On 18/09/2011 10:46, Will Robertson wrote:
>> I guess an
>> alternative is to set up the code such that a stream which has been
>> closed will give a suitable error message if one tries to use it.
> 
> I prefer this option; I think it's natural to want to write
> 
> \iow_new:N \g_foo_iow
> ...
> \cs_new:Nn \foo_internal_something:n
>   {
>     \iow_open:Nn \g_foo_iow {#1.output}
>     ...
>     \iow_close:N
>   }

That's easy to do, leaving only the question of what happens with

  \iow_new:N \l_my_stream
  \iow_open:N \l_my_stream { <file> }
  ...
  \iow_close:N \l_my_stream
  ..
  \iow_open:N \l_my_stream { <file> }

The easiest approach is that we just overwrite the <file>.
-- 
Joseph Wright