四种不同的函数，具有不同的返回类型，相同的错误

Question

I am studying scala and slick. And I got an error like this:

found :   Option[Int]
required: Int
  def update(c: Color): Int = findById(c.id).update(c)

I am not sure what is found and required stand for. So I add other functions:

def update(c: Color): Int          = findById(c.id).update(c)
def update2(c: Color): Option[Int] = findById(c.id).update(c)
def update3(c: Color): String      = findById(c.id).update(c)
def update4(c: Color): Unit        = findById(c.id).update(c)

And expecting different found and required , but same error came out:

found :   Option[Int]
required: Int
  def update(c: Color): Int = findById(c.id).update(c)

found :   Option[Int]
required: Int
  def update2(c: Color): Option[Int] = findById(c.id).update(c)

found :   Option[Int]
required: Int
  def update3(c: Color): String      = findById(c.id).update(c)

found :   Option[Int]
required: Int
  def update4(c: Color): Unit        = findById(c.id).update(c)

Why same error came out? What is found and required stand for? Thanks.

Answer 1

The source of the error message is this part of your code

findById(c.id)

This part is the same for all four examples given. Thus the same error message.

findById( id ) expects an Int as an argument but c.id returns an Option[Int] .

A possible solution would be to map over c.id

c.id map ( id => findById(id) update c  )

then it would return an Option[Int] as required by your update2 .

Or you could use a for comprehension which would return an Int as your function update requires.

for {
  id <- c.id
  elem = findBy(id)
} yield elem.update(c)