Does Scala have record update syntax for making modified clones of immutable data structures?

Question

In Mercury I can use:

A = B^some_field := SomeValue

to bind A to a copy of B, except that some_field is SomeValue instead of whatever it was in B. I believe the Haskell equivalent is something like:

a = b { some_field = some_value }

Does Scala have something like this for "modifying" immutable values. The alternative seems to be to have a constructor that directly sets every field in the instance, which isn't always ideal (if there are invarients the constructor should be maintaining). Plus it would be really clunky and much more fragile if I had to explicitly pass every other value in the instance I want to have a modified copy of.

I couldn't find anything about this by googling, or in a brief survey of the language reference manual or "Scala By Example" (which I have read start-to-finish, but haven't absorbed all of yet, so it may well be in there).

I can see that this feature could have some weird interactions with Java-style access protection and subclasses though...

Isn't case classes what you're looking for? http://daily-scala.blogspot.com/2010/01/case-classes-in-28.html — Arjan Blokzijl, Jul 12 '11 at 09:41
See also http://stackoverflow.com/questions/3900307/cleaner-way-to-update-nested-structures — Knut Arne Vedaa, Jul 12 '11 at 10:45

score 28 · Answer 1 · edited Jul 12 '11 at 11:42

28

If you define your class as a case class, a convenient copy method is generated, and calling it you can specify with named parameters new values for certain fields.

scala> case class Sample(str: String, int: Int)
defined class Sample

scala> val s = Sample("text", 42)
s: Sample = Sample(text,42)

scala> val s2 = s.copy(str = "newText")
s2: Sample = Sample(newText,42)

It even works with polymorphic case classes:

scala> case class Sample[T](t: T, int: Int)
defined class Sample

scala> val s = Sample("text", 42)
s: Sample[java.lang.String] = Sample(text,42)

scala> val s2 = s.copy(t = List(1,2,3), 42)
s2: Sample[List[Int]] = Sample(List(1, 2, 3),42)

Note that s2 has a different type than s.

edited Jul 12 '11 at 11:42

Lukas Rytz

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answered Jul 12 '11 at 09:38

Jean-Philippe Pellet

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Thanks a lot for the edit, Lukas, I've just learnt something new! – Jean-Philippe Pellet Jul 12 '11 at 13:31
1

Thanks for the excellent examples! The polymorphic copy is very nifty. – Ben Jul 12 '11 at 23:39

score 12 · Accepted Answer · edited Nov 26 '11 at 06:06

12

You can use case classes for this, but you don't have to. Case classes are nothing magical - the modifier case just saves you a lot of typing. The copy method is realized by the use of named and default parameters. The names are the same as the fields and the defaults are the current values of the fields. Here's an example:

class ClassWithCopy(val field1:String, val field2:Int) {
    def copy(field1:String = this.field1, field2:Int = this.field2) = {
        new ClassWithCopy(field1,field2);
    }
}

You can use this just like the copy method on case classes. Named and default parameters are a very useful feature, and not only for copy methods.

edited Nov 26 '11 at 06:06

Robin Green

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answered Jul 12 '11 at 10:38

KIMA

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Wow, how'd I miss that Scala has has named parameter passing? That'll be very useful. I'm accepting this answer, since it explains how the copy method on case classes works, as well as how I can apply the ideas even when case classes are not appropriate. – Ben Jul 12 '11 at 23:38
as an alternative to this, the _autogenerated_ `copy` method mentioned below behaves differently regarding DRY principle – Hartmut Pfarr Apr 22 '20 at 10:21

score 6 · Answer 3 · answered Jul 12 '11 at 09:38

6

If the object you're planning on modifying is a case class then you can use the autogenerated copy method:

scala> val user = User(2, "Sen")
user: User = User(2,Sen)

scala> val corrected = user.copy(name = "Sean")
corrected: User = User(2,Sean)

answered Jul 12 '11 at 09:38

Sean Parsons

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Does Scala have record update syntax for making modified clones of immutable data structures?

3 Answers3