Parsing declarations and procedure calls
16.1.5 Parsing declarations and procedure calls
The extensions needed to the attributed grammar to describe the process of parsing procedure declarations and calls can be understood with reference to the Cocol extract below, where, for temporary clarity, we have omitted the actions needed for code generation, while retaining those needed for constraint analysis: PRODUCTIONS Clang = . TABLE_entries entry; . PROGRAM Idententry.name . entry.idclass = TABLE_progs; Table-enterentry; Table-openscope; . WEAK ; Block . . Block = SYNC { ConstDeclarations | VarDeclarations | ProcDeclaration SYNC } CompoundStatement . Table-closescope; . . ProcDeclaration = . TABLE_entries entry; . PROCEDURE Idententry.name . entry.idclass = TABLE_procs; Table-enterentry; Table-openscope; . WEAK ; Block ; . Statement = SYNC [ CompoundStatement | AssignmentOrCall | IfStatement | WhileStatement | ReadStatement | WriteStatement ] . AssignmentOrCall = . TABLE_entries entry; . DesignatorclasssetTABLE_vars, TABLE_procs, entry . if entry.idclass = TABLE_vars SemError210; . := Expression SYNC | . if entry.idclass = TABLE_procs SemError210; . . The reader should note that: Variables declared local to a Block will be associated with a level one higher than the block identifier itself. In a hand-crafted parser we can resolve the LL1 conflict between the assignments and procedure calls within the parser for Statement, on the lines of void Statementsymset followers { TABLE_entries entry; bool found; if FirstStatement.membSYM.sym allow for empty statements { switch SYM.sym { case SCAN_identifier: must resolve LL1 conflict Table-searchSYM.name, entry, found; look it up if found Report-error202; undeclared identifier if entry.idclass == TABLE_procs ProcedureCallfollowers, entry; else Assignmentfollowers, entry; break; case SCAN_ifsym: other statement forms IfStatementfollowers; break; as needed } testfollowers, EmptySet, 32; synchronize if necessary } Exercises 16.7 In Exercise 14.50 we suggested that undeclared identifiers might be entered into the symbol table and assumed to be variables at the point where they were first encountered. Investigate whether one can do better than this by examining the symbol which appears after the offending identifier. 16.8 In a hand-crafted parser, when calling Block from within ProcDeclaration the semicolon symbol has to be added to Followers, as it becomes the legal follower of Block. Is it a good idea to do this, since the semicolon a widely used and abused token will then be an element of all Followers used in parsing parts of that block? If not, what does one do about it?16.2 Storage management
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