Scala: using foldl to add pairs from list to a map? - scala

Question

I am trying to add pairs from list to a map using foldl. I get the following error:
"missing arguments for method /: in trait TraversableOnce; follow this method with `_' if you want to treat it as a partially applied function"
code:
val pairs = List(("a", 1), ("a", 2), ("c", 3), ("d", 4))
def lstToMap(lst:List[(String,Int)], map: Map[String, Int] ) = {
(map /: lst) addToMap ( _, _)
}
def addToMap(pair: (String, Int), map: Map[String, Int]): Map[String, Int] = {
map + (pair._1 -> pair._2)
}
What is wrong?

Answer 1

scala> val pairs = List(("a", 1), ("a", 2), ("c", 3), ("d", 4))
pairs: List[(String, Int)] = List((a,1), (a,2), (c,3), (d,4))
scala> (Map.empty[String, Int] /: pairs)(_ + _)
res9: scala.collection.immutable.Map[String,Int] = Map(a -> 2, c -> 3, d -> 4)
But you know, you could just do:
scala> pairs.toMap
res10: scala.collection.immutable.Map[String,Int] = Map(a -> 2, c -> 3, d -> 4)

Answer 2

You need to swap the input values of addToMap and put it in parenthesis for this to work:
def addToMap( map: Map[String, Int], pair: (String, Int)): Map[String, Int] = {
map + (pair._1 -> pair._2)
}
def lstToMap(lst:List[(String,Int)], map: Map[String, Int] ) = {
(map /: lst)(addToMap)
}
missingfaktor's answer is much more concise, reusable, and scala-like.

Answer 3

If you already have a collection of Tuple2s, you don't need to implement this yourself, there is already a toMap method that only works if the elements are tuples!
The full signature is:
def toMap[T, U](implicit ev: <:<[A, (T, U)]): Map[T, U]
It works by requiring an implicit A <:< (T, U) which is essentially a function that can take the element type A and cast/convert it to tuples of type (T, U). Another way of saying this is that it requires an implicit witness that A is-a (T, U). Therefore, this is completely type-safe.
Update: which is what #missingfaktor said

Answer 4

This is not a direct answer to the question, which is about folding correctly on the map, but I deem it important to emphasize that
a Map can be treated as a generic Traversable of pairs
and you can easily combine the two!
scala> val pairs = List(("a", 1), ("a", 2), ("c", 3), ("d", 4))
pairs: List[(String, Int)] = List((a,1), (a,2), (c,3), (d,4))
scala> Map.empty[String, Int] ++ pairs
res1: scala.collection.immutable.Map[String,Int] = Map(a -> 2, c -> 3, d -> 4)
scala> pairs.toMap
res2: scala.collection.immutable.Map[String,Int] = Map(a -> 2, c -> 3, d -> 4)