Question on Scala Closure (From "Programming in Scala")
- by Ekkmanz
I don't understand why authors said that Code Listing 9.1 from "Programming in Scala" use closure. In chapter 9, they show how to refactor code into more less duplicated form, from this original code:
object FileMatcher {
private def filesHere = (new java.io.File(".")).listFiles
def filesEnding(query: String) =
for (file <- filesHere; if file.getName.endsWith(query))
yield file
def filesContaining(query: String) =
for (file <- filesHere; if file.getName.contains(query))
yield file
def filesRegex(query: String) =
for (file <- filesHere; if file.getName.matches(query))
yield file
}
To the second version:
object FileMatcher {
private def filesHere = (new java.io.File(".")).listFiles
def filesMatching(query: String,
matcher: (String, String) => Boolean) = {
for (file <- filesHere; if matcher(file.getName, query))
yield file
}
def filesEnding(query: String) =
filesMatching(query, _.endsWith(_))
def filesContaining(query: String) =
filesMatching(query, _.contains(_))
def filesRegex(query: String) =
filesMatching(query, _.matches(_))
}
Which they said that there is no use of closure here. Now I understand until this point. However they introduced the use of closure to refactor even some more, shown in Listing 9.1:
object FileMatcher {
private def filesHere = (new java.io.File(".")).listFiles
private def filesMatching(matcher: String => Boolean) =
for (file <- filesHere; if matcher(file.getName))
yield file
def filesEnding(query: String) =
filesMatching(_.endsWith(query))
def filesContaining(query: String) =
filesMatching(_.contains(query))
def filesRegex(query: String) =
filesMatching(_.matches(query))
}
Now they said that query is a free variable but I don't really understand why they said so? Since ""query"" seems to be passed from top method down to string matching function explicitly.
