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From Paolo Di Tommaso <>
Subject Add a marker interface to bypass Collections and Maps formatting
Date Sat, 28 May 2016 16:26:37 GMT
Hi all,

Groovy implements a built-in formatting strategy for collection and map
objects that is surely nicer and more useful than the one provided by the
default Java implementation for these classes.

However there are use cases in which custom collection or map classes need
to implement their own formatting rule.

Currently in Groovy this is quite painful and may lead to inconsistent
results. Take in consideration the following example:

class MyList extends ArrayList {
 String toString() {

def x = new MyList()
x << 1 << 2 << 3

println x.toString()
println x
println "$x"

Which prints:

[1, 2, 3]
[1, 2, 3]

Both the second and third `println` use the Groovy built-in formatting
method and there's no easy way to override this behaviour. Also there's not
a clear reason why the first and the second print return a different

The only options I've found is to define `MyList` with a @Delegate without
implementing the `List` interface. But this leads to other weird side
effects. The remaining possibility is to use some bytecode manipulation to
bypass the default Groovy formatting, but it looks to me a really
overkilling solution for such problem.

For this reason a would like to propose to introduce a mechanism that would
allow custom collection and map classes to bypass the default formatting
method. This should not be too difficult. The current Groovy built-in
formatting is implemented by formatList
and formatMap

It would be enough to add a marker interface (or an annotation) that when
applied to a class it would be used to by-pass the logic in the formatList
and formatMap methods and simply return the string provided by the object
`toString` method.

I could easily contribute this patch however I would know the opinion of
the Groovy core committers. In particular:

1) What name should have this marker interface? groovy.lagn.Something?
2) Are formatList and formatMap methods the right place to add this logic?
3) A similar problem exists also when using the `equals` (and hashCode?)
method for collections and maps. Should this mechanism be extended also to
this case?


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