Unclear return value in Scala function

Question

This is my Scala function:

def combine(occur: Occurrences): List[List[Word]] = {
  if (occur.isEmpty) List(List()) 
  else {
    for {
      combintaion <- combinations(occur)
      words = getWords(combintaion)
      if !words.isEmpty
      word: Word <- words
      rest <- combine(subtract(occur, combintaion))
    } yield List(word) :: rest
  }
}

I expect it to return List[List[Word]], but compiler says, that for expression returns List[List[Object]]. Why so, and what should I do?

You should change yield List(word) :: rest to yield word :: rest — Eastsun
– Eastsun, Commented Feb 1, 2014 at 13:28

Eastsun · Accepted Answer · 2014-02-01 13:30:10Z

5

You should change

yield List(word) :: rest

to

yield word :: rest

since rest is type of List[Word] and word :: rest is still type of List[Word], hence for(..) yield word :: rest is List[List[Word]].

answered Feb 1, 2014 at 13:30

Eastsun

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Comments

Ravi · Accepted Answer · 2014-02-01 12:48:34Z

0

You are returning two type of values here.

Use

val result:List[List[Word]]=List()
return result;

Insted of

if (occur.isEmpty) List(List())

because its return type is List[List[Nothing]]

answered Feb 1, 2014 at 12:48

Ravi

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