Re: unnecessary object creation in copy with changes methods

There are several exceptions. For example IntMap and LongMap seem to
do the right thing when using filter:

scala> val i0=IntMap(1->2, 3->4)
i0: scala.collection.immutable.IntMap[Int] = IntMap(1 -> 2, 3 -> 4)

scala> val i1=i0.filter(_=>true)
i1: scala.collection.immutable.IntMap[Int] = IntMap(1 -> 2, 3 -> 4)

scala> i0 eq i1
res32: Boolean = true

On Wed, Nov 2, 2011 at 4:42 PM, Daniel Sobral wrote:
> On Wed, Nov 2, 2011 at 12:07, rklaehn wrote:
>> Hi all,
>>
>> I noticed that the scala collections make no attempts to reduce
>> unnecessary object creation when a collection is unchanged by an
>> operation. For example, Seq.filter:
>
> There's one exception to this, which is scala.xml.transform. See
> method transform at
> https://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src//library/scal....
> Note that it needs to create the alternate version anyway. See also
> SI-3689 (https://issues.scala-lang.org/browse/SI-3689), where I
> explain how that simple code has the nasty consequence of turning
> transformations exponential in their depth.
>
> With very few exceptions -- such as filter -- there must be object
> creation if only to compare to the previous version. If in your own
> code it is likely that the collection is unchanged, then adapt your
> algorithm to apply an exists test before filtering.
>
> --
> Daniel C. Sobral
>
> I travel to the future all the time.
>