Better lambdas!!!!!!!!!!

[Prudence via Digitalmars-d]

On Saturday, 12 September 2015 at 02:13:11 UTC, Pierre Krafft 
wrote:
> On Saturday, 12 September 2015 at 01:03:54 UTC, Prudence wrote:
>> On Thursday, 10 September 2015 at 18:02:36 UTC, Ali Çehreli 
>> wrote:
>>> On 09/10/2015 10:55 AM, Prudence wrote:
>>> > How bout this:
>>> >
>>> > void myfunc(double delegate(int i, int z, float f)) {....}
>>> >
>>> >
>>> > myfunc((int i, int z, float f) { return i*z*f; } }
>>> >
>>> > vs
>>> >
>>> > myfunc({ return i*z*f; })   // Names of parameters are
>>> inferred from
>>> > signature.
>>>
>>> Considering other features of the language, that's pretty 
>>> much impossible in D. What if there is another i in scope:
>>>
>>> int i;
>>> myfunc({ return i*z*f; });
>>>
>>> Now, should it call another overload of myfunc that takes 
>>> (int z, int f) because i is something else?
>>>
>>> Should the compiler analyze the body of the code and decide 
>>> which symbols could be parameters? And then go through all 
>>> overloads of myfunc? etc.?
>>>
>>> Ali
>>
>> As I said, it could throw a warning or error. It, in some 
>> sense, is already a a problem with nested blocks that hide 
>> outside variables, is it not?
>>
>> The compiler doesn't need to scan anything. It knows the which 
>> parameters from the definition!
>>
>>
>> -> void myfunc(double delegate(int i, int z, float f))  <- 
>> Compiler knows to use the names here as the default names in 
>> for the parameters when.
>>
>>
>> when used:
>>
>> myfunc({ return i*z*f; }); <- Oh, there are the names, we know 
>> what they are because the signature is tells us. The compiler 
>> does the following:
>>
>>
>> 1. Sees we have a block without any parameters defined. i.e., 
>> a lambda.
>>
>> 2. It looks up the signature of myfunc to find out what the 
>> names are
>>
>> 3. It sees that they are i z and f
>>
>> 4. Now it knows and it effectively rewrites the code as
>>
>> myfunc((i,z,f) { return i*z*f; });
>>
>> Surely this is not difficult, 4 steps?
>
> You're making your code more brittle for a small gain. The 
> suggestion makes parameter usage order important and the 
> compiler can't warn about my typos.
> Consider:
> myfunc({return "x:"~x~"y:"-y;}) getting changed to 
> myfunc({return "y:"~y~"x:"~x;});
> Or the typo in
> myfunc({return i*z+f*j;});
>
> Lambdas are already very concise. This proposal doesn't give 
> any benefits outside of very simple lambdas. Such lambdas are 
> already so simple that they could use some standard functions 
> instead (like sum, to!T, and bind).


What does this have to do with my proposal? Those issues exist 
regardless of the simplification.

myfunc({return "x:"~x~"y:"-y;}) getting changed to
myfunc({return "y:"~y~"x:"~x;});

huh? What do you mean the suggestion makes parameter usage order 
important? They are important, it has nothing to do with the 
suggestion? Are you saying that you want to reserve the right to 
do something like

myfunc(string delegate(string x, string y));

and

myfunc((y,x){ "y:"~y~"x:"~x; })

? If so, or unless I'm missing something, that's bad no matter 
what. Changing the order and reversing the names is more than 
just confusing, it's hard to read and most people will gloss over 
that fact. Be consistent with your parameters and maybe you'll 
have less bugs?



Or the typo in

myfunc({return i*z+f*j;});

Again, what does this have to do with anything? A typo is a typo 
and is always a mistake. The above example has the same effect 
regardless if the parameters are explicit or deduced.


myfunc((i,z,f) {return i*z+f*j;});

j is still a problem. If j is defined outside the lambda then 
regardless of specific or implicit parameter names, it will not 
cause any problems.

In either case, the compiler can see that j is either referenced 
outside the scope or undefined. It has nothing to do with the 
parameters used.


Of course maybe I'm missing something, but essentially are not 
almost all uses of lambda's simply copying the parameter 
signature of the delegate. It already infers types... you could 
say that leads to typo's too...