CODEWITHSUNDEEP
scalafunctional-programming
Hanfei Sun
Hanfei Sun

Reputation: 47061

In Scala, how to convert FilterMonadic to a List?

I got the FilterMonadic from the following line:

val f = ((2 to 10) withFilter (_ > 5) withFilter( _ < 8))

However, I found f doesn't have a toList method. Does anyone have ideas about how to convert it into a List?

And what are the main different between withFilter and filter method?

Upvotes: 1

Views: 602

Answers (2)

Kolmar
Kolmar

Reputation: 14224

The easiest way is probably f.map(identity), which doesn't necessarily return a List, but an appropriate type of sequence, based on the original sequence type before filtering.

If you want strictly a List, convert the result to List afterwards: f.map(identity).toList.

As for the difference, for most collections filter immediately performs the filtering, builds a new collection in memory and returns it, and withFilter returns an object, which stores the original collection and does the filtering only when an element is requested.

Upvotes: 2

tuxdna
tuxdna

Reputation: 8487

You can use flatMap:

scala> val f = ((2 to 10) withFilter (_ > 5) withFilter( _ < 8))
f: scala.collection.generic.FilterMonadic[Int,scala.collection.immutable.IndexedSeq[Int]] = scala.collection.TraversableLike$WithFilter@60b8da0

scala> f.flatMap(List(_)).toList
res0: List[Int] = List(6, 7)

Upvotes: 0

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