DevoxxUK Lambdas Lab

[Maurizio Cimadamore]

Hi Stephen,
these problems are known, and are caused by the fact that the structural 
most specific check only kicks in when the two candidates' SAM have 
descriptors with the _same_ argument types. Unfortunately the signature 
of those methods:

<T> comparing(ToLongFunction<? super T>)
<T>comparing(ToDoubleFunction<? super T>)

do not satisfy this property, as the argument types of the two 
descriptor are _two_ different type-variables.
We are looking into ways to overcome this limitation.

Maurizio

On 03/04/13 11:34, Stephen Colebourne wrote:
> My experience (part 1):
>
> The afternoon did not start well, with some pretty basic inference problems:
> This works:
>   ToIntFunction<String> ref = String::length;
>   strs.sort(Comparators.comparing(ref).thenComparing(Comparators.naturalOrder()));
> This doesn’t work:
>   strs.sort(Comparators.comparing(String::length).thenComparing(Comparators.naturalOrder()));
> Nor does this work:
>   strs = strs.stream().sorted(Comparators.comparing(String::length)).sorted().collect(Collectors.toList());
>
> This works:
>   strs.sort(Comparators.comparing((String s) ->
> s.length()).thenComparing(Comparators.naturalOrder()));
> This doesn’t work:
>   strs.sort(Comparators.comparing(s ->
> s.length()).thenComparing(Comparators.naturalOrder()));
> Nor does this work:
>   strs = strs.stream().sorted(Comparators.comparing(s ->
> s.length())).sorted().collect(Collectors.toList());
>
> In other words, both of the obvious (to me) ways to code this problem
> (sort by length then alphabetically) failed. If these can't be fixed,
> I'd be really worried about the usability of lambdas.
> Stephen
>
>
> On 3 April 2013 09:12, Richard Warburton <richard.warburton at gmail.com> wrote:
>> Last week at DevoxxUK we ran a brief lambdas hackday.  People were
>> encouraged to focus on the collectors component of the API through setting
>> a few problems to solve.  I appreciate its a bit late in the game as far as
>> API changes, but some of these issues are fixable through
>> documentation/improved compiler error changes rather than API changes.
>>
>> 1. No one complained about the move from “into(new ArrayList<Foo>());” ->
>> “collect(toList())” when you explained that the change had been made.
>>   However, people didn’t naturally find Collectors.toList() and they did
>> express frustration around that.  At least one request for an abbreviated
>> toList() method on a stream - more for findability/fluency reasons rather
>> than brevity of code.
>>
>> 2. ToIntFunction, ToDoubleFunction etc. are all usable with flatMap, but
>> the naming confused people as to why.
>>
>> 3. Several people requested a way to transform a boxed stream into an
>> unboxed stream.  Its pretty easy to go the other way around, but there
>> didn’t seem to be any utility methods for making the boxed -> unboxed
>> transformation.
>>
>> 4. People found “groupingBy” to be a hard conceptual leap.  We had set an
>> exercise where people were asked to count the frequency of words in a
>> document, in order to force them to use it.
>>
>> a. Quite a few people didn’t initially look for a function that collects a
>> stream into a map.
>>
>> b. When you suggest that they should look for that, they didn’t look for
>> something called “groupingBy”.
>>
>> c. They did get the concept once you bring up SQL.  With hindsight I wished
>> I had enquired about how many people had used LINQ.
>>
>> d. People then didn’t grok that they needed to use the multiple argument
>> overload of groupingBy, with a reducingBy, in order to complete the task.
>>   I suspect that this method needs more documentation examples in order to
>> be easily understandable by people.
>>
>> 5. People are beginning to get confused by old documentation on the
>> internet being out of date.  I hadn’t seen this in previous hackdays.  Even
>> an article in the latest Java Magazine is out of date due to the API moving
>> so much recently, and so is the official tutorial:
>> http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html.
>>   Probably not a long term concern - but might be a concern for the first
>> few months.
>>
>> 6. If anyone else runs this kind of thing and they are a day-to-day eclipse
>> user, word of advice to make sure you know how to set the preferred JVM
>> location in netbeans and intellij before you run the event!
>>
>> Thanks to everyone that attended, and especially to Stuart Marks, Maurice
>> Naftalin, Graham Allan and John Oliver for helping out with running the lab.
>>
>> There’s a full link with code that people wrote and pasted at, and it also
>> contains some more comments by people:
>>
>> https://docs.google.com/document/d/1riMDt_JkAX74X30lHuOiSBjSxa7ifjxaynOA4LttOCk/edit
>>
>> regards,
>> *
>>
>>    Richard Warburton
>>
>>    http://insightfullogic.com
>>    @RichardWarburto <http://twitter.com/richardwarburto>
>>