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get rid of Rational's 'private val g'?

7 replies on 1 page. Most recent reply: Dec 23, 2010 10:55 AM by Jim Balter

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Rob Dickens

Posts: 15
Nickname: robcd
Registered: Feb, 2008

get rid of Rational's 'private val g'? Posted: Dec 18, 2010 8:27 AM
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Dear fellow readers,

I refer to Listing 6.3 on p148 (p138 of 1st edition).

g is only required in order to calculate numer and denom, and therefore doesn't deserve its broader scope, and to be kept around.

In Java, g could be got rid of using blank finals (for numer and denom).

The next best thing in Scala would be to put numer and denom in a tuple, which would get rid of g, but be less satisfactory otherwise.

Anyone have a better solution?

Thanks,
Rob


Jim Balter

Posts: 13
Nickname: jibal
Registered: Dec, 2010

Re: get rid of Rational's 'private val g'? Posted: Dec 21, 2010 10:14 PM
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I would expect the compiler to eliminate g.

Rob Dickens

Posts: 15
Nickname: robcd
Registered: Feb, 2008

Re: get rid of Rational's 'private val g'? Posted: Dec 21, 2010 11:30 PM
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Hi. In which case g wouldn't be 'kept around', taking up space. That would indeed solve half the problem.

However, from the programmer's point of view, the unwarranted scope is still off-putting.

Jim Balter

Posts: 13
Nickname: jibal
Registered: Dec, 2010

Re: get rid of Rational's 'private val g'? Posted: Dec 22, 2010 12:51 AM
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Try

val (numer, denom) = { val g = gcd(n.abs, d.abs); (n/g, d/g) }

Rob Dickens

Posts: 15
Nickname: robcd
Registered: Feb, 2008

Re: get rid of Rational's 'private val g'? Posted: Dec 22, 2010 2:33 AM
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Right - this is the one solution I mention in my post - use a tuple.

However, I didn't really explain why I thought it was unsatisfactory.

On second thoughts, 'val (numer, denom)' does after all define a 'val numer' and a 'val denom'.

But is it truly equivalent to defining two separate vals, when viewed from the outside?

Jim Balter

Posts: 13
Nickname: jibal
Registered: Dec, 2010

Re: get rid of Rational's 'private val g'? Posted: Dec 22, 2010 12:22 PM
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Yes, it is equivalent. val (numer, denom) is a pattern match, not a tuple. It is matched against the tuple (n/g, d/g) to decompose it. (If for some reason you object to using tuples as intermediate values, don't use Scala.)

Jim Balter

Posts: 13
Nickname: jibal
Registered: Dec, 2010

Re: get rid of Rational's 'private val g'? Posted: Dec 23, 2010 10:35 AM
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However, as noted on the Scala mailing list where you inquired, and at http://stackoverflow.com/questions/1118669/how-do-you-define-a-local-var-val-in-the-primary-constructor-in-scala/1118984, that secretly adds a useless field to the class.

Jim Balter

Posts: 13
Nickname: jibal
Registered: Dec, 2010

Re: get rid of Rational's 'private val g'? Posted: Dec 23, 2010 10:55 AM
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See http://stackoverflow.com/questions/1218872/avoiding-scala-memory-leaks-scala-constructors/1451024#1451024 for a solution that avoids extra fields (by having a private primary constructor that takes g as a parameter).

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