Difference between mapValues and transform in Map

Question

In Scala Map (see API) what is the difference in semantics and performance between mapValues and transform ?

For any given map

Answer 1

collection.Map doesn't provide transform: it has a different signature for mutable and immutable Maps.

$ scala
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scala> val im = Map('a -> 1, 'b -> 2, 'c -> 3)
im: scala.collection.immutable.Map[Symbol,Int] = Map('a -> 1, 'b -> 2, 'c -> 3)

scala> im.mapValues(_ * 7) eq im
res0: Boolean = false

scala> im.transform { case (k,v) => v*7 } eq im
res2: Boolean = false

scala> val mm = collection.mutable.Map('a -> 1, 'b -> 2, 'c -> 3)
mm: scala.collection.mutable.Map[Symbol,Int] = Map('b -> 2, 'a -> 1, 'c -> 3)

scala> mm.mapValues(_ * 7) eq mm
res3: Boolean = false

scala> mm.transform { case (k,v) => v*7 } eq mm
res5: Boolean = true

Mutable transform mutates in place:

scala> mm.transform { case (k,v) => v*7 }
res6: mm.type = Map('b -> 98, 'a -> 49, 'c -> 147)

scala> mm.transform { case (k,v) => v*7 }
res7: mm.type = Map('b -> 686, 'a -> 343, 'c -> 1029)

So mutable transform doesn't change the type of the map:

scala> im mapValues (_ => "hi")
res12: scala.collection.immutable.Map[Symbol,String] = Map('a -> hi, 'b -> hi, 'c -> hi)

scala> mm mapValues (_ => "hi")
res13: scala.collection.Map[Symbol,String] = Map('b -> hi, 'a -> hi, 'c -> hi)

scala> mm.transform { case (k,v) => "hi" }
:9: error: type mismatch;
 found   : String("hi")
 required: Int
              mm.transform { case (k,v) => "hi" }
                                           ^

scala> im.transform { case (k,v) => "hi" }
res15: scala.collection.immutable.Map[Symbol,String] = Map('a -> hi, 'b -> hi, 'c -> hi)

...as can happen when constructing a new map.