Re: [boost] [next gen future-promise] What to callthemonadicreturntype?

Subject: Re: [boost] [next gen future-promise] What to callthemonadicreturntype?
From: Gottlob Frege (gottlobfrege_at_[hidden])
Date: 2015-06-01 11:32:25

On Sun, May 31, 2015 at 6:25 PM, Vicente J. Botet Escriba
<vicente.botet_at_[hidden]> wrote:
> Le 30/05/15 22:49, Peter Dimov a écrit :
>>
>> Vicente J. Botet Escriba wrote:
>>>
>>> >> Surely for catch_error() one must always return the same type of
>>> >> future >> as the input? Otherwise it couldn't work when f1 has a value.
>>> >
>>> > Vicente in N4048 says that it's not required to return the same type.
>>> >
>>> If I said that it was an error. Could you point me where?
>>
>>
>> Input Parameters:
>> • Lambda function: The lambda function on map()/bind() takes a
>> future<t>::value_type. The lambda
>> function on catch_error() takes an exception_ptr. Both could return
>> whatever type. This makes propagating
>> exceptions straightforward. This approach also simplifies the chaining of
>> continuations.
>>
>>
> Thanks. I suspect I was referring to map and bind. future<T>::cath_error
> must return T or future<T>. I recognize that it is not clear in the N4048.
>
> Initially catch_error() was called recover(). It can be used to recover from
> errors and then the function returns a value or a future ready with a value
> and also to change the reported error, in this case the function should
> return future<T> ready with an exception.
>

I can imagine catch_error() returning something _convertible_

>     future<X> f1 = async( ... );
>     future<Y> f2 = f1.catch_error( []( exception_ptr e ) { return Y(); } );

Y needs to be convertible from X.

Not sure there is much value in that, however.

P.S. can I specialize std::future<MyType> ?  And does that just cause
headaches for the STL?

Tony