Scala Option[(A, B)] pattern matching

Question

I am writing a Java code generator.

I have an immutable Map that contains a mapping from java.sql.Types [Int] to a tuple of (String, String) where the first value is a Java type and the second a Java package from which to import the type if it is not imported by default ( java.lang ):

val SqlTypesToJavaTypeNames =
    Map(Types.BIGINT -> ("Long", None),
        Types.BINARY -> ("byte[]", None),
        Types.BIT -> ("Boolean", None),
        Types.BOOLEAN -> ("Boolean", None),
        Types.CHAR -> ("String", None),
        Types.DATE -> ("Date", Some("java.sql.Date")),
        Types.DECIMAL -> ("BigDecimal", Some("java.math.BigDecimal")),
        Types.DOUBLE -> ("Double", None),
        Types.FLOAT -> ("Float", None),
        Types.INTEGER -> ("Integer", None),
        Types.LONGNVARCHAR -> ("String", None),
        Types.LONGVARCHAR -> ("String", None),
        Types.NCHAR -> ("String", None),
        Types.NUMERIC -> ("BigDecimal", None),
        Types.NVARCHAR -> ("String", None),
        Types.REAL -> ("Float", None),
        Types.SMALLINT -> ("Short", None),
        Types.SQLXML -> ("String", None),
        Types.TIME -> ("Time", Some("java.sql.Time")),
        Types.TIMESTAMP -> ("Timestamp", Some("java.sql.Timestamp")),
        Types.TINYINT -> ("Byte", None),
        Types.VARCHAR -> ("String", None))

I am trying to pattern match on a search of this map, where dataType is the java.sql.Types value from a database metadata:

val (javaType, importType) =
  SqlTypesToJavaTypeNames.get(dataType) match {
    case Some(jType, Some(iType)) => (jType, iType)
    case Some(jType, None) => (jType, null)
    case None => throw new IllegalStateException("Unknown translation to Java type for SQL type " + dataType)
  }

The compiler is giving me an error on the first case (starts with case Some(jType, Some(iType)) ): error: wrong number of arguments for <none>: (x: (java.lang.String, Option[java.lang.String]))Some[(java.lang.String, Option[java.lang.String])]

I'm not sure what is wrong.

Answer 1

Some doesn't extract to two values, it extracts to one. If you want to match some pair, then you need to double-up on the parentheses:

case Some( (jType, Some(iType)) ) => (jType, iType)

It would be nice if you could use the arrow convention as an extractor, but that sadly doesn't seem to work:

case Some(jType -> Some(iType)) => (jType, iType)

UPDATE

Alternatively, given that you're using an Option, you could take advantage of its monadic nature and simply map over the thing:

val tpes = SqlTypesToJavaTypeNames.get(dataType)
val (javaType, importType) =
  tpes map { case (a,b) => (a, b.orNull) } getOrElse { throw ... }

Answer 2

You are missing the inner parens (because you have an Option[(A, B)] :

case Some( (jType, Some(iType)) ) => 
case Some( (jType, _) )           =>
case None                         =>

From the looks of your method, it seems like you could simplify even more:

SqlTypesToJavaTypeNames.get(dataType) map { case (jType, maybeIType) => jType -> maybeIType.orNull } getOrElse error("Unmapped : " + dataType)