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Show CHAPTER 3 GCS'S IMPLEMENTATION 2·1 The Parser The MINI [8] parser generator system was used to generate t he parser to parse conditional expressions. Specification of a parser using MINI consists of defining the syntax with BNF-like rules and semantics with LISP expressions. The parsers used in GCS fall in two classes, and are discussed as follows. The first type of parser is used for parsing conditional expressions when a designer creates a global input variable, a state or a transition. When a designer selects menu items for these items, GCS allows him to specify logical expressions for transition or output conditions. At this time, GCS invokes a parser to parse these expressions which are typed into the input window by the designer. This parser only parses logical expressions. Another type of parser is used when the designer wants to modify information about global inputs, states or transitions as have been described in section 2.3. Basically, when menu items are selected for these features, GCS displays the corresponding information in a tabular form on an edit pad so that the user can modify them. After the designer has done the editing, a specific parser (depending on whether it is an input, state or transition that is being modified) will be invoked to parse this new information and update the database. |
