[m-dev.] logging proposal

[Ralph Becket]

Ian MacLarty, Sunday, 12 March 2006:
> On Mon, Mar 06, 2006 at 11:41:59AM +1100, Julien Fischer wrote:
> > 
> > On Sun, 5 Mar 2006, Ian MacLarty wrote:
> > 
> > > On 2 Mar 2006, at 05:54, Ralph Becket wrote:
> > >
> > > > Mark Brown, Thursday,  2 March 2006:
> > > >> but I think I prefer the list syntax.
> > > >
> > > > Seconded.
> > >
> > > I'm happy with the list syntax.  I'm not sure when I'll get time to
> > > implement this though :-)
> > >
> > > I was thinking of implementing it in the following way:
> > >
> > 
> > I suggest that you first of all write the reference manual entry for
> > this feature and *then* we can discuss how to implement it.
> 
> Index: doc/reference_manual.texi
> ===================================================================
> RCS file: /home/mercury1/repository/mercury/doc/reference_manual.texi,v
> retrieving revision 1.345
> diff -u -r1.345 reference_manual.texi
> --- doc/reference_manual.texi	8 Mar 2006 02:25:38 -0000	1.345
> +++ doc/reference_manual.texi	11 Mar 2006 14:29:07 -0000
> @@ -642,55 +642,6 @@
>  
> +
> + at item @code{debug @var{Vars} @var{Goal}}
> +A goal used only for the purposes of debugging.
> + at var{Vars} is a list containing exactly two variables of the
> +type io.state.  
> + at var{Goal} is a deterministic goal that may perform I/O using the
> +io.state variables in @var{Vars}.
> +When debug goals are disabled they are equivalent to the goal @samp{true}.
> +When debug goals are enabled they are equivalent to the impure goal
> + at code{impure make_io_state(IO0), Goal, impure consume_io_state(IO)}
> +where IO0 and IO are the names of the variables given in @var{Vars} and
> + at code{make_io_state/1} constructs a fake @code{io.state} variable and 
> + at code{comsume_io_state/1} destroys the fake io.state, with the exception that
> +the impurity need not be propogated to the predicate the debug goal
> +appears in.

s/propogated/propagated/

> +
>  @end table
>  
>  @node State variables
> @@ -8819,6 +8833,50 @@
>  		impure Ys = map(ImpureFunc, Ys).
>  @end example
>  
> + at node Debug goals
> + at section Debug goals
> +
> +Sometimes it is useful for a program to have side effects only for the purposes
> +of debugging.
> +For example you may wish to observe the state of an executing
> +program by printing out the values of certain variables at different points
> +of execution.  
> +In these cases it is unnessecarily restrictive to either thread
> +an I/O state through all of your program

s/an I/O state/the I/O state/
s/through all of your program/to the appropriate goals/

> +(the program may be non-determinstic),

s/the program may be non-determinstic/the predicates of interest may not
be deterministic/

> +or to use the impurity system to achive the desired side effects
> +(since then the impurity must be propogated to all ancestors of the

s/propogated/propagated/

s/all ancestors of the/all callers of/

> +predicate(s) containing the impure debugging goals,

s/containing the impure debugging goals, ...)./containing impure debugging
goals)./

> +even when the program is not being debugged).
> +
> +The Mercury language therefore supports a special type of goal called a 
> + at emph{debug goal} (@pxref{Goals}).
> +Each debug goal has access to its own pair of io.state variables.

s/io.state/@code{io.state}/

> +
> +For example:
> +
> + at example
> +	:- pred perform_computation(float::in, float::out) is det.
> +
> +	perform_computation(Input, Output) :-
> +		do_complex_calculation(Input, Ouput),
> +		debug [!IO] (
> +			io.format("input = %.8f, output = %.8f\n",
> +				[f(Input), f(Output)], !IO)
> +		).
> + at end example
> +
> +Mercury compiler implemetations are required to support a boolean switch
> +which allows the user to enable or disable debug goals at compile time.
> +Debug goals only produce side effects if the compiler option to enable them is
> +given at compilation time.
> +If debug goals are disabled then they are equivalent
> +to the empty conjunction.

I think you should mention the switch name here.

> +Debug goals must pass type and mode checking regardless of whether they are
> +enabled or not.
> +For the purposes of mode checking the debug goals are 
> +treated as impure.
> +
>  @node Pragmas
>  @chapter Pragmas

Otherwise, that looks good.

-- Ralph
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