Is there a lambda function in Scala?

Question

I have a Java class that has access to a transaction context that I'd like to use from Scala. So I thought that I'd write a Java method that takes a Scala function and calls it inside the transaction -

class Context {
    @Transactional public void runInTx(Function0<scala.Unit> f) {
        f.apply();
    }

So far so good. I'd like to be able to pass a Scala closure to it

def deleteItems(ids: Set[Long]) = {
  context.runInTx { ids foreach { dao.delete(_) } }
}

but can't because runInTx isn't actually call by name, and ids foreach { dao.delete(_) } isn't actually a Function0.

Now I can solve this problem with a small thunk

def deleteItems(ids: Set[Long]) = {
  def localApply(f: => Unit) = context.applyInTx(f _)
  localApply { ids foreach { dao.delete(_) } }
}

but it seems to me that I need a lambda function to produce an unnamed Function0 out of a code block.

Does such a thing exist in the API, or how would I go about writing it?

Duncan McGregor · Accepted Answer · 2011-11-18 18:03:13Z

11

You can tell the compiler to interpret the block as a function rather than a statement to be called immediately by adding the (in this case lack of) parameters.

So in this case:

def deleteItems(ids: Set[Long]) = {
  context.runInTx {() => ids foreach { dao.delete(_) } }
}

() => ids foreach { dao.delete(_) } is the full form of the function literal. The compiler allows the parameter list to be omitted in places where it can infer that a function is required - this is true for call by name, but doesn't seem to be true when passing to a method taking a Function

edited Nov 18, 2011 at 18:03

answered Nov 18, 2011 at 17:44

Duncan McGregor

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