|
|
|
|
@@ -6306,6 +6306,162 @@ Unify the current value of mutable term @var{M} with term @var{D}.
|
|
|
|
|
Set the current value of mutable term @var{M} to term @var{D}.
|
|
|
|
|
@end table
|
|
|
|
|
|
|
|
|
|
<<<<<<< yap.tex
|
|
|
|
|
@node Global Variables, Profiling, Profiling, Term Modification, Top
|
|
|
|
|
@section Global Variables
|
|
|
|
|
|
|
|
|
|
@cindex global variables
|
|
|
|
|
|
|
|
|
|
Global variables are associations between names (atoms) and
|
|
|
|
|
terms. They differ in various ways from storing information using
|
|
|
|
|
@node{assert/1} or @node{recorda/3}.
|
|
|
|
|
|
|
|
|
|
@itemize @bullet
|
|
|
|
|
@item The value lives on the Prolog (global) stack. This implies that
|
|
|
|
|
lookup time is independent from the size of the term. This is
|
|
|
|
|
particularly interesting for large data structures such as parsed XML
|
|
|
|
|
documents or the CHR global constraint store.
|
|
|
|
|
|
|
|
|
|
@item They support both global assignment using nb_setval/2 and
|
|
|
|
|
backtrackable assignment using b_setval/2.
|
|
|
|
|
|
|
|
|
|
@item Only one value (which can be an arbitrary complex Prolog term) can be associated to a variable at a time.
|
|
|
|
|
|
|
|
|
|
@item Their value cannot be shared among threads. Each thread has its own
|
|
|
|
|
namespace and values for global variables.
|
|
|
|
|
@end itemize
|
|
|
|
|
|
|
|
|
|
Currently global variables are scoped globally. We may consider module
|
|
|
|
|
scoping in future versions. Both @code{b_setval/2} and
|
|
|
|
|
@code{nb_setval/2} implicitly create a variable if the referenced name
|
|
|
|
|
does not already refer to a variable.
|
|
|
|
|
|
|
|
|
|
Global variables may be initialised from directives to make them
|
|
|
|
|
available during the program lifetime, but some considerations are
|
|
|
|
|
necessary for saved-states and threads. Saved-states to not store
|
|
|
|
|
global variables, which implies they have to be declared with
|
|
|
|
|
@code{initialization/1} to recreate them after loading the saved
|
|
|
|
|
state. Each thread has its own set of global variables, starting with
|
|
|
|
|
an empty set. Using @code{thread_initialization/1} to define a global
|
|
|
|
|
variable it will be defined, restored after reloading a saved state
|
|
|
|
|
and created in all threads that are created after the
|
|
|
|
|
registration. Finally, global variables can be initialised using the
|
|
|
|
|
exception hook called @code{exception/3}. The latter technique is by
|
|
|
|
|
CHR.
|
|
|
|
|
|
|
|
|
|
@table @code
|
|
|
|
|
@item b_setval(+@var{Name}, +@var{Value})
|
|
|
|
|
@findex b_setval/2
|
|
|
|
|
@snindex b_setval/2
|
|
|
|
|
@cnindex b_setval/2
|
|
|
|
|
Associate the term @var{Value} with the atom @var{Name} or replaces
|
|
|
|
|
the currently associated value with @var{Value}. If @var{Name} does
|
|
|
|
|
not refer to an existing global variable a variable with initial value
|
|
|
|
|
[] is created (the empty list). On backtracking the assignment is
|
|
|
|
|
reversed.
|
|
|
|
|
|
|
|
|
|
@item b_getval(+@var{Name}, -@var{Value})
|
|
|
|
|
@findex b_getval/2
|
|
|
|
|
@snindex b_getval/2
|
|
|
|
|
@cnindex b_getval/2
|
|
|
|
|
Get the value associated with the global variable @var{Name} and unify
|
|
|
|
|
it with @var{Value}. Note that this unification may further
|
|
|
|
|
instantiate the value of the global variable. If this is undesirable
|
|
|
|
|
the normal precautions (double negation or @var{copy_term/2}) must be
|
|
|
|
|
taken. The @var{b_getval/2} predicate generates errors if @var{Name} is not
|
|
|
|
|
an atom or the requested variable does not exist.
|
|
|
|
|
|
|
|
|
|
@item nb_setval(+@var{Name}, +@var{Value})
|
|
|
|
|
@findex nb_setval/2
|
|
|
|
|
@snindex nb_setval/2
|
|
|
|
|
@cnindex nb_setval/2
|
|
|
|
|
Associates a copy of @var{Value} created with @var{duplicate_term/2} with
|
|
|
|
|
the atom @var{Name}. Note that this can be used to set an initial
|
|
|
|
|
value other than @code{[]} prior to backtrackable assignment.
