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# engines - pmq

This folder contains an implementation of a message priority queue using
a perpetual threaded engine holding the priority queue. At any moment, we
ask for a list of the pending messages ordered by priority.

A variant is also provided that splits top messages from normal messages
into separate queues. In this case, asking for a list of the pending
messages returns a list with top messages before the normal messages but
keeping the message sent order otherwise.

Print Logtalk, Prolog backend, and kernel versions (if running as a notebook):

```logtalk
%versions
```

Start by loading the example and the required libraries:

```logtalk
logtalk_load(pmq(loader)).
```

<!--
true.
-->

Send some messages to the priority queue:

```logtalk
pmq::(send(13-let), send(5-out), send(11-the), send(17-who), send(7-dogs)).
```

<!--
true
-->

Retrieve the current messages sorted by priority:

```logtalk
pmq::messages(Messages).
```

<!--
Messages = [who, let, the, dogs, out].
-->

After retrieving the messages the priority queue is empty until new messages
are received:

```logtalk
pmq::messages(Messages).
```

<!--
Messages = [].
-->

Send the next batch of messages:

```logtalk
pmq::(send(8-fun), send(11-have), send(3-':-)')).
```

Retrieve the current messages sorted by priority:

```logtalk
pmq::messages(Messages).
```

<!--
Messages = [have, fun, ':-)'].
-->

Same messages but to an alternative implementation that splits top messages
from normal messages instead of sorting all messages by priority:

```logtalk
split::(send(13-let), send(5-out), send(11-the), send(17-who), send(7-dogs)).
```

<!--
true.
-->

```logtalk
split::messages(List).
```

<!--
List = [let, the, who, out, dogs].
-->

```logtalk
split::messages(List).
```

<!--
List = [].
-->

```logtalk
split::(send(8-fun), send(11-have), send(3-':-)')).
```

<!--
true.
-->

```logtalk
split::messages(List).
```

<!--
List = [have, fun, ':-)'].
-->
