A "Simple' Scala question but took me long time to debug - scala

Question

Please check the two pieces of script as above.
genComb4(lst) works since I put z <- genComb4(xs) before i <- 0 to x._2 in the for-comprehension; genComb(lst) does not work since I change the order of these two lines in for-comprehension.
It took me almost half day to find this bug, but I cannot explain it by myself. Could you tell me why this happened?
Thank you very much in advance.
// generate combinations
val nums = Vector(1, 2, 3)
val strs = Vector('a', 'b', 'c')
val lst: List[(Char, Int)] = strs.zip(nums).toList
def genComb4(lst: List[(Char, Int)]): List[List[(Char, Int)]] = lst match {
case Nil => List(List())
case x :: xs =>
for {
z <- genComb4(xs) // correct
i <- 0 to x._2 // correct
} yield ( (x._1, i) :: z)
}
genComb4(lst)
def genComb(lst: List[(Char, Int)]): List[List[(Char, Int)]] = lst match {
case Nil => List(List())
case x :: xs =>
for {
i <- (0 to x._2) // wrong
z <- genComb(xs) // wrong
} yield ( (x._1, i) :: z)
}
genComb(lst)

Answer 1

It's because of different types of container in for comprehension. When you start for-comprehension from line: i <- (0 to x._2) it's set type of result container as IndexedSeq but in case where first line is z <- genComb4(xs) the type of result container is List, take a look:
val x = 'a' -> 2
val indices: Seq[Int] = 0 to x._2
val combs: List[List[(Char, Int)]] = genComb4(List(x))
// indexed sequence
val indicesInFor: IndexedSeq[(Char, Int)] = for {
i <- 0 to x._2
} yield (x._1, i)
// list
val combsInFor: List[List[(Char, Int)]] = for {
z <- genComb4(List(x))
} yield z
so for make your second case is working, you should cast (0 to x._2).toList:
val indicesListInFor: List[(Char, Int)] = for {
i <- (0 to x._2).toList
} yield (x._1, i)
result code should be:
def genComb(lst: List[(Char, Int)]): List[List[(Char, Int)]] = lst match {
case Nil => List(List())
case x :: xs =>
for {
i <- (0 to x._2).toList
z <- genComb(xs)
} yield ( (x._1, i) :: z)
}
genComb(lst)
You should remember about type of starting line in for-comprehension and inheritance of scala collections. If next types in for-comprehension can't be converted by inheritance rules to the first expression line type you should take care about it by yourself.
Good practise is unwrap for-expression into flatMap, map and withFilter functions, then you will find miss-typing or something else faster.
useful links:
how does yield work, scala documentation
quick review of scala for-comprehensions
for-expressions, scala documentation