Scala collections

Eric,

2.7.2 does this already:
scala> List(1,2,3).map(_ + 1)
res62: List[Int] = List(2, 3, 4)

scala> Array(1,2,3).map(_ + 1)
res63: Array[Int] = Array(2, 3, 4)

What collections are not returning the correct type?

Thanks,

David

On Mon, Dec 29, 2008 at 5:35 PM, Eric Torreborre <etorreborre@yahoo.com> wrote:

Hi,

I think I've read that Scala collection library is undergoing change.

I had this issue today where I needed Seq.map to return the underlying type
of my seq:

List(1, 2).map(_ + 1) // returns a Seq and not a List

Is it planned to have the Seq trait methods return the underlying collection
type when possible?
What kind of side effects would there be with such a design?
Is my question irrelevant and the sign that something is wrong in the way I
handle collections in my program?

Thanks.


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--
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You're right, I need to be more explicit with what I'm doing,...

My issue is that this doesn't compile:

val l1: Seq[Int] = List(1, 2)
val l2: List[Int] = l1.map((x:Int) => x)

:5: error: type mismatch;
found : Seq[Int]
required: List[Int]
val l2: List[Int] = l.map((x:Int) => x)

If I change the type of l1 to List[Int] this is ok.

Reading this I realize that this is perfectly understandable,...

So let's transform that to another question because my objective was to add
a mapFilter method on Seq objects which would combine a map and a filter in
one pass:

something like:

def mapFilter(seq: Seq[T], f: U => Option[U]) = {
// to be implemented with something like
seq.map(f(u)).filter(_.isDefined).map(_.get)
}

Oh,... time to answer my own question. This works ok:

List(1, 2).flatMap((x:Int) => if (x%2 == 0) Some(x*2) else None)

// returns List(4)

There's no need for a mapFilter function and I need more of this Monadic
stuff in my veins.

Thanks,

Eric.

David Pollak-4 wrote:
>
> Eric,
>
> 2.7.2 does this already:
> scala> List(1,2,3).map(_ + 1)
> res62: List[Int] = List(2, 3, 4)
>
> scala> Array(1,2,3).map(_ + 1)
> res63: Array[Int] = Array(2, 3, 4)
>
> What collections are not returning the correct type?
>
> Thanks,
>
> David
>
> On Mon, Dec 29, 2008 at 5:35 PM, Eric Torreborre
> wrote:
>
>>
>> Hi,
>>
>> I think I've read that Scala collection library is undergoing change.
>>
>> I had this issue today where I needed Seq.map to return the underlying
>> type
>> of my seq:
>>
>> List(1, 2).map(_ + 1) // returns a Seq and not a List
>>
>> Is it planned to have the Seq trait methods return the underlying
>> collection
>> type when possible?
>> What kind of side effects would there be with such a design?
>> Is my question irrelevant and the sign that something is wrong in the way
>> I
>> handle collections in my program?
>>
>> Thanks.
>>
>>
>> --
>> View this message in context:
>> http://www.nabble.com/Scala-collections-tp21211931p21211931.html
>> Sent from the Scala mailing list archive at Nabble.com.
>>
>>
>
>

Take a flatMap and to foldLeft's and call me in the morning. ;-)

On Dec 29, 2008 7:37 PM, "Eric Torreborre" <etorreborre@yahoo.com> wrote:


You're right, I need to be more explicit with what I'm doing,...

My issue is that this doesn't compile:

val l1: Seq[Int] = List(1, 2)
val l2: List[Int] = l1.map((x:Int) => x)

<console>:5: error: type mismatch;
 found   : Seq[Int]
 required: List[Int]
      val l2: List[Int] = l.map((x:Int) => x)

If I change the type of l1 to List[Int] this is ok.

Reading this I realize that this is perfectly understandable,...

So let's transform that to another question because my objective was to add
a mapFilter method on Seq objects which would combine a map and a filter in
one pass:

something like:

def mapFilter(seq: Seq[T], f: U => Option[U]) = {
 // to be implemented with something like
seq.map(f(u)).filter(_.isDefined).map(_.get)
}

Oh,... time to answer my own question. This works ok:

List(1, 2).flatMap((x:Int) => if (x%2 == 0) Some(x*2) else None)

// returns List(4)

There's no need for a mapFilter function and I need more of this Monadic
stuff in my veins.

Thanks,

Eric.








David Pollak-4 wrote:
>
> Eric,
>
> 2.7.2 does this already:
> scala> List(1,2,3).map(_ + 1)
> res62: List[Int] = List(2, 3, 4)
>
> scala> Array(1,2,3).map(_ + 1)
> res63: Array[Int] = Array(2, 3, 4)
>
> What collections are not returning the correct type?
>
> Thanks,
>
> David
>
> On Mon, Dec 29, 2008 at 5:35 PM, Eric Torreborre
> <etorreborre@yahoo.com>wrote:
>
>>
>> Hi,
>>
>> I think I've read that Scala collection library is undergoing change.
>>
>> I had this issue today where I needed Seq.map to return the underlying
>> type
>> of my seq:
>>
>> List(1, 2).map(_ + 1) // returns a Seq and not a List
>>
>> Is it planned to have the Seq trait methods return the underlying
>> collection
>> type when possible?
>> What kind of side effects would there be with such a design?
>> Is my question irrelevant and the sign that something is wrong in the way
>> I
>> handle collections in my program?
>>
>> Thanks.
>>
>>
>> --
>> View this message in context:
>> http://www.nabble.com/Scala-collections-tp21211931p21211931.html
>> Sent from the Scala mailing list archive at Nabble.com.
>>
>>
>
>
> --
> Lift, the simply functional web framework http://liftweb.net
> Collaborative Task Management http://much4.us
> Follow me: http://twitter.com/dpp
> Git some: http://github.com/dpp
>
>

--
View this message in context: http://www.nabble.com/Scala-collections-tp21211931p21212744.html
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If flatMap didn't already do the trick, the higher-kinded collection library would let you write your mapFilter function, looking maybe something like:

  def mapFilter[S, T, C[X] <: Iterable[X]](c: C[S], f: S => Option[T]): C[T] = {
    val b = c.newBuilder[T]
    for (x <- c; y <- f(x)) b ++= y
    b.result
  }

--j

On Mon, Dec 29, 2008 at 9:37 PM, Eric Torreborre <etorreborre@yahoo.com> wrote:

You're right, I need to be more explicit with what I'm doing,...