|
|
|
|
|
|
|
|
|
|
@item nb_getval(+@var{Name}, -@var{Value})
|
|
|
|
|
@findex nb_getval/2
|
|
|
|
|
@snindex nb_getval/2
|
|
|
|
|
@cnindex nb_getval/2
|
|
|
|
|
The @code{nb_getval/2} predicate is a synonym for @code{b_getval/2},
|
|
|
|
|
introduced for compatibility and symmetry. As most scenarios will use
|
|
|
|
|
a particular global variable either using non-backtracable or
|
|
|
|
|
backtrackable assignment, using @code{nb_getval/2} can be used to
|
|
|
|
|
document that the variable is used non-backtracable.
|
|
|
|
|
|
|
|
|
|
@item nb_linkval(+@var{Name}, +@var{Value})
|
|
|
|
|
@findex nb_linkval/2
|
|
|
|
|
@snindex nb_linkval/2
|
|
|
|
|
@cnindex nb_linkval/2
|
|
|
|
|
Associates the term @var{Value} with the atom @var{Name} without
|
|
|
|
|
copying it. This is a fast special-purpose variation of @code{nb_setval/2}
|
|
|
|
|
intended for expert users only because the semantics on backtracking
|
|
|
|
|
to a point before creating the link are poorly defined for compound
|
|
|
|
|
terms. The principal term is always left untouched, but backtracking
|
|
|
|
|
behaviour on arguments is undone if the original assignment was
|
|
|
|
|
trailed and left alone otherwise, which implies that the history that
|
|
|
|
|
created the term affects the behaviour on backtracking. Please
|
|
|
|
|
consider the following example:
|
|
|
|
|
|
|
|
|
|
@example
|
|
|
|
|
demo_nb_linkval :-
|
|
|
|
|
T = nice(N),
|
|
|
|
|
( N = world,
|
|
|
|
|
nb_linkval(myvar, T),
|
|
|
|
|
fail
|
|
|
|
|
; nb_getval(myvar, V),
|
|
|
|
|
writeln(V)
|
|
|
|
|
).
|
|
|
|
|
@end example
|
|
|
|
|
|
|
|
|
|
@item nb_set_shared_val(+@var{Name}, +@var{Value})
|
|
|
|
|
@findex nb_set_shared_val/2
|
|
|
|
|
@snindex nb_set_shared_val/2
|
|
|
|
|
@cnindex nb_set_shared_val/2
|
|
|
|
|
Associates the term @var{Value} with the atom @var{Name}, but sharing
|
|
|
|
|
non-backtrackable terms. This may be useful if you want to rewrite a
|
|
|
|
|
global variable so that the new copy will survive backtracking, but
|
|
|
|
|
you want to share structure with the previous term.
|
|
|
|
|
|
|
|
|
|
The next example shows the differences between the three built-ins:
|
|
|
|
|
@example
|
|
|
|
|
demo_nb_linkval :-
|
|
|
|
|
T = nice(N),
|
|
|
|
|
( N = world,
|
|
|
|
|
nb_linkval(myvar, T),
|
|
|
|
|
fail
|
|
|
|
|
; nb_getval(myvar, V),
|
|
|
|
|
writeln(V)
|
|
|
|
|
).
|
|
|
|
|
@end example
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
@item nb_current(?@var{Name}, ?@var{Value})
|
|
|
|
|
@findex nb_current/2
|
|
|
|
|
@snindex nb_current/2
|
|
|
|
|
@cnindex nb_current/2
|
|
|
|
|
Enumerate all defined variables with their value. The order of
|
|
|
|
|
enumeration is undefined.
|
|
|
|
|
|
|
|
|
|
@item nb_delete(+@var{Name})
|
|
|
|
|
@findex nb_delete/2
|
|
|
|
|
@snindex nb_delete/2
|
|
|
|
|
@cnindex nb_delete/2
|
|
|
|
|
Delete the named global variable.
|
|
|
|
|
@end table
|
|
|
|
|
|
|
|
|
|
Global variables have been introduced by various Prolog
|
|
|
|
|
implementations recently. We follow the implementation of them in
|
|
|
|
|
SWI-Prolog, itself based on hProlog by Bart Demoen.
|
|
|
|
|
|
|
|
|
|
GNU-Prolog provides a rich set of global variables, including
|
|
|
|
|
arrays. Arrays can be implemented easily in YAP and SWI-Prolog using
|
|
|
|
|
@code{functor/3} and @code{setarg/3} due to the unrestricted arity of
|
|
|
|
|
compound terms.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
@node Profiling, Call Counting, Global Variables, Top
|
|
|
|
|
=======
|
|
|
|
|
@node Global Variables, Profiling, Term Modification, Top
|
|
|
|
|
@section Global Variables
|
|
|
|
|
|
|
|
|
|
@@ -6511,6 +6667,7 @@ compound terms.
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
@node Profiling, Call Counting, Global Variables, Top
|
|
|
|
|
>>>>>>> 1.222
|
|
|
|
|
@section Profiling Prolog Programs
|
|
|
|
|
|
|
|
|
|
@cindex profiling
|
|
|
|
|
|
vsc