Why Scala's traits execute from right to left?

Question

For example, this:

val queue = new BasicIntQueue with Doubling with Incrementing with Filtering
queue.put(1)
println(queue.get())

will print:

Filtering
Incrementing
Doubling
put
4

As for me it would be more readable if it executed from left to right, in the order I wrote operations.

I'm not terribly familiar with Scala, but I'm guessing just how that code is parsed. So it's `(((new BasicIntQueue with Doubling) with Incrementing) with Filtering)`, and expressions like that are evaluated innermost-paren-first. — millimoose, Jul 12 '13 at 17:25
Yes, see [this](http://stackoverflow.com/questions/7230808/inheriting-a-trait-twice) — S.R.I, Jul 12 '13 at 17:25

score 7 · Answer 1 · answered Jul 12 '13 at 17:48

Because it follows the same pattern as inheritance. Imagine that you had something like this:

class BasicIntQueue 
class Doubling extends BasicIntQueue
class Incrementing extends Doubling
class Filtering extends Incrementing

val queue = new Filtering

You'll get the same results as you saw: Filtering gets executed first, then pass on to Incrementing, then Doubling and finally BasicIntQueue.

score 6 · Accepted Answer · answered Jul 12 '13 at 17:25

6

Don't think 'executing', because it's not... think 'layering': if you add another with XYZ on the right, you've added another layer on top.

answered Jul 12 '13 at 17:25

Richard Sitze

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score 3 · Answer 3 · answered Jul 12 '13 at 17:50

3

The truth is more complicated, see the Language Reference "5.1.2 Class Linearization" (page 56), or the explanation by Jim McBeath

answered Jul 12 '13 at 17:50

Landei

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score 0 · Answer 4 · answered Jul 12 '13 at 17:44

0

To solve the diamond problem of multiple inharitance

answered Jul 12 '13 at 17:44

Kfir Bloch

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Why Scala's traits execute from right to left?

4 Answers4