When to use parenthesis in Scala infix notation

Question

When programming in Scala, I do more and more functional stuff. However, when using infix notation it is hard to tell when you need parenthesis and when you don't.

For example the following piece of code:

def caesar(k:Int)(c:Char) = c match {
    case c if c isLower => ('a'+((c-'a'+k)%26)).toChar
    case c if c isUpper => ('A'+((c-'A'+k)%26)).toChar
    case _ => c
}

def encrypt(file:String,k:Int) = (fromFile(file) mkString) map caesar(k)_

The (fromFile(file) mkString) needs parenthesis in order to compile. When removed I get the following error:

Caesar.scala:24: error: not found: value map
    def encrypt(file:String,k:Int) = fromFile(file) mkString map caesar(k)_
                                                                 ^
one error found

mkString obviously returns a string on which (by implicit conversion AFAIK)I can use the map function.

Why does this particular case needs parentheses? Is there a general guideline on when and why you need it?

Answer 1

This is what I put together for myself after reading the spec:

• Any method which takes a single parameter can be used as an infix operator: a.m(b) can be written a m b.
• Any method which does not require a parameter can be used as a postfix operator: a.m can be written a m.

For instance a.##(b) can be written a ## b and a.! can be written a!

• Postfix operators have lower precedence than infix operators, so foo bar baz means foo.bar(baz) while foo bar baz bam means (foo.bar(baz)).bam and foo bar baz bam bim means (foo.bar(baz)).bam(bim).
• Also given a parameterless method m of object a, a.m.m is valid but a m m is not as it would parse as exp1 op exp2.

Because there is a version of mkString that takes a single parameter it will be seen as an infix opertor in fromFile(file) mkString map caesar(k)_. There is also a version of mkString that takes no parameter which can be used a as postfix operator:

scala> List(1,2) mkString
res1: String = 12

scala> List(1,2) mkString "a"
res2: String = 1a2

Sometime by adding dot in the right location, you can get the precedence you need, e.g. fromFile(file).mkString map { }

And all that precedence thing happens before typing and other phases, so even though list mkString map function makes no sense as list.mkString(map).function, this is how it will be parsed.