Default value for structure

[Manfred Lotz]

Hi,
Can I have a default value for a member of a structure which is not
specified when doing a struct-map?

Minimal example: I have (defstruct st :a :b) and always when I define
something like this (struct-map st :a 4) omitting :b I would like to
have :b set automatically to 0.0 instead having nil.

Is this possible?

--
Manfred

[ataggart]

No, but if you need to do that, then you can do what deftype sort-of
does:

user=> (defstruct St :a :b)
#'user/St
user=> (defn st ([a] (struct St a 0.0)) ([a b] (struct St a b)))
#'user/st
user=> (st 5)
{:a 5, :b 0.0}

[Manfred Lotz]

On Mon, 1 Mar 2010 11:01:36 -0800 (PST)
ataggart <alex.t...@gmail.com> wrote:

> No, but if you need to do that, then you can do what deftype sort-of
> does:
>
> user=> (defstruct St :a :b)
> #'user/St
> user=> (defn st ([a] (struct St a 0.0)) ([a b] (struct St a b)))
> #'user/st
> user=> (st 5)
> {:a 5, :b 0.0}
>
>

Thanks for this.

Now I tried a different way:

(defstruct st :a :b)

(defn my-struct-map [s & inits]
(let [sm (struct-map s inits)]
(if (= nil (sm :b))
(assoc sm :b 0.0)
sm))
)

Unfortunately, the part sm (struct-map s inits) doesn't work. I have no
idea what is wrong with my code.

--
Manfred

[Mike Mazur]

Hi,

On Sat, Mar 6, 2010 at 20:36, Manfred Lotz <manfre...@arcor.de> wrote:
> Now I tried a different way:
>
> (defstruct st :a :b)
>
> (defn my-struct-map [s & inits]
>  (let [sm (struct-map s inits)]
>    (if (= nil (sm :b))
>      (assoc sm :b 0.0)
>      sm))
>  )
>
> Unfortunately, the part sm (struct-map s inits) doesn't work. I have no
> idea what is wrong with my code.

The problem is with the inits argument passed to struct-map. Your
method will pass a sequence. For instance, if you invoke
(my-struct-map st :a 1 :b 2), struct-map is invoked like this:
(struct-map st (:a 1 :b 2)).

I didn't know how to fix this myself, but nteon on the IRC helped out
and pointed me to apply. So, this definition now works for me:

(defn my-struct-map [s & inits]

(let [sm (apply struct-map s inits)]

(if (= nil (sm :b))
(assoc sm :b 0.0)
sm)))

Hope that helps,
Mike

[b2m]

Hi,

depending on your application you might use defnk (http://
richhickey.github.com/clojure-contrib/def-api.html#clojure.contrib.def/
defnk) to build your struct with default values.

Benjamin

[Manfred Lotz]

On Sun, 7 Mar 2010 12:35:27 +0800
Mike Mazur <mma...@gmail.com> wrote:

> Hi,
>
> On Sat, Mar 6, 2010 at 20:36, Manfred Lotz <manfre...@arcor.de>
> wrote:
> > Now I tried a different way:
> >
> > (defstruct st :a :b)
> >
> > (defn my-struct-map [s & inits]
> >  (let [sm (struct-map s inits)]
> >    (if (= nil (sm :b))
> >      (assoc sm :b 0.0)
> >      sm))
> >  )
> >
> > Unfortunately, the part sm (struct-map s inits) doesn't work. I
> > have no idea what is wrong with my code.
>
> The problem is with the inits argument passed to struct-map. Your
> method will pass a sequence. For instance, if you invoke
> (my-struct-map st :a 1 :b 2), struct-map is invoked like this:
> (struct-map st (:a 1 :b 2)).
>

Oh yes, I was not aware of this, although it is pretty logical.

> I didn't know how to fix this myself, but nteon on the IRC helped out
> and pointed me to apply. So, this definition now works for me:
>
> (defn my-struct-map [s & inits]
> (let [sm (apply struct-map s inits)]
> (if (= nil (sm :b))
> (assoc sm :b 0.0)
> sm)))
>

Yep, helped.

--
Thanks,
Manfred