Why do I get a “missing parameter for expanded function” in one case and not the other?

Question

Case this works:

Seq(fromDir, toDir) find (!_.isDirectory) foreach (println(_))

Whereas this doesn\'t:

Seq(fromDir, toDir)          


        

Answer 1

The difference is whether _ stands for the whole parameter, or is part of an expression. Depending on which, it falls into one of the two following categories:

Partially Applied Function

Seq(fromDir, toDir) find (!_.isDirectory) foreach (println(_))

translates into

Seq(fromDir, toDir) find (!_.isDirectory) foreach ((x$1) => println(x$1))

Anonymous Function Parameter

Seq(fromDir, toDir) find (!_.isDirectory) foreach (throw new Exception(_.toString))

translates into

Seq(fromDir, toDir) find (!_.isDirectory) foreach (throw new Exception((x$1) => x$1.toString))