightly modified one that I found (on c.l.l)

Here it is
(defun flatten (tree)
(labels ((flatten-with-nil (tree)
(typecase tree
(null (list nil))
(list (mapcan #'flatten-with-nil tree))
(atom (list tree)))))
(when tree
(flatten-with-nil tree))))

CL-USER> (flatten '(1 2 (5 3 6) nil 9))
(1 2 5 3 6 NIL 9)
CL-USER> (flatten nil)
NIL

This was mostly a cut-past-change exercise, so any feedback would be useful.

-Antony

[Chris Riesbeck]

Although I'm betting you don't really need or want a flatten at all, I
don't see why you need a typecase, or why you need the first branch
given the third. So

(defun flatten (tree)
(labels ((flatten-with-nil (tree)

(if (atom tree) (list tree)
(mapcan #'flatten-with-nil tree))))
(when tree
(flatten-with-nil tree))))

[Antony]

On 9/21/2011 2:29 PM, Chris Riesbeck wrote:
> On 9/21/2011 8:39 AM, Antony wrote:
>> I needed a flatten that keeps the nils
>> All the ones I found seem to drop or ignore nils
>> So, I slightly modified one that I found (on c.l.l)

>>...

>> Here it is
>> (defun flatten (tree)
>> (labels ((flatten-with-nil (tree)
>> (typecase tree
>> (null (list nil))
>> (list (mapcan #'flatten-with-nil tree))
>> (atom (list tree)))))
>> (when tree
>> (flatten-with-nil tree))))
>>
>> CL-USER> (flatten '(1 2 (5 3 6) nil 9))
>> (1 2 5 3 6 NIL 9)
>> CL-USER> (flatten nil)
>> NIL

>>...

>
> Although I'm betting you don't really need or want a flatten at all, I
> don't see why you need a typecase, or why you need the first branch
> given the third. So
>
> (defun flatten (tree)
> (labels ((flatten-with-nil (tree)
> (if (atom tree) (list tree)
> (mapcan #'flatten-with-nil tree))))
> (when tree
> (flatten-with-nil tree))))

Thanks for the feedback.
Changed to
(defun flatten (tree &optional keep-nil)

(labels ((flatten-with-nil (tree)
(typecase tree

(null (when keep-nil
(list nil)))

(list (mapcan #'flatten-with-nil tree))
(atom (list tree)))))
(when tree
(flatten-with-nil tree))))

This should be pluggable in place of what people are already using
-Antony

[Antony]

On 9/21/2011 2:29 PM, Chris Riesbeck wrote:

> On 9/21/2011 8:39 AM, Antony wrote:
>> I needed a flatten that keeps the nils
>> All the ones I found seem to drop or ignore nils
>> So, I slightly modified one that I found (on c.l.l)

>>...

>> Here it is
>> (defun flatten (tree)
>> (labels ((flatten-with-nil (tree)
>> (typecase tree
>> (null (list nil))
>> (list (mapcan #'flatten-with-nil tree))
>> (atom (list tree)))))
>> (when tree
>> (flatten-with-nil tree))))
>>
>> CL-USER> (flatten '(1 2 (5 3 6) nil 9))
>> (1 2 5 3 6 NIL 9)
>> CL-USER> (flatten nil)
>> NIL

>>...

>
> Although I'm betting you don't really need or want a flatten at all, I
> don't see why you need a typecase, or why you need the first branch
> given the third. So
>
> (defun flatten (tree)
> (labels ((flatten-with-nil (tree)
> (if (atom tree) (list tree)
> (mapcan #'flatten-with-nil tree))))
> (when tree
> (flatten-with-nil tree))))

Thanks for the feedback.
Changed to
(defun flatten (tree &optional keep-nil)

(labels ((flatten-with-nil (tree)
(typecase tree

(null (when keep-nil
(list nil)))

(list (mapcan #'flatten-with-nil tree))
(atom (list tree)))))
(when tree
(flatten-with-nil tree))))

[Tim Bradshaw]

On 2011-09-22 07:33:38 +0100, Antony said:

> This should be pluggable in place of what people are already using

What I want to know is: has anyone, ever, used a flatten function in
anger? I always assume they are only set as exercises (I've set them
as such).

[Zach Beane]

Tim Bradshaw <t...@tfeb.org> writes:

I used one once when I wanted to construct a list to pass as arguments
to a shell command, and I wanted to do something like this:

(list "--foo" 42
(when bar
"--bar" 91)
"--output" "baz.txt")

Of course, in this case, I wanted the semantics of NIL "disappearing".

Zach

[Antony]

My use case is for building cons pairs for http parameters to drakma
I have stuff like
(list
"foo" (do-somehting-to-get-value)
(when (some-condition)
(list x y))
...)
where foo and x are http query parameters.
Then do a flatten and then construct the pairs.

So I needed to keep the nils.
Mostly likely in-efficient, but works.

I guess using backquote and ,@ might be another way, even though this is
runtime need.


-Antony

[Pascal J. Bourguignon]

Right, but you could instead write:

(append '("--foo" 42)
(when bar
'("--bar" 91))
'("--output" "baz.txt"))

or even:

`("--foo" 42 ,@(when bar '("--bar" 91)) "--output" "baz.txt")


--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.

[Zach Beane]

"Pascal J. Bourguignon" <p...@informatimago.com> writes:

> Zach Beane <xa...@xach.com> writes:
>
>> Tim Bradshaw <t...@tfeb.org> writes:
>>
>>> On 2011-09-22 07:33:38 +0100, Antony said:
>>>
>>>> This should be pluggable in place of what people are already using
>>>
>>> What I want to know is: has anyone, ever, used a flatten function in
>>> anger? I always assume they are only set as exercises (I've set them
>>> as such).
>>
>> I used one once when I wanted to construct a list to pass as arguments
>> to a shell command, and I wanted to do something like this:
>>
>> (list "--foo" 42
>> (when bar
>> "--bar" 91)
>> "--output" "baz.txt")
>>
>> Of course, in this case, I wanted the semantics of NIL "disappearing".
>
> Right, but you could instead write:
>
> (append '("--foo" 42)
> (when bar
> '("--bar" 91))
> '("--output" "baz.txt"))
>
> or even:
>
> `("--foo" 42 ,@(when bar '("--bar" 91)) "--output" "baz.txt")

I consider both of those uglier.

Zach

[Tim Bradshaw]

On 2011-09-22 13:09:09 +0100, Zach Beane said:

> I consider both of those uglier.

Me to, that's a good use for flatten.

[Peter Keller]

If I recall, the book Let Over Lambda uses them to collect specific
symbol names out of bodies of code during macro expansion. I believe
this is to implement the defmacro/g! form specified in the book. I
don't have a copy handy at the moment though to cross reference.

-pete

[WJ]

NewLisp:

> (flat '(2 nil (3 (4))))
(2 nil 3 4)