The following BNF grammar gives a concise description of the syntax of
class definitions. It assumes an understanding of Prolog syntax for
the following items: variable, atom, compound_term,
and constant. Slot types, particularly the address, class and
pointer types, were discussed in an earlier section.
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| class_def | ::= class_begin { clause | method } class_end
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| class_begin | ::= :- class class_name opt_class_spec .
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| opt_class_spec | ::= empty | = class_spec
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| class_spec | ::= multi_parent_or_slots | term_class_spec
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| clause | ::= head opt_body .
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| head | ::= atom | compound_term .
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| method | ::= message_head opt_body .
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| message_head | ::= message_goal
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| class_end | ::= :- end_class opt_class_name .
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| | empty /* if followed by class_begin or eof */
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| message | ::= atom | compound_term
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| multi_parent_or_slots | ::= parent_or_slots { + parent_or_slots }
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| parent_or_slots | ::= class_name | [] | [ slot_def {, slot_def } ]
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| slot_def | ::= opt_visibility slot_name : slot_type opt_init_value
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| opt_visibility | ::= empty | private | protected | public
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| opt_init_value | ::= empty | = constant
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| term_class_spec | ::= term(term opt_goal_essence)
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| opt_goal_essence | ::= empty | , goal opt_essence
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| opt_essence | ::= empty | , essence
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| essence | ::= [ variable : slot_type { , variable : slot_type } ]
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| opt_body | ::= empty | :- body
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| body | ::= message_or_goal { , message_or_goal }
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| message_or_goal | ::= message_goal | goal
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| message_goal | ::= variable message_operator message
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| message_operator | ::= << | >> | <-
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| opt_class_name | ::= empty | class_name
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| class_name | ::= atom
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| slot_name | ::= atom
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| slot_type | ::= integer
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| short
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| char
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| unsigned_short
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| unsigned_char
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| float
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| double
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| atom
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| address
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| term
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| | class_name
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| pointer(atom)
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