Bitten by the lambda parameter name

[Zhong Yu]

On Mon, Jul 15, 2013 at 11:33 AM, Remi Forax <forax at univ-mlv.fr> wrote:
> On 07/15/2013 05:59 PM, Zhong Yu wrote:
>>
>> On Mon, Jul 15, 2013 at 10:34 AM, Maurizio Cimadamore
>> <maurizio.cimadamore at oracle.com> wrote:
>>>
>>> On 15/07/13 16:32, Maurizio Cimadamore wrote:
>>>>
>>>> On 15/07/13 16:28, Remi Forax wrote:
>>>>>
>>>>> On 07/15/2013 05:13 PM, Maurizio Cimadamore wrote:
>>>>>>
>>>>>> On 15/07/13 15:52, Remi Forax wrote:
>>>>>>>
>>>>>>> This snippet not compile,
>>>>>>>      Kind kind = ...
>>>>>>>      partySetMap.computeIfAbsent(kind, kind -> new
>>>>>>> HashSet<>()).add(party);
>>>>>>>
>>>>>>> Each time I write more than a hundred lines of codes that use some
>>>>>>> lambdas,
>>>>>>> I fall into this trap.
>>>>>>>
>>>>>>> It's very annoying !
>>>>>>>
>>>>>>> Rémi
>>>>>>>
>>>>>>>
>>>>>> Annoying yes - but there is a reason for it? If we provide special
>>>>>> scoping for lambda parameters then we will never be able to add
>>>>>> control abstraction syntax in a nice way; not saying that it's
>>>>>> something we want - but it's good to have option open at least.
>>>>>
>>>>> It's a crystal ball argument, in the future if we do that then ...
>>>>> It usually doesn't work because between now and the future, the way
>>>>> the feature will be introduced will change.
>>>>>
>>>> Well, yes and no - I remember we discussed a lot whether a lambda
>>>> should look (semantically) more like a block or an inner class. We
>>>> decided it should look like the former. This is a consequence of that
>>
>> It is also very annoying that this doesn't compile
>>
>>          int x = 1;
>>
>>          {
>>              int x = 2;
>>          }
>>
>> It is too hard to give a distinct name to every variable. Remi is
>> right this is a big PITA.
>
>
> No, I'm fine with this.
> You don't need two different things to be named with the same name,
> so the rule on block helps to catch bugs.
>
> With a lambda, it's the opposite, you need to find two names for the same
> thing,

Yes. Ideally we shouldn't create two variables for the same thing; the
API could have been designed as

    computeIfAbsent( key, ()->compute(key) )

but for efficiency we need to repeat "key" as the input parameter.
This is going to a common problem with lambda and annoy a lot of
people.

Zhong Yu



> and this stupid rule bugs me.
>
> Rémi
>
>
>>
>>>> decision. I think that mixing and matching semantics on a by-need
>>>> basis is not a good idea.
>>>
>>> And - one might argue the code you are trying to write is not that
>>> readable in the first place (adding random suffixes just to get it
>>> through javac is not very elegant readability-wise, but it does convery
>>> the concept that the two references of 'kind' which occur very close one
>>> to the other are indeed unrelated).
>>
>>
>>
>>
>>> Maurizio
>>>>
>>>> Maurizio
>>>>>
>>>>> In this peculiar case, if we add control abstraction syntax we will
>>>>> use a different syntax,
>>>>> so it's very annoying for no reason.
>>>>>
>>>>>> Maurizio
>>>>>
>>>>> Rémi
>>>>>
>>>
>