[alsa-devel] Type safe Haskell interface to ALSA sequencer
alsa at henning-thielemann.de
Tue May 25 16:11:17 CEST 2010
Dear ALSA developers,
I read the ALSA-Sequencer-API-Doxygen documentation, various
ALSA-MIDI-How-Tos and their examples and succeeded to work with ALSA
sequencers in Haskell. However, I'm still uncertain about the meaning of
the system components and how they interact. It is extensively documented,
what works, but not, what does not work. Whenever I do a mistake, ALSA
leaves me with an unspecific "operation not permitted" or "invalid
argument" error code. Thus I try to encode conditions in Haskell types.
For instance once I got an "Invalid argument" error code and only after
some investigation, I found that I opened the sequencer in Output mode but
later called event_input. (Actually I only wanted to output something, but
used event_input for synchronization using Echo events. Unfortunately the
intention of Echo messages is even not explained in the Doxygen stuff.) In
order to prevent another occurrence of this problem, I defined distinct
Haskell types for sequencers depending on the input mode. Now, event_input
is only allowed on sequencers that were opened in Output or Duplex mode.
However I wondered, why I could create writeable ports in a output-only
sequencer. Does this make sense? Would it be reasonable to forbid
writeable ports in output sequencers and readable ports in input
sequencers by Haskell types? Is there a difference between
cap_read|cap_write and cap_duplex?
I also wondered why there can be several ports in one sequencer, since
multiple clients can connect to one port, and event_input and event_output
have no port argument. Further I wondered why queues are not bound to a
specific port. Can queues be shared amongst ports? Is it sensible to have
multiple queues in one sequencer? Why has sync_output_queue no queue
argument? The more I think about it, it seems to me that a port is more an
identifier, a part of an address, than a mailbox. I could use ports as
tags, that tell which part of an application to control. A queue looks
like a mail company and I can use the same sender and receiver addresses
across different companies. If this is true, then I think it would be
worth to mention this in the API documentation.
In another example I called sync_output_queue in order to wait for all
events being sent, but it aborted with "Operation not permitted", whereas
I have seen example code where it works. What are the preconditions to
I really like to see a picture of how sequencers, clients, ports,
port_infos, queues, timers, events are related.
More information about the Alsa-devel