Understanding `andThen`
function-composition, scala
Solution
`andThen` is just function composition. Given a function `f`
val f: String => Int = s => s.length
`andThen` creates a new function which applies `f` followed by the argument function
val g: Int => Int = i => i * 2
val h = f.andThen(g)
`h(x)` is then `g(f(x))`
Problem
I encountered `andThen`, but did not properly understand it. To look at it further, I read the Function1.andThen docs ``` def andThen[A](g: (R) ⇒ A): (T1) ⇒ A ``` `mm` is a MultiMap instance. ``` scala> mm res29: scala.collection.mutable.HashMap[Int,scala.collection.mutable.Set[String]] with scala.collection.mutable.MultiMap[Int,String] = Map(2 -> Set(b) , 1 -> Set(c, a)) scala> mm.keys.toList.sortWith(_ < _).map(mm.andThen(_.toList)) res26: List[List[String]] = List(List(c, a), List(b)) scala> mm.keys.toList.sortWith(_ < _).map(x => mm.apply(x).toList) res27: List[List[String]] = List(List(c, a), List(b)) ``` Note - code from DSLs in Action Is `andThen` powerful? Based on this example, it looks like `mm.andThen` de-sugars to `x => mm.apply(x)`. If there is a deeper meaning of `andThen`, then I haven’t understood it yet.