String concatenation gone functional

Question

Suppose there are 3 strings:

protein, starch, drink 

Concatenating those, we could say what is for dinner:

Example:

val protein = "fish" val starch = "chips" val drink = "wine"  val dinner = protein + ", " + starch + ", " + drink 

But what if something was missing, for example the protein, because my wife couldn't catch anything. Then, we will have: ,chips, drink for dinner.

There is a slick way to concatenate the strings to optionally add the commas - I just don't know what it is ;-). Does anyone have a nice idea?

I'm looking for something like:

val dinner = protein +[add a comma if protein is not lenth of zero] + starch ..... 

It's just a fun exercise I'm doing, so now sweat if it can't be done in some cool way. The reason that I'm trying to do the conditional concatenation in a single assignment, is because I'm using this type of thing a lot in XML and a nice solution will make things..... nicer.

Answer 1

When you say "it may be absent", this entity's type should be Option[T]. Then,

def dinner(components: List[Option[String]]) = components.flatten mkString ", " 

You would invoke it like this:

scala> dinner(None :: Some("chips") :: Some("wine") :: Nil) res0: String = chips, wine 

In case you absolutely want checking a string's emptiness,

def dinner(strings: List[String]) = strings filter {_.nonEmpty} mkString ", "  scala> dinner("" :: "chips" :: "wine" :: Nil) res1: String = chips, wine 

Answer 2

You're looking for mkString on collections, maybe.

val protein = "fish" val starch = "chips" val drink = "wine"  val complete = List (protein, starch, drink)  val partly =  List (protein, starch)   complete.mkString (", ") partly.mkString (", ") 

results in:

res47: String = fish, chips, wine res48: String = fish, chips 

You may even specify a start and end:

scala> partly.mkString ("<<", ", ", ">>") res49: String = <<fish, chips>>