Debugging tips: nothing - Revision history

Thomas Linder Puls: category

2016-04-28T17:14:12Z

Thomas Linder Puls: released

2014-07-23T20:39:26Z

released

← Older revision		Revision as of 22:39, 23 July 2014
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	~~{{NonReleased}}~~

	This article contains a few debug tips. For all of them the solution is/involves <vp>core::nothing</vp>.		This article contains a few debug tips. For all of them the solution is/involves <vp>core::nothing</vp>.

Thomas Linder Puls: correction

2013-06-13T19:50:57Z

correction

← Older revision		Revision as of 21:50, 13 June 2013
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	<vip>clauses		<vip>clauses
	~~rrr~~() :-		ppp() :-
	qqq(). % "last call" disappears in the run stack and variables window</vip>		qqq(). % "last call" disappears in the run stack and variables window

			clauses
			qqq() :-
			rrr(),
			...</vip>

	When placing <vp>nothing</vp> last in a clause it is the one that receives last call optimizations, so the "old" last call is no longer optimized:		When placing <vp>nothing</vp> last in a clause it is the one that receives last call optimizations, so the "old" last call is no longer optimized:

	<vip>clauses		<vip>clauses
	~~rrr~~() :-		ppp() :-
	qqq(), % no longer last call, will appear in run stack		qqq(), % no longer last call, will appear in run stack
	core::nothing().</vip>		core::nothing().</vip>


	=== Non visible variable values ===		=== Non visible variable values ===

Thomas Linder Puls: initial

2013-06-13T13:10:05Z

initial

New page

{{NonReleased}}

This article contains a few debug tips. For all of them the solution is/involves <vp>core::nothing</vp>.

'''Caveat''': The tips all involves recompiling the program, so they may not be applied to all kind of customer scenarios.

<vp>core::nothing</vp> is a predicate that takes any arguments and does nothing</vp>.

<vip>predicates
nothing : (...) [deprecated("Should not be used in production code")]. % Not really deprecated
% @short This predicate does nothing, but is useful in debug context.
% For example to make sure that the variables are visible in the variables window.
% @end</vip>

It is declared as deprecated so that usage issues a deprecated warning, which reminds you about removing it again, when it has served its debugging purpose.

=== Last call optimization ===

Due to last call optimization entries may be missing in the run stack window (and therefore the local variables are lost). It looks as if <vp>ppp</vp> called <vp>rrr</vp>, but actualy <vp>ppp</vp> called <vp>qqq</vp> which then called <vp>rrr</vp>; the <vp>qqq</vp> entry and the local variables there are not present at that debug time.

<vip>clauses
rrr() :-
qqq(). % "last call" disappears in the run stack and variables window</vip>

When placing <vp>nothing</vp> last in a clause it is the one that receives last call optimizations, so the "old" last call is no longer optimized:

<vip>clauses
rrr() :-
qqq(), % no longer last call, will appear in run stack
core::nothing().</vip>

=== Non visible variable values ===

Due to other optimizations (this problem may become worse in the future, who knows) the values of variables may be "lost" at points where you would think they should still exist.

A typical example are semi-anonymous variables (e.g. <vp>_X</vp>) which often don’t show up in the variables window.

<vip>clauses
rrr() :-
get_some_things(_X, Y),
qqq(Y). % _X is not shown in the variables window</vip>

By passing variables as argument to <vp>nothing</vp>, you can keep/make them alive, so that they can be seen in the variables window:

<vip>clauses
rrr() :-
get_some_things(X, Y),
qqq(Y),
core::nothing(X). % keep X alive/visible even though we don't really need it</vip>

=== Conditional break points ===

People often suggest that breakpoints should be conditional. Actually they are, but only very few knows the language, in which you write the conditions.

<vip>clauses
rrr(X) :-
qqq(X). % only break when X is "Special"</vip>

There is however one language you know, and in which you can write the appropriate condition: Visual Prolog.

<vip>clauses
rrr(X) :-
if "Special" = X then
core::nothing() % "conditional" break point here, i.e. only break when X is "Special"
end if,
qqq(X).</vip>

I used to write <vp>succeed</vp> instead of <vp>nothing</vp>. It is just as good for breaking, but unfortunately it has a tendency to stay in the code, when it goes into production. <vp>nothing</vp> is deprecated, so there is a much larger chance that it is found in due time.

← Older revision		Revision as of 19:14, 28 April 2016
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	I used to write <vp>succeed</vp> instead of <vp>nothing</vp>. It is just as good for breaking, but unfortunately it has a tendency to stay in the code, when it goes into production. <vp>nothing</vp> is deprecated, so there is a much larger chance that it is found in due time.		I used to write <vp>succeed</vp> instead of <vp>nothing</vp>. It is just as good for breaking, but unfortunately it has a tendency to stay in the code, when it goes into production. <vp>nothing</vp> is deprecated, so there is a much larger chance that it is found in due time.

			[[Category:Tutorials]]