Scala Unit type

Question

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?

Answer 1

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)

Answer 2

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} ) { .... }