defvar vs defconst

defvar vs defconst

[Thorsten Jolitz]

Hi List,

when you define a variable that should not be touched/set by the user of
your library (which is most likely a programmer), but will be frequently
set to a new value by your own program - would you call that a VAR or a
CONST?

(defconst ...) signals "don't touch this", which seems to the right
thing in this case, but OTOH it is a bit strange to call something a
constant that not only can be changed, but will actually be changed
quite often.

-- 
cheers,
Thorsten

Re: defvar vs defconst

[Emanuel Berg]

Thorsten Jolitz <tjolitz@gmail.com> writes:

> (defconst ...) signals "don't touch this", which seems to the
> right thing in this case, but OTOH it is a bit strange to call
> something a constant that not only can be changed, but will
> actually be changed quite often.

A constant should not be changed, not by a human, and not by a
computer, either. This is my firm intuition.

#00FF00 for green and #FFFF00 for yellow; 3.1415... for PI;
etc. This is the stuff you always make constants of.

number_of_attempts *could* be a constant, because once set, it
would never be changed in an execution run, and actually not at
all, unless there is a policy decision to change the number of
attempts. Still, I'd prefer it a variable.

In your particular case, I don't know. Are there different
implications using the methods, if you don't consider what signals
they send to a human user?

-- 
Emanuel Berg - programmer (hire me! CV below)
computer projects: http://user.it.uu.se/~embe8573
internet activity: http://home.student.uu.se/embe8573

Re: defvar vs defconst

[Stefan Monnier]

> set to a new value by your own program - would you call that a VAR or a
> CONST?

defconst should be used for things which don't change, so I'd call it a VAR.


        Stefan

Re: defvar vs defconst

[Eric Abrahamsen]

Thorsten Jolitz <tjolitz@gmail.com> writes:

> Hi List,
>
> when you define a variable that should not be touched/set by the user of
> your library (which is most likely a programmer), but will be frequently
> set to a new value by your own program - would you call that a VAR or a
> CONST?
>
> (defconst ...) signals "don't touch this", which seems to the right
> thing in this case, but OTOH it is a bit strange to call something a
> constant that not only can be changed, but will actually be changed
> quite often.

In addition to the other answers, which make perfect sense, you can also
look in existing libraries and see what other people do. I've found that
pretty much all "internal" variables -- last-abbrev-location,
org-capture-current-plist, basically any variables used to preserve
state -- are defined with defvar.

HTH,
Eric

Re: defvar vs defconst

[Thorsten Jolitz]

Eric Abrahamsen <eric@ericabrahamsen.net> writes:

> Thorsten Jolitz <tjolitz@gmail.com> writes:
>
>> Hi List,
>>
>> when you define a variable that should not be touched/set by the user of
>> your library (which is most likely a programmer), but will be frequently
>> set to a new value by your own program - would you call that a VAR or a
>> CONST?
>>
>> (defconst ...) signals "don't touch this", which seems to the right
>> thing in this case, but OTOH it is a bit strange to call something a
>> constant that not only can be changed, but will actually be changed
>> quite often.
>
> In addition to the other answers, which make perfect sense, you can also
> look in existing libraries and see what other people do. I've found that
> pretty much all "internal" variables -- last-abbrev-location,
> org-capture-current-plist, basically any variables used to preserve
> state -- are defined with defvar.

Pretty clear picture - I will use 'defvar then. Thanks to everybody. 

-- 
cheers,
Thorsten