Type mismatch in Scala's for-comprehension - scala

Question

I have tried to define a recursive Scala function that looks something like this:
def doSomething: (List[List[(Int, Int)]], List[(Int, Int)], Int, Int) => List[Int] =
(als, rs, d, n) =>
if (n == 0) {
for (entry <- rs if (entry._1 == d)) yield entry._2
} else {
for (entry <- rs; adj <- als(entry._1)) yield doSomething(als, rs.::((adj._1, adj._2 + entry._2)), d, n - 1)
}
Now, the compiler tells me:
| | | | | | <console>:17: error: type mismatch;
found : List[List[Int]]
required: List[Int]
for (entry <- rs; adj <- als(entry._1)) yield doSomething(als, rs.::((adj._1, adj._2 + entry._2)), d, n - 1)
^
I cannot figure out what the problem is. I'm sure that I'm using <- correctly. On the other hand, I'm a Scala newbie coming from the Java world...
Regarding the types of the input:
als : List[List[(Int,Int)]],
rs : List[(Int,Int)],
d and n : Int
The compiler error appears as soon as I tell IntelliJ to send my code to the Scala console.

Answer 1

When you yield an A when iterating on a List, you return a List[A]. doSomething returns a List[Int], so by yielding that you return a List[List[Int]]. You can unroll that like this:
def doSomethingElse(als: List[List[(Int, Int)]], rs: List[(Int, Int)], d: Int, n: Int): List[Int] =
if (n == 0) {
for ((k, v) <- rs if k == d) yield v
} else {
for {
(k, v) <- rs
(adjk, adjv) <- als(k)
item <- doSomethingElse(als, (adjk, adjv + v) :: rs, d, n - 1)
} yield item
}
Notice that I also used a method notation for brevity and destructured the pairs and leveraged the right-associativity of methods whose name ends in : for readability, feel free to use whatever convention you might want (but I don't see really a reading why having a method that returns a constant function (maybe you'd want to just use a val to declare it).
As a further note, you are using random access on a linear sequence (als(k)), you may want to consider an indexed sequence (like a Vector). More info on the complexity characteristics of the Scala Collection API can be found here.

Answer 2

for test purpose I created some sample data that meets the input datatypes as
val als = List(List((1,2), (3,4)), List((1,2), (3,4)), List((1,2), (3,4)))
//als: List[List[(Int, Int)]] = List(List((1,2), (3,4)), List((1,2), (3,4)), List((1,2), (3,4)))
val rs = List((1,2), (2,3))
//rs: List[(Int, Int)] = List((1,2), (2,3))
val d = 1
//d: Int = 1
val n = 3
//n: Int = 3
And in you doSomething function when n == 0 you are doing
for (entry <- rs if (entry._1 == d)) yield entry._2
//res0: List[Int] = List(2)
You can see that the return type is List[Int]
And for the else part you are calling recursively doSomething.
I have created dummy doSomething method of yours as your doSomething function definition lacks input variables as
def dosomething(nn: Int)={
for (entry <- rs if (entry._1 == d)) yield entry._2
}
and I call the method recursively as
for (entry <- rs; adj <- als(entry._1)) yield dosomething(0)
//res1: List[List[Int]] = List(List(2), List(2), List(2), List(2))
Clearly you can see that the second nested for loop is returning List[List[Int]]
And thats what the compiler is warning you
error: type mismatch;
found : List[List[Int]]
required: List[Int]
I hope the answer is helpful