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

1 Jun 2015


      Le 01/06/15 17:32, Gottlob Frege a écrit :
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On Sun, May 31, 2015 at 6:25 PM, Vicente J. Botet Escriba
 wrote:
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Le 30/05/15 22:49, Peter Dimov a écrit :
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Vicente J. Botet Escriba wrote:
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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_
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future<X> f1 = async( ... );
     future<Y> f2 = f1.catch_error( []( exception_ptr e ) { return Y(); } );
Y needs to be convertible from X.
The opposite is also possible. The continuation can return a type Y 
convertible to X
future<X> f1 = async( ... );
     future<X> f2 = f1.catch_error( []( exception_ptr e ) { return Y(); } );
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Not sure there is much value in that, however.
I have not needed it.
P.S. can I specialize std::future<MyType> ?  And does that just cause
headaches for the STL?
No and I don't know. Do you have a use case that would need specialization?

Vicente