scala: not possible to match on value expressions? -

why isn't possible place expressions in case statements? :

  x match {     case <value expr> => {}   } 

for example

  x match {     case (1+2) => {}   } 

is not allowed, but

  val someval = (1+new java.util.random().nextint())    x match {     case someval => {}   } 

is. seems if second case allowed, should first.

i'm trying add list of mappings this:

  val typ:type = symbol.typesignature    typ match {     case typeof[collection.immutable.list[any]] => { return function handle type }     case typeof[...] => {}     case typeof[...] => {}     ... many   } 

it defies definition of patterns in pattern matching.

from scala language specification, chapter 8:

a pattern built constants, constructors, variables , type tests

"expressions" not in list, , makes sense - think - when expression evaluated? if has side-effects, evaluated if case preceding used return result?

a valid workaround using guard, is evaluated:

x match {   case if == (1+2) => {} // works expected... }