Structured data in Emacs Lisp

Structured data in Emacs Lisp

[PT]

I'm writing a moderately complex Emacs package and not being a Lisp guru I  
wonder what is the best way to handle data structures in Elisp. The  
emphasis is not on efficiency, rather on readability.

For example, from a function I want to return three values. How should I  
do this?

Using a list? (Value1 Value2 Value3)?

This has the disadvantage of storing specific values on specific  
positions, so the caller must now the first element of the list is Value1,  
etc. And what if the return value is changed later and Value2 is not  
returned anymore? Then I have to fix every invocation of the function.

Or maybe an association list? '((value1 . 3) (value2 . 4) (value3 . 5))
It's certainly more resistant to code changes, but feels a bit  
heavyweight. (Maybe its just me.)

Or is there an other Lispish way to handle structured data I don't know  
about?

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Re: Structured data in Emacs Lisp

[Denis Bueno]

On 4/26/05, PT <mailshield.gg@mailnull.com> wrote:
<snip explanation>

> Or maybe an association list? '((value1 . 3) (value2 . 4) (value3 . 5))
> It's certainly more resistant to code changes, but feels a bit
> heavyweight. (Maybe its just me.)

You could use a plist:

  (:key1 1 :key2 2)

Then to get a particular value:

(getf '(:key1 1 :key2 2) :key1)
=> 1

According to C-h f getf RET:

  getf is a compiled Lisp function in `cl-extra'.
  (getf plist tag &optional def)

So you might need to do (require 'cl) before using it....

-Denis
PGP: http://pgp.mit.edu:11371/pks/lookup?search=0xA1B51B4B&op=index

Re: Structured data in Emacs Lisp

[PT]

On Tue, 26 Apr 2005 17:28:48 +0200, Denis Bueno <dbueno@gmail.com> wrote:
>
> You could use a plist:
>
>   (:key1 1 :key2 2)
>
> Then to get a particular value:
>
> (getf '(:key1 1 :key2 2) :key1)
> => 1
>

Nice! I like it better than association lists. Thanks!

> So you might need to do (require 'cl) before using it....

I already require 'cl, so it's no problem. I checked Info and getf was  
listed under the Symbols node, that's why I didn't know about it.

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Re: Structured data in Emacs Lisp

[Phillip Lord]

>>>>> "PT" == PT  <mailshield.gg@mailnull.com> writes:

  PT> I'm writing a moderately complex Emacs package and not being a
  PT> Lisp guru I wonder what is the best way to handle data
  PT> structures in Elisp. The emphasis is not on efficiency, rather
  PT> on readability.

  PT> For example, from a function I want to return three values. How
  PT> should I do this?

  PT> Using a list? (Value1 Value2 Value3)?

  PT> This has the disadvantage of storing specific values on specific
  PT> positions, so the caller must now the first element of the list
  PT> is Value1, etc. And what if the return value is changed later
  PT> and Value2 is not returned anymore? Then I have to fix every
  PT> invocation of the function.

  PT> Or maybe an association list? '((value1 . 3) (value2 . 4)
  PT> (value3 . 5)) It's certainly more resistant to code changes, but
  PT> feels a bit heavyweight. (Maybe its just me.)

  PT> Or is there an other Lispish way to handle structured data I
  PT> don't know about?


plists, or a hashtable would also work. 


Another solution would be to look at eieio, which gives you objects
that you can then pass around. This doesn't directly answer your
question, but it gets away from some of the nastiness of having
multiply nested list structures.


Phil

Re: Structured data in Emacs Lisp

[Jim Ottaway]

>>>>> PT  <mailshield.gg@mailnull.com> writes:

> On Tue, 26 Apr 2005 17:28:48 +0200, Denis Bueno <dbueno@gmail.com> wrote:
>> 
>> You could use a plist:
>> 
>> (:key1 1 :key2 2)
>> 
>> Then to get a particular value:
>> 
>> (getf '(:key1 1 :key2 2) :key1)
>> => 1
>> 

Or you could use defstruct [also in 'cl]

-- 
Jim Ottaway

Re: Structured data in Emacs Lisp

[PT]

On Tue, 26 Apr 2005 18:03:45 +0200, Phillip Lord <p.lord@cs.man.ac.uk>  
wrote:
>
> plists, or a hashtable would also work.

Are there any portability considerations regarding these constructs? Any  
XEmacs guys here?

I develop on GNU Emacs, but I want to be on the safe side if the need of  
supporting XEmacs arises in the future, so I'd avoid using GNU Emacs  
specific data structures if possible.


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Re: Structured data in Emacs Lisp

[Ian Zimmerman]

Denis> You could use a plist:
Denis> 
Denis> (:key1 1 :key2 2)
Denis> 
Denis> Then to get a particular value:
Denis> 
Denis> (getf '(:key1 1 :key2 2) :key1) => 1
Denis> 

Jim> Or you could use defstruct [also in 'cl]

I don't know how many different structures you have.  If not too many, I
would just use lists (or even better, vectors) and for each field define
an accessor macro:

(defsubst get-key1 (result) (nth 0 result))

or

(defsubst get-key1 (result) (aref result 0))

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Re: Structured data in Emacs Lisp

[Phillip Lord]

>>>>> "PT" == PT  <mailshield.gg@mailnull.com> writes:

  PT> On Tue, 26 Apr 2005 18:03:45 +0200, Phillip Lord
  PT> <p.lord@cs.man.ac.uk>
  PT> wrote:

  >>
  >> plists, or a hashtable would also work.

  PT> Are there any portability considerations regarding these
  PT> constructs?  Any XEmacs guys here?

plist should work fine I think. hashtables were new for Emacs 21. But
I've used them in packages which apparently work on XEmacs, so you
should be alright. 


  PT> I develop on GNU Emacs, but I want to be on the safe side if the
  PT> need of supporting XEmacs arises in the future, so I'd avoid
  PT> using GNU Emacs specific data structures if possible.


Sensible enough. My experience with trying to support both emacs is
that it's often small differences which cause problems rather than
missing features. For one package I wrote, I spent an hour trying to
debug an XEmacs problem which turned out to be because the `subseq'
function behaves differently between the two. What a nightmare!

In general, now, I write for GNU Emacs. If it's a nice package,
someone will port it to XEmacs sooner or later. 

Cheers

Phil

Re: Structured data in Emacs Lisp

[Klaus Berndl]

If you want to move around datas which belong together i recommend `defstruct'
which is in both Emacs and XEmacs available! It is a macro of the cl-package
so you can use it safely also with GNU Emacs ;-)

See the info-manual of `defstruct' - It#s in the cl-info-manual!

defstruct allows you to get and set slots via key-names so there are position
independed!


Does this help?
Klaus

On Tue, 26 Apr 2005, mailshield.gg@mailnull.com wrote:

>  I'm writing a moderately complex Emacs package and not being a Lisp
>  guru I  wonder what is the best way to handle data structures in
>  Elisp. The  emphasis is not on efficiency, rather on readability.
>  
>  For example, from a function I want to return three values. How should
>  I  do this?
>  
>  Using a list? (Value1 Value2 Value3)?
>  
>  This has the disadvantage of storing specific values on specific
>  positions, so the caller must now the first element of the list is
>  Value1,  etc. And what if the return value is changed later and Value2
>  is not  returned anymore? Then I have to fix every invocation of the
>  function.
>  
>  Or maybe an association list? '((value1 . 3) (value2 . 4) (value3 . 5))
>  It's certainly more resistant to code changes, but feels a bit
>  heavyweight. (Maybe its just me.)
>  
>  Or is there an other Lispish way to handle structured data I don't
>  know  about?

-- 
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sd&m AG				http://www.sdm.de
software design & management	
Carl-Wery-Str. 42, 81739 Muenchen, Germany
Tel +49 89 63812-392, Fax -220

Re: Structured data in Emacs Lisp

[PT]

On Wed, 27 Apr 2005 17:27:16 +0200, Klaus Berndl <klaus.berndl@sdm.de>  
wrote:
>
> If you want to move around datas which belong together i recommend  
> `defstruct'
[..]
>
> Does this help?
> Klaus

Yep. Someone else also recommended defstruct earlier in the thread, I  
checked it out and that's exactly what I need. The code is much more  
readable without those deep association lists.

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Re: Structured data in Emacs Lisp

[Christopher C. Stacy]

PT <mailshield.gg@mailnull.com> writes:

> For example, from a function I want to return three values. 
> How should I  do this?

> Using a list? (Value1 Value2 Value3)?

> Or maybe an association list? '((value1 . 3) (value2 . 4) (value3 . 5))

> Or is there an other Lispish way to handle structured
> data I don't know about?

This isn't going to help with your Emacs problem, but just so 
you know: Emacs Lisp is a very simple kind of Lisp, lacking many
features present in most modern (post-1979) dialects of Lisp.  
For example, in ANSI Common Lisp, you can directly pass mutiple 
values around without packing them into some kind of container.  
And for representing structured data, you can use either structs 
or object-oriented programming. There are of course many other 
data types and a zillion other important features in "real" 
Lisp that Emacs Lisp doesn't have.   I'm telling you this
because you asked for a "Lispy" way to do things.

I think the "CL" library in Emacs Lisp might give 
you some version of DEFSTRUCT, which is something
like the one in Common Lisp.

Re: Structured data in Emacs Lisp

[Phillip Lord]

>>>>> "Christopher" == Christopher C Stacy <cstacy@news.dtpq.com> writes:

  Christopher> PT <mailshield.gg@mailnull.com> writes:

  >> For example, from a function I want to return three values.  How
  >> should I do this?

  >> Using a list? (Value1 Value2 Value3)?

  >> Or maybe an association list? '((value1 . 3) (value2 . 4) (value3
  >> . 5))

  >> Or is there an other Lispish way to handle structured data I
  >> don't know about?

  Christopher> This isn't going to help with your Emacs problem, but
  Christopher> just so you know: Emacs Lisp is a very simple kind of
  Christopher> Lisp, lacking many features present in most modern
  Christopher> (post-1979) dialects of Lisp.  For example, in ANSI
  Christopher> Common Lisp, you can directly pass mutiple values
  Christopher> around without packing them into some kind of
  Christopher> container.  And for representing structured data, you
  Christopher> can use either structs or object-oriented
  Christopher> programming. There are of course many other data types
  Christopher> and a zillion other important features in "real" Lisp
  Christopher> that Emacs Lisp doesn't have.  I'm telling you this
  Christopher> because you asked for a "Lispy" way to do things.

  Christopher> I think the "CL" library in Emacs Lisp might give you
  Christopher> some version of DEFSTRUCT, which is something like the
  Christopher> one in Common Lisp.

Much of what you say about emacs lisp is true. But it is worth
mentioning that you can do all three of the things you mention
here. Multiple values you can pass around with the :construct, structs
with CL and classes with eieio (which is not a standard part of
emacs).

Phil