{trace}
Switches the execution trace on for just the part of the page it wraps.
Syntax
{trace} ... {/trace}Description
{trace} ... {/trace} records how the engine processes its content: every level that starts and ends, every occurrence of a loop, the tags met and the content each level produced, with line numbers. It is the trace that $padInfo = 'trace' writes for a whole request, narrowed to the part you are interested in - a slow loop, a condition that takes the wrong branch - so the trace stays readable.
The page itself is not changed: the content renders exactly as without the tag. The trace goes to files under DATA/trace/<page>/<request id>-<n>/ - root.txt with the outline, one line per event, and a local.txt per tag below it. On the way in the tag parks the request's own info settings and loads the default trace set (pad/config/info/trace.php: levels, occurrences, line numbers and results on; the noisier internals off); at {/trace} it closes the scope and puts back what was parked, so nothing outside the pair is traced.
A request cannot switch tracing on by itself - $padInfo is never filled from the query string. For a whole page use $padInfo = 'trace' in _config/config.php, or render it with php editors/render.php app page --trace. To look at a value rather than the execution, use {debug}.
Examples
Trace one loop
Only the loop inside the pair is traced; the output is the same as without the tag.
<h2>Orders</h2>
{trace}
{orders where='$total gt 100'}
<p>{$number}: {$total}</p>
{/orders}
{/trace}
<p>Not traced.</p>What the trace writes
root.txt for a two-row loop: the level path, the event number, what it started from, the event and the result.
{trace}{sequence '1..2'}{$sequence}{/sequence}{/trace}
{# DATA/trace/<page>/<id>-0/root.txt - beside it trace/local.txt and trace/sequence/local.txt
1 1 trace start
1/1 2 occur start
2 3 level start {sequence '1..2'}
2/1 4 occur start
2/1 5 4 occur end 1
2/2 6 occur start
2/2 7 6 occur end 2
2 8 3 level end 12
1/1 9 2 occur end 12
1 10 1 trace end 12
#}