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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.
class_def | ::= class_begin { clause | method } class_end |
class_begin | ::= :- class class_name opt_class_spec . |
opt_class_spec | ::= empty | = class_spec |
class_spec | ::= multi_parent_or_slots | term_class_spec |
clause | ::= head opt_body . |
head | ::= atom | compound_term . |
method | ::= message_head opt_body . |
message_head | ::= message_goal |
class_end | ::= :- end_class opt_class_name . |
| empty /* if followed by class_begin or eof */ | |
message | ::= atom | compound_term |
multi_parent_or_slots | ::= parent_or_slots { + parent_or_slots } |
parent_or_slots | ::= class_name | [] | [ slot_def {, slot_def } ] |
slot_def | ::= opt_visibility slot_name : slot_type opt_init_value |
opt_visibility | ::= empty | private | protected | public |
opt_init_value | ::= empty | = constant |
term_class_spec | ::= term( term opt_goal_essence) |
opt_goal_essence | ::= empty | , goal opt_essence |
opt_essence | ::= empty | , essence |
essence | ::= [ variable : slot_type { , variable : slot_type } ] |
opt_body | ::= empty | :- body |
body | ::= message_or_goal { , message_or_goal } |
message_or_goal | ::= message_goal | goal |
message_goal | ::= variable message_operator message |
message_operator | ::= << | >> | <- |
opt_class_name | ::= empty | class_name |
class_name | ::= atom |
slot_name | ::= atom |
slot_type | ::= integer |
| short | |
| char | |
| unsigned_short | |
| unsigned_char | |
| float | |
| double | |
| atom | |
| address | |
| term | |
| class_name | |
| pointer(atom) | |