When to use parenthesis in Scala infix notation

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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.

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