Error Handling: SYNTAX

Transcription

1 Error Handling: SYNTAX Basic Model Parse tree + w w S treep t 1 t 2 t 3 t n wp $ Cursors: treep & wp Error condition treep & wp are out of synchronisation Error recovery Re synch treep & wp Assumptions Missing symbol in w do nothing Extra symbol in w skip symbol 1DFR - PL - Errors

2 Missing Symbol do nothing Example testb.pas (keyword program missing) W is testb ( input, output ) ;. $ wp testb parser: prog() { prog_header(); } treep prog_header() prog_header() { match(program); syntax error: expected program found ID match(id); wp and treep are re synchronised! match( ( ); } 2DFR - PL - Errors

3 Missing Symbol do nothing This works for test cases testb.pas up to testk.pas (see the outputs and trace outputs) The trace output is useful for checking re synchronisation read these trace outputs! The first problem comes in testl.pas Check the output & trace output! 3DFR - PL - Errors

4 Missing symbol testl.pas program testl (input, output); var A B, C: integer; begin A := B + C * 2 end. The comma is missing between A and B, wp B