[GROOVY-11143] Add extension method to source unit (same locality as static import)

• Type: New Feature
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: groovy-jdk

Language

groovy

• Assignee: Unassigned
• Reporter: Eric Milles
• Created: 31/Jul/23 18:56
• Updated: 23/Aug/23 02:26

Description

I have been poking around with the many ways to extend a class/object's functionality within groovy. To add a new method that can be called like any other instance method, there are a few possibilities:

1. modify the MetaClass

String.metaClass.ext = { ->
  print delegate
}
"works".ext()

2. use a category class

@Category(String) class Cat {
  void ext() {
    println this
  }
}
use (Cat) {
  "works".ext()
}

3. apply a mixin
first create class similar to Cat in (2)

def str = "works"; str.metaClass.mixin(Cat)
str.ext()

4. apply a trait

trait Ext {
  void ext() { println this.toLowerCase() } // see GROOVY-11142
}
"works".withTraits(Ext).ext()

5. create an extension moodule

• create class similar to Cat in (2)
• create META-INF/groovy/org.codehaus.groovy.runtime.ExtensionModule file
• build and deploy separately from current script / project

What I'm after is something that works as easily in a script or source file as static imports but lets me call like an extension method. And does not modify state outside of the current script (like MetaClass) or introduce a thread barrier (like use). And works with static type checking.

Scala has an "extension" mechanism that is similar to static import but there is a keyword on the category class that lets compiler know the method is used like any other instance method. Scala used to provide "implicit" methods – I've lost my familiarity with those.

I've seen "import static extension foo.Bar.baz;" for some language. (citation required)

And lastly, Kotlin provides a local extension like this:

fun foo.Bar.baz() { x ->
  // ...
}

For any new users or just to add something quickly that can be used in a file, I think I'm looking for an enhancement to "import static ..." that will get substituted in the source unit, like StaticImportVisitor does today for static method calls. For example:

import static foo.Bar.baz
import static /*something*/ foo.Bar.baz
baz("") // supported by first import
"".baz() // supported by second import

I have been considering whether "extension", "default", some annotation or something else should be used for /*something*/ in the script above.

Similar to Kotlin, this could let someone declare an extension in a single script with no dynamic runtime impact:

import static /*something*/ ThisScript.*
static ext(String self) {
  print self
}
"works".ext()

Has anyone looked at something like this in Groovy or other languages and has some feedback?

Issue Links

• is related to: GROOVY-2521 keyword 'use' like 'import (static)' to use categories in a complete compilation unit
• is related to: GROOVY-8417 Add @UseCategory annotation for classes/methods instead of use() everywhere
• is related to: GROOVY-3387 Add @Use Annotation
• relates to: GROOVY-11161 Support extension methods inside the same compilation unit