Scala Default Args

gistfile1.scala

scala> def addStringOneParam(input: String): String = input + "foo"
addStringOneParam: (input: String)String

scala> def addStringDefParam(input: String, toAdd: String = "foo"):String = input + toAdd
addStringDefParam: (input: String, toAdd: String)String

scala> addStringOneParam("baz")
res3: String = bazfoo

scala> addStringDefParam("baz")
res4: String = bazfoo

scala> val myMap = Map("one" -> "foo", "two" -> "bar")
myMap: scala.collection.immutable.Map[String,String] = Map(one -> foo, two -> bar)

scala> myMap.mapValues(addStringOneParam)
res5: scala.collection.immutable.Map[String,String] = Map(one -> foofoo, two -> barfoo)

scala> myMap.mapValues(addStringDefParam)
<console>:11: error: type mismatch;
 found   : (String, String) => String
 required: String => ?
              myMap.mapValues(addStringDefParam)
              
 scala> val mySeq = Array("one", "two", "three")
mySeq: Array[String] = Array(one, two, three)

scala> mySeq.map(addStringOneParam)
res8: Array[String] = Array(onefoo, twofoo, threefoo)

scala> mySeq.map(addStringDefParam)
<console>:11: error: type mismatch;
 found   : (String, String) => String
 required: String => ?
              mySeq.map(addStringDefParam)

Here's the answer to "why": http://stackoverflow.com/a/5582400