My issue is that this doesn't compile:

val l1: Seq[Int] = List(1, 2)
val l2: List[Int] = l1.map((x:Int) => x)

<console>:5: error: type mismatch;
 found   : Seq[Int]
 required: List[Int]
      val l2: List[Int] = l.map((x:Int) => x)

If I change the type of l1 to List[Int] this is ok.

Reading this I realize that this is perfectly understandable,...

So let's transform that to another question because my objective was to add
a mapFilter method on Seq objects which would combine a map and a filter in
one pass:

something like:

def mapFilter(seq: Seq[T], f: U => Option[U]) = {
 // to be implemented with something like
seq.map(f(u)).filter(_.isDefined).map(_.get)
}

Oh,... time to answer my own question. This works ok:

List(1, 2).flatMap((x:Int) => if (x%2 == 0) Some(x*2) else None)

// returns List(4)

There's no need for a mapFilter function and I need more of this Monadic
stuff in my veins.

Thanks,

Eric.








David Pollak-4 wrote:
>
> Eric,
>
> 2.7.2 does this already:
> scala> List(1,2,3).map(_ + 1)
> res62: List[Int] = List(2, 3, 4)
>
> scala> Array(1,2,3).map(_ + 1)
> res63: Array[Int] = Array(2, 3, 4)
>
> What collections are not returning the correct type?
>
> Thanks,
>
> David
>
> On Mon, Dec 29, 2008 at 5:35 PM, Eric Torreborre
> <etorreborre@yahoo.com>wrote:
>
>>
>> Hi,
>>
>> I think I've read that Scala collection library is undergoing change.
>>
>> I had this issue today where I needed Seq.map to return the underlying
>> type
>> of my seq:
>>
>> List(1, 2).map(_ + 1) // returns a Seq and not a List
>>
>> Is it planned to have the Seq trait methods return the underlying
>> collection
>> type when possible?
>> What kind of side effects would there be with such a design?
>> Is my question irrelevant and the sign that something is wrong in the way
>> I
>> handle collections in my program?
>>
>> Thanks.
>>
>>
>> --
>> View this message in context:
>> http://www.nabble.com/Scala-collections-tp21211931p21211931.html
>> Sent from the Scala mailing list archive at Nabble.com.
>>
>>
>
>
> --
> Lift, the simply functional web framework http://liftweb.net
> Collaborative Task Management http://much4.us
> Follow me: http://twitter.com/dpp
> Git some: http://github.com/dpp
>
>

--
View this message in context: http://www.nabble.com/Scala-collections-tp21211931p21212744.html
Sent from the Scala mailing list archive at Nabble.com.

Yes, I also tried doing something like that when I remembered that I needed
the "Builder" abstraction described in the "Generics of a higher kind"
paper.

And this is going to be part of the rework of the Scala collections, right?

E.

Yup, Builder is baked in to the new collections.

You can check out the (in progress) code:

http://lampsvn.epfl.ch/trac/scala/browser/scala/trunk/src/library/scalax/collection/

--j

On Tue, Dec 30, 2008 at 1:19 AM, Eric Torreborre <etorreborre@yahoo.com> wrote:

Yes, I also tried doing something like that when I remembered that I needed
the "Builder" abstraction described in the "Generics of a higher kind"
paper.

And this is going to be part of the rework of the Scala collections, right?

E.

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View this message in context: http://www.nabble.com/Scala-collections-tp21211931p21213910.html
Sent from the Scala mailing list archive at Nabble.com.

On Tue, Dec 30, 2008 at 8:19 AM, Eric Torreborre wrote:
>
> Yes, I also tried doing something like that when I remembered that I needed
> the "Builder" abstraction described in the "Generics of a higher kind"
> paper.
>
> And this is going to be part of the rework of the Scala collections, right?
>
Yes, it is.

But I don't think you need to define mapFilter in any case. flatMap
alone does the trick:

scala> val xs = List(1, 2, 3)
xs: List[Int] = List(1, 2, 3)

scala> def f(x: Int) = if (x % 2 == 1) Some(x) else None
f: (Int)Option[Int]

scala> xs flatMap f
res0: List[Int] = List(1, 3)

Or, if you prefer:

for (xo <- List(xs); x <- xo) yield x

The for expression expands into the same flatMap.

Cheers

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Eric Torreborre wrote:
> Oh,... time to answer my own question. This works ok:
>
> List(1, 2).flatMap((x:Int) => if (x%2 == 0) Some(x*2) else None)
>
> // returns List(4)
>
> There's no need for a mapFilter function and I need more of this
> Monadic stuff in my veins.
>
> Thanks,
>
> Eric.
>

Hi Eric,
You can write mapFilter using Scalaz which has a MonadEmptyPlus data
structure. I suggest you do not use the available subversion of the
List.flatMap type signature from which to learn ("Monadic stuff in my
veins").

- --
Tony Morris
http://tmorris.net/

S, K and I ought to be enough for anybody.

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