scala: how to avoid mutables inside try catch blocks

Question

I'm trying to avoid mutables variables, but the problem is that I have to access a val that I need to initialize inside the try (it's a db operation that migth fail) and I need that var in the finally block

I tried with several alternatives:

declaring the val inside the try block

try {
  val resultSet = SQL(sql).resultSet
  return ColumnInfo(resultSet.getMetaData)
} catch {
  case e => throw new ColumnInfoException("Error getting metadata")
} finally {
  resultSet.close
}

error: not found: value resultSet

declaring the val outside the try block without initializing it

val resultSet: java.sql.ResultSet
try {
  resultSet = SQL(sql).resultSet
  return ColumnInfo(resultSet.getMetaData)
} catch {
  case e => throw new ColumnInfoException("Error getting metadata")
} finally {
  resultSet.close
}

error: only classes can have declared but undefined members

using a var, which seems to work

var resultSet: java.sql.ResultSet = null
try {
  resultSet = SQL(sql).resultSet
  return ColumnInfo(resultSet.getMetaData)
} catch {
  case e => throw new ColumnInfoException("Error getting metadata")
} finally {
  resultSet.close
}

and finally nesting try-catch block, which seems rather dirty

try {
  val resultSet = SQL(sql).resultSet
  try {
    return ColumnInfo(resultSet.getMetaData)
  } catch {
    case e => throw new ColumnInfoException("Error getting metadata")
  } finally {
    resultSet.close
  }
} catch {
  case e => throw new ColumnInfoException("Error opening resultSet")
}

Is there some better approach I can take to avoid using vars and nesting try-catch blocks?

score 3 · Answer 1 · answered Sep 11 '12 at 06:08

import scala.util.control.Exception._

allCatch.either(SQL(sql).resultSet).right.flatMap{ resultSet =>
  val ans = allCatch.either(ColumnInfo(resultSet.getMetaData))
  resultSet.close
  ans
}.fold(e => throw e, identity)

or you can skip the fold and keep the exception(s) packaged in Left.

score 2 · Answer 2 · edited May 23 '17 at 11:55

2

from this question functional try & catch w/ Scala

I learned about the loan pattern: https://wiki.scala-lang.org/display/SYGN/Loan

Play framework itself seems to use it with the DB.withConnection method

edited May 23 '17 at 11:55

Community

1
1

answered Sep 11 '12 at 05:17

opensas

60,462
79
252
386

score 2 · Answer 3 · answered Oct 08 '14 at 10:05

In Scala, try block is an expression. In your case it can evaluate to a tuple containing both vals:

val res: (java.sql.ResultSet, ColumnInfo) = try {
  val rs = SQL(sql).resultSet
  (rs, ColumnInfo(rs.getMetaData))
} catch {
  case _: Throwable => throw new Exception("Error getting metadata")
} finally {
  res._1.close
}
val columnInfo = res._2

score 0 · Answer 4 · answered Sep 11 '12 at 05:17

0

Get rid of the first catch block:

try {
  val resultSet = SQL(sql).resultSet
  try {
    ColumnInfo(resultSet.getMetaData)
  } finally {
    resultSet.close
  }
} catch {
  case e => throw new ColumnInfoException("Error opening resultSet")
}

answered Sep 11 '12 at 05:17

nilskp

3,097
1
30
34

the second catch would also catch any exception from the first try, right? (upvoted, cause it improves my own version...) – opensas Sep 12 '12 at 15:08
Yes, it will close the result set, but leave the catching to the outer try block. – nilskp Sep 12 '12 at 17:32

scala: how to avoid mutables inside try catch blocks

4 Answers4