Scala Unit type | 易学教程

问题

I use opencsv to parse csv files, and my code is

while( (line = reader.readNext()) != null ) { .... }

I got a compiler warning saying:

 comparing values of types Unit and Null using `!=' will always yield true
 [warn]     while( (aLine = reader.readNext()) != null ) {

How should I do the while loop?

回答1:

In your case (line = reader.readNext()) is a functional literal that returns type Unit. You may rewrite the code as follows:

while( {line = reader.readNext();  line!= null} ) { .... }

回答2:

An assignment expression has type Unit in Scala. That is the reason for your compiler warning.

There is a nice idiom in Scala that avoids the while loop:

val iterator = Iterator.continually(reader.readNext()).takeWhile(_ != null)

This gives you an iterator over whatever reader.readNext returns.

The continually method returns an "infinite" iterator and takeWhile takes the prefix of that, up to but not including the first null.

(Scala 2.8)

回答3:

You can use a Stream to get what you want:

Stream.continually(reader.readLine()).takeWhile( _ ne null) foreach { line =>
  //do Stuff
}

This has the added advantage of other cool stuff as well:

Stream.continually(reader.readLine()).takeWhile( _ ne null) match {
  case head #:: tail => //perhaps you need to do stuff with the first element?
  case _             => //empty
}

EDIT - thanks to mkneissl for pointing out I should have included this warning:

scala> Stream.continually(1).take(100000000).foreach(x=>()) 

scala> val s = Stream.continually(1).take(100000000) 
s: scala.collection.immutable.Stream[Int] = Stream(1, ?) 

scala> s.foreach(x=>()) java.lang.OutOfMemoryError: Java heap space

回答4:

You are writing Scala code they way you would write it in Java. Try doing it in a more Scala-like way. To read a text file line by line and do something with each line, try this:

import java.io.File
import scala.io.Source

Source.fromFile(new File("myfile.txt")).getLines.foreach { line =>
    // Do something with the line, for example print it
    println(line)
}

回答5:

Assignment in Scala doesn't return a value, Unit is similar to void in C or C++.

try

var line = ""

while ({line = reader.readNext(); line != null}) { ... }

This works because the value of the last expression in a block is returned and in this case it is a Boolean which is required by the while

来源：https://stackoverflow.com/questions/3062804/scala-unit-type

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scala

while-loop