37 lines
1.3 KiB
Markdown
37 lines
1.3 KiB
Markdown
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Attributed Variables and Co-Routining {#attributes}
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=======================================
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@ingroup extensions
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YAP supports attributed variables, originally developed at OFAI by
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Christian Holzbaur. Attributes are a means of declaring that an
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arbitrary term is a property for a variable. These properties can be
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updated during forward execution. Moreover, the unification algorithm is
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aware of attributed variables and will call user defined handlers when
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trying to unify these variables.
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Attributed variables provide an elegant abstraction over which one can
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extend Prolog systems. Their main application so far has been in
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implementing constraint handlers, such as Holzbaur's CLPQR, Fruewirth
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and Holzbaur's CHR, and CLP(BN).
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Different Prolog systems implement attributed variables in different
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ways. Originally, YAP used the interface designed by SICStus
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Prolog. This interface is still
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available through the <tt>atts</tt> library, and is used by CLPBN.
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From YAP-6.0.3 onwards we recommend using the hProlog, SWI style
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interface. We believe that this design is easier to understand and
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work with. Most packages included in YAP that use attributed
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variables, such as CHR, CLP(FD), and CLP(QR), rely on the SWI-Prolog
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interface.
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+ @ref SICS_attributes
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+ @ref New_Style_Attribute_Declarations
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+ @ref CohYroutining
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+ @ref AttributeVariables_Builtins
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@}
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