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blocks

This folder contains an example of representation and handling of relations using events. We have instances of class block and a binary block_stack relation between the blocks. Every time we move a block, we want the blocks on top of it to move along. If we break the stack by moving a middle block, we want to automatically destroy the corresponding relation tuple.

It's instructive to use the debugger to better understand this example. Set spy points in all block instances and then activate the debugger.

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

%versions

Start by loading the example and the required library files:

logtalk_load(blocks(loader)).

Create four blocks, all standing on the "ground" (use your imagination... ;-) (don't use message broadcasting syntax in order to workaround a XSB parser bug):

block::new(a, [position-(8, 1)]), block::new(b, [position-(6, 1)]), block::new(c, [position-(4, 1)]), block::new(d, [position-(2, 1)]).

<!-- true. -->

Set up an ASCII stack monitor so we can watch the blocks moving:

define_events(after, _, move(_,_), _, stack_monitor).

<!-- true. -->

Ensure that top-level message goals generate events:

set_logtalk_flag(events, allow).

<!-- true. -->

Make the stack (don't use message broadcasting syntax in order to workaround a XSB parser bug):

block_stack::add_tuple(c-d), block_stack::add_tuple(b-c), block_stack::add_tuple(a-b).

<!-- |.c...... |.d...b.a --------- |.b...... |.c...... |.d.....a --------- |.a |.b |.c |.d ---

true. -->

List all tuples:

block_stack::tuples(Tuples).

<!-- Tuples = [c-d, b-c, a-b]. -->

List all after events and monitors:

%%table
current_event(after, Object, Message, Sender, Monitor).

<!-- Message = move(_, _), Monitor = stack_monitor ; Object = d, Message = move(_, _), Monitor = block_stack ; Object = c, Message = move(_, _), Monitor = block_stack ; Object = a, Message = move(_, _), Monitor = block_stack ; Object = b, Message = move(_, _), Monitor = block_stack ; false. -->

Move all stack to new position by moving bottom block:

d::move(9, 1).

<!-- |.a....... |.b....... |.c....... |........d ---------- |.a....... |.b....... |........c |........d ---------- |.a....... |........b |........c |........d ---------- |........a |........b |........c |........d ----------

true. -->

Check results:

a::position(Xa, Ya), b::position(Xb, Yb), c::position(Xc, Yc), d::position(Xd, Yd).

<!-- Xa = 9, Xb = 9, Xc = 9, Xd = 9, Ya = 4, Yb = 3, Yc = 2, Yd = 1. -->

List all tuples:

block_stack::tuples(Tuples).

<!-- Tuples = [c-d, b-c, a-b]. -->

Break stack in half by moving b to the "ground":

b::move(3, 1).

<!-- |........a |......... |........c |..b.....d ---------- |..a.....c |..b.....d ----------

true. -->

Check results:

a::position(Xa, Ya), b::position(Xb, Yb), c::position(Xc, Yc), d::position(Xd, Yd).

<!-- Xa = 3, Xb = 3, Xc = 9, Xd = 9, Ya = 2, Yb = 1, Yc = 2, Yd = 1. -->

List all tuples:

block_stack::tuples(Tuples).

<!-- Tuples = [c-d, a-b]. -->

Create a new block_stack tuple:

block_stack::add_tuple(d-a).

<!-- |..d...... |..a.....c |..b...... ---------- |..c |..d |..a |..b ----

true. -->

Check results:

a::position(Xa, Ya), b::position(Xb, Yb), c::position(Xc, Yc), d::position(Xd, Yd).

<!-- Xa = 3, Xb = 3, Xc = 3, Xd = 3, Ya = 2, Yb = 1, Yc = 4, Yd = 3. -->

List all tuples:

block_stack::tuples(Tuples).

<!-- Tuples = [c-d, a-b, d-a]. -->

Move all stack to new position by moving bottom block:

b::move(5, 1).

<!-- |..c.. |..d.. |..a.. |....b ------ |..c.. |..d.. |....a |....b ------ |..c.. |....d |....a |....b ------ |....c |....d |....a |....b ------

true. -->

Check results:

a::position(Xa, Ya), b::position(Xb, Yb), c::position(Xc, Yc), d::position(Xd, Yd).

<!-- Xa = 5, Xb = 5, Xc = 5, Xd = 5, Ya = 2, Yb = 1, Yc = 4, Yd = 3. -->

List all tuples:

block_stack::tuples(Tuples).

<!-- Tuples = [c-d, a-b, d-a]. -->

Clean up instances, tuples and monitors:

block_stack::remove_all_tuples.

<!-- true. -->

Delete all blocks:

block::delete_all.

<!-- true. -->