These may seem a bit simple for DGMs, but they do solve a couple problems as 
compared to "list ? list.first() ?: defaultValue".  First, no repetition of the 
"list" expression, which may be complex.  Second, content assist will show 
proposals for these methods.  Third, the Groovy truth problems with "list?[0] 
?: defaultValue", "list?.getAt(0) ?: defaultValue", "list.find() ?: 
defaultValue" and others do not exist for these.  If the first element is null 
or otherwise falsy, it will be returned as desired.  The only case for me that 
is unresolved is a null array or iterable.  In this case, Groovy throws "Cannot 
invoke method firstOrElse() on null obect" instead of running the method 
allowing null tolerance and return of default value.


    public static <T> T firstOrElse(Iterable<T> self, T defaultValue) {
        Iterator<T> iter = self.iterator();
        if (iter.hasNext()) {
            return iter.next();
        }
        return defaultValue;
    }
    public static <T> T firstOrElse(T[] self, T defaultValue) {
        if (self.length > 0) {
            return self[0];
        }
        return defaultValue;
    }
    // and similarly for the Closure (or java.util.function.Supplier if Java 8+ 
only) variants


Since safe navigation is being explored for by-index access, is there a 
possibility for including some for of "or supplied default" in any of the 
safe-navigation cases?  I personally find the new safe-indexing syntax to be 
unnecessary when "list?.getAt(i)" appears to be the equivalent to "list?[i]".


Alternate proposal, what if the DGM.getAt(self, int idx) had variants that 
included a default value return instead of hard-coded null?  Like this:

    public static <T> T getAt(List<T> self, int idx, T def) {
        int size = self.size();
        int i = normaliseIndex(idx, size);
        if (i < size) {
            return self.get(i);
        } else {
            //return null;
            return def;
        }
    }



________________________________
From: Mario Garcia <mario.g...@gmail.com>
Sent: Thursday, October 18, 2018 6:19 PM
To: dev@groovy.apache.org
Subject: Re: DGM for first or default

Good point OC:

[0,'',[],[:]].find()?:'not quite what you wanted here'
[0,1,2].find()?:'nor in this case'

The more I think on this the more I think is an interesting topic. I fully 
understand your frustration with first(), but apart from the example with Cocoa 
you mentioned, looking in the JVM it turns out there're plenty of examples of 
language collections behaving that way:

In scala the head of an empty list does throw an exception
-------------------
scala> var empty = List[Int]()
empty: List[Int] = List()

scala> empty.head
java.util.NoSuchElementException: head of empty list
  at scala.collection.immutable.Nil$.head(List.scala:426)
  at scala.collection.immutable.Nil$.head(List.scala:423)
  ... 28 elided

scala>
---------------------

and so does kotlin when calling to first()
----------------------
Welcome to Kotlin version 1.2.71 (JRE 1.8.0_171-b11)
Type :help for help, :quit for quit
>>> val num: List<Int> = listOf()
>>> num.first()
java.util.NoSuchElementException: List is empty.
at kotlin.collections.CollectionsKt___CollectionsKt.first(_Collections.kt:184)
>>>
---------------------
in Kotlin they have firstOrNull(), but I haven't found any overloaded function 
with a default value. They also have "find", but it's not possible to call it 
without parameter

However Clojure returns null whether:

  *   The first element was nil
  *   The list was empty
  *   Or the list was nil

--------------------
user=> (def a nil)
#'user/a
user=> a
nil
user=> (first a)
nil
user=> (def a '(nil))
#'user/a
user=> a
(nil)
user=> (first a)
nil
user=> (def a '())
#'user/a
user=> a
()
user=> (first a)
nil
user=>
-------------------

BTW I forgot to mention that Groovy 3 will have safe indexing meaning an 
expression like the following:

  *   will return the first element of a non empty list which I guess it will 
be the Kotlin firstOrNull() equivalent
  *   or null if the list was null or empty

---------
// trying to get first element from null list
nullList?[0] ==> null

// trying to get an inexistent element from a non empty list (but this is not 
new, this is how a non empty list indexing works in Groovy)
nonNullList?[9999] => null
----------

Outside the JVM, Haskell, when asking for the head of an empty list, throws an 
exception (There is an explanation in stackoverflow which I'm afraid I don't 
understand). So in the end Groovy's first() seems not to be the exception among 
other modern languages out there.

Another point of view, could be thinking about returning null consistently. 
Lets say a list returns null using first():

  *   Does it mean the first element is a null value or is an empty list and 
that's why is giving me a null value ?
  *   What if null is a valid value, with some meaning in my process ? With 
that context a method like firstOrNull() (even first(defaultValue) with a null 
list) could be considered ambiguous.

My guess is that in the case of languages throwing an exception using first() 
on an empty list, when they designed the language collections they didn't have 
any other way to express that kind of semantics. But this is just a lucky 
guess. I'm probably wrong. I only can think of pattern matching as a complete 
solution, where the terminal result in the case of an empty or null list, is a 
default value different than any of the elements of the expected result set ?

I apologize in advance for the lengthy e-mail, but it seemed interesting to 
think why first() was designed like that, not only in Groovy, but in some other 
languages as well.
Mario

El jue., 18 oct. 2018 a las 20:27, Milles, Eric (TR Technology & Ops) 
(<eric.mil...@thomsonreuters.com<mailto:eric.mil...@thomsonreuters.com>>) 
escribió:

I think first() exists so there is a semantic pair for functional programming: 
first()/head() and tail()  or init() and last()

________________________________
From: ocs@ocs <o...@ocs.cz<mailto:o...@ocs.cz>>
Sent: Thursday, October 18, 2018 1:20 PM
To: dev@groovy.apache.org<mailto:dev@groovy.apache.org>
Subject: Re: DGM for first or default

Well I thought first is smart enough to return null for an empty list, same as 
my firstObject in Cocoa does. If it throws, what's on earth point of having the 
thing at all? In that case it can be replaced by list[0] without any drawback 
at all.

All the best,
OC

On 18 Oct 2018, at 7:19 PM, Milles, Eric (TR Technology & Ops) 
<eric.mil...@thomsonreuters.com<mailto:eric.mil...@thomsonreuters.com>> wrote:

"list?.first() ?: defaultValue" is not the equivalent.  If the collection is 
empty, first() throws an IndexOutOfBoundsException is thrown.  That's why I'm 
asking if there is a simple equivalent.  I suppose this is the equivalent now 
that I think about it:

list ? list.first() : defaultValue


________________________________
From: ocs@ocs <o...@ocs.cz<mailto:o...@ocs.cz>>
Sent: Thursday, October 18, 2018 12:07 PM
To: dev@groovy.apache.org<mailto:dev@groovy.apache.org>
Subject: Re: DGM for first or default

Myself, I am not a huge fan of adding not-often-needed functionalities (and 
actually would add almost none of those discussed lately); nevertheless...

On 18 Oct 2018, at 6:48 PM, Paolo Di Tommaso 
<paolo.ditomm...@gmail.com<mailto:paolo.ditomm...@gmail.com>> wrote:

-1, it can be easily done as:
list.first() ?: defaultValue

... this won't work in case the first object is a Groovy False (e.g., an empty 
string, or a plethora of others).

All the best,
OC



p

On Thu, Oct 18, 2018 at 6:45 PM Daniel.Sun 
<sun...@apache.org<mailto:sun...@apache.org>> wrote:
+0 from me.
P.S. we should add similar DGM for `last` too?

Cheers,
Daniel.Sun




-----
Daniel Sun
Apache Groovy committer
Blog: 
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Twitter: @daniel_sun

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