First page of tutorial should start with how ordering guarantees work

[ned14]

With examples of acquire-release and seq_cst.

With examples of ordering in response to errors e.g.

In the sequence:

auto a=async_file();
auto b=async_read(a);
auto c=async_truncate(b);
auto d=async_rmfile(c);
auto e=async_close(d);

... you are explicitly saying do not execute later steps if an earlier step fails. It defaults to this as this is probably safest and least surprising.

If on the other hand you DO always want to delete and close the file no matter what happened before you would write:

auto a=async_file();
auto b=async_read(a);
auto c=async_truncate(b);
auto d=async_rmfile(depends(c, a));
auto e=async_close(d);

The depends(precondition, input) function swaps the second future for the first when the first completes. It is implemented as:

precondition.then([input](future &&f) { return input; }