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Beginning with version 2.5, LispMe supports multiple sessions in contrast
to the single session in earlier versions.
Each LispMe session is like a single process in a multiprocessing operating
system. You can switch between sessions at any time (even during evaluation)
and restart the session later.
All sessions are totally separate from each other, there's no way of
communication (except external resources like MemoPad I/O) between them.
That's not a problem as the idea behind them is not concurrent programming
(you could do this using continuations in a single session much better), but
to provide the framework to switch between many small utility programs
written in LispMe without having to pop and reload Memos in a single session.
A session database holds of course not only the Scheme program loaded, but
also all data, so for simple applications you don't have to write code
to save your data e.g. to a memo, as your data won't be lost when
switching to another session. Have a look how the
Calculator sample program does
not explicitly save its state on exit (it's not handling
the frm-close event)
This is even more faciliated by the option to create starter icons, which
start LispMe together with a specific session database. So now your dream
has come true: Write Scheme programs on your Pilot, create dialogs for them,
and execute them by a simple tap in the application launcher!
A starter application is a tiny program which
appears in the Pilot application launcher and starts LispMe with a
selected session database and simulates tapping the
So the steps to create a stand-alone LispMe program are:
Normally starter icons show in the application launcher as
in list view), but you can
create your own ones by including a tAIB resource with id
9000 (instead of 1000 for big icon) and/or a tAIB resource
with id 9001 (instead of 1001 for small icon) in the resource DB used
by your app. The resource DB must be open at the moment you create the
starter app by checking the checkbox, since LispMe looks
in this moment for icons with IDs 9000 and 9001 in all open
resource DBs. If you didn't define icons, LispMe will find its own
default icons depicted above.
- Go to the Session dialog, create
a new session and switch to it.
- Change its memory settings
- Load all needed memos
- Enter the start expression into the input field
- Test it
- Go to the Session dialog again and
check the icon checkbox.
- You might want to select another category for it in the application
Backing up and beaming sessions
Each LispMe session is stored in a separate Pilot database, which has the
same name as the session. You can backup and restore these databases
like any other .pdb file and transfer it to other Pilots
running LispMe (please make sure that LispMe version numbers match!)
with some tools.
A LispMe session is (almost) fully self-contained, so you can backup
and restore (or beam to other Palm devices) a LispMe session
and it will continue in the exactly same state when it
left your Pilot. This always works for a self-contained session,
the problems are external references. In case of LispMe, these can be:
- Open ports: These are identified by their Unique IDs in the MemoDB
database. A broken reference will be indicated by the error
message memo vanished
- Open GUI resource DBs, which store the entire
name of the database. When a database with this name exists on the
other Pilot, all is assumed OK, when not, no open GUI database
is assumed which may cause the code to fail in benign way.
- The latest-loaded memo, which is simple a record number in the
MemoDB database. This record number most certainly identifies
another memo on the target Pilot, it may even not exist.
- Other foreign types, like open sockets.
When you check the icon option in the session list to create a starter
icon, a new program is created in fact, which simply starts LispMe
using a special launch code and the name of the session database to
start. The database name of the starter is the same as the session database
name, but the high bit of the first char in the name is flipped. So don't
be alarmed when you find those strange looking .prc names on
your Pilot. You can delete a starter app any time you want and recreate
it via the session list.
The creator ID of all starter apps must be distinct, so LispMe
searches an unused one beginning with fbLA. So you should not
try to transfer a starter app to another Pilot, as the creator ID might
already be in use there. Just transfer the session database and create
the starter app on the other Pilot.