Scala function partial application

Question

I'm trying to understand how function partial application works in Scala.

To do that, I've built this simple code:

object Test extends App { myCustomConcat("General", "Public", "License") foreach print GeneralPublicLicenceAcronym(myCustomConcat(_)) foreach print def myCustomConcat(strings: String*): List[Char] = { val result = for (s <- strings) yield { s.charAt(0) } result.toList } def GeneralPublicLicenceAcronym (concatFunction: (String*) => List[Char] ) = { myCustomConcat("General", "Public", "License") } }

myCostumConcat function takes in input an array of String and it returns a list containing the first letter of each string.

So, the code

myCustomConcat("General", "Public", "License") foreach print

will print on console: GPL

Suppose now that I want to write a function to generate the GPL acronym, using (as input parameter) my previous function extracting the first letter of each string:

def GeneralPublicLicenceAcronym (concatFunction: (String*) => List[Char] ): List[Char] = { myCustomConcat("General", "Public", "License") }

Running this new function with partial application:

GeneralPublicLicenceAcronym(myCustomConcat(_)) foreach print

I get this error:

Error:(8, 46) type mismatch; found : Seq[String] required: String GeneralPublicLicenceAcronym(myCustomConcat(_)) foreach print

Why? Can I use partial application in this case?

DNA · Accepted Answer · 2015-06-18 20:19:53Z

All you need to do is change myCustomConcat(_) to myCustomConcat _, or indeed just myCustomConcat

What you are doing isn't exactly partial application - it's just using a method as a function value.

In some cases (where a function value is expected) the compiler will work out what you mean, but in other contexts you often need to tell the compiler your intention, using the _ suffix.

"partial application" means that we are supplying some, but not all, of the arguments to a function, to create a new function, for example:

 def add(x: Int, y: Int) = x + y //> add: (x: Int, y: Int)Int val addOne: Int => Int = add(1, _) //> addOne : Int => Int = <function1> addOne(2) //> res0: Int = 3

I suppose your case could be seen as partial application, but applying none of the arguments - you can use partial application syntax here, but you need to give a _* hint to the compiler because of the repeated parameters (String*), which ends up a bit ugly:

myCustomConcat(_:_*)

See also: Scala type ascription for varargs using _* cause error

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