Equal Sign and Type Mismatch

Question

When working with the standard widget toolkit (SWT), I usually use something like this to define my GridLayout:

layout.marginTop = layout.marginBottom = 
    layout.marginLeft = layout.marginRight =
        layout.horizontalSpacing = layout.verticalSpacing = 20

It works in java but not in scala. It gives me type mismatch; Found: Unit Required: Int.

So how can this solve it?

score 7 · Accepted Answer · edited May 23 '17 at 12:04

7

You cannot do this in one line in scala because the result type of an assignment expression (e.g. a = b) is Unit. You'd have to have 6 separate calls:

layout.marginTop = 20
layout.marginBottom = 20 
... etc

Why is the result type of an assignment Unit and nmot the assigned value? I believe this was chosen for performance reasons as outlined in this question.

There is a related question on assignment which points out that at declaration site, it is possible via:

val a, b, c = X

edited May 23 '17 at 12:04

Community

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answered May 17 '11 at 13:15

oxbow_lakes

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...and I thought scala is about syntactic sugar and "type less, do more" philosophy :/ – f4lco May 17 '11 at 13:17
@phineas If you want to type less to do more in SWT, be sure to check out Dave Orme's [XScalaWT](http://www.coconut-palm-software.com/the_new_visual_editor/doku.php?id=blog:simplifying_swt_with_scala) library. – Jean-Philippe Pellet May 17 '11 at 13:19
@phineas - this is one of those very few areas where Java can be more concise. Note that the example you gave is a rather nasty case of bad encapsulation, however! It's a code pattern that I would see immediately as a "smell" – oxbow_lakes May 17 '11 at 13:20
I have to admit, Swing is modelled a bit better (but lacks certain features). – f4lco May 17 '11 at 14:24

score 1 · Answer 2 · answered May 17 '11 at 13:14

1

You have to write multiple assignments separately. As the compiler says, an assignment in Scala returns Unit, which can be seen as Java's void.

answered May 17 '11 at 13:14

Jean-Philippe Pellet

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Landei · Answer 3 · 2011-05-17T19:45:39.837

1

You could do

def assign[A](a:A)(fs: (A => Unit)*) = fs.foreach(_(a))

val r = new java.awt.Rectangle
assign(20)(r.x=_, r.y=_, r.width=_, r.height=_)

But this is clearly worse than writing everything separately. But at least you don't have to type "layout" every time in Scala:

val rectangle = new java.awt.Rectangle
import rectangle._
x = 20
y = 20
width = 20
height = 20

edited May 17 '11 at 19:45

answered May 17 '11 at 19:35

Landei

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Equal Sign and Type Mismatch

3 Answers3