scala newbie having troubles with Option, what's the equivalent of the ternary operator

Question

I've already read that the if statement in scala always returns an expression

So I'm trying to do the following (pseudo code)

sql = "select * from xx" + iif(order.isDefined, "order by " order.get, "")

I'm trying with

val sql: String = "select * from xx" + if (order.isDefined) {" order by " + order.get} else {""}

But I get this error:

illegal start of simple expression

order is an Option[String]

I just want to have an optional parameter to a method, and if that parameter (in this case order) is not passed then just skip it

what would be the most idiomatic way to achieve what I'm trying to do?

-- edit --

I guess I hurried up too much to ask

I found this way,

val orderBy = order.map( " order by " + _ ).getOrElse("")

Is this the right way to do it?

I thought map was meant for other purposes...

The reason your code isn't compiling is because you need parentheses around the `if` expression. But as Tomasz Nurkiewicz points out, there are better ways to write this anyway. — Tom Crockett, Apr 01 '12 at 21:54
Anyone want to comment on *why* we need brackets around the if-expression (other than "because it compiles")? — Luigi Plinge, Apr 01 '12 at 22:00
@LuigiPlinge this is just speculation, but it may be that they wanted to avoid programmer confusion as to how an expression like `1 + if (b) 2 else 3 + 4` would associate. Is it `1 + (if (b) 2 else 3) + 4` or `1 + (if (b) 2 else 3 + 4)`? Certainly you could have a grammar which resolves this ambiguity (Haskell does), but that doesn't mean people wouldn't make mistakes because they assumed a different behavior. — Tom Crockett, Apr 01 '12 at 22:31

Tomasz Nurkiewicz · Accepted Answer · 2012-04-01T21:53:36.167

11

First of all you are not using Option[T] idiomatically, try this:

"select * from xx" + order.map(" order by " + _).getOrElse("")

or with different syntax:

"select * from xx" + (order map {" order by " + _} getOrElse "")

Which is roughly equivalent to:

"select * from xx" + order match {
  case Some(o) => " order by " + o
  case None => ""
}

Have a look at scala.Option Cheat Sheet. But if you really want to go the ugly way of ifs (missing parentheses around if):

"select * from xx" + (if(order.isDefined) {" order by " + order.get} else {""})

edited Apr 01 '12 at 21:53

answered Apr 01 '12 at 21:48

Tomasz Nurkiewicz

Thanks a lot, Tomasz, I just found it on google... And yes, I'm trying to learn the most idiomatic way to work with Scala... – opensas Apr 01 '12 at 21:52

score 0 · Answer 2 · answered Apr 02 '12 at 17:56

0

...or, if you really want to impress your friends:

order.foldLeft ("") ((_,b)=>"order by  " + b)

(I would still recommend Tomasz's answer, but I think this one is not included in the scala.Option cheat sheet, so i thought I'd mention it)

answered Apr 02 '12 at 17:56

Paolo Falabella

1

If you're gonna fold, why not use the prefix as the initial value, and do the whole thing in one go? `val sql = order.foldLeft("select * from xx")((_,b)=>" order by " + b) ` – Andrzej Doyle Apr 23 '12 at 10:13
@AndrzejDoyle you're right, that's a way to do it... I was imagining this to be part of a larger dynamically constructed query where you have a function that builds only the "order by" part, another one that builds the "where" clauses and you put them all together at the end – Paolo Falabella Apr 23 '12 at 10:50

2 Answers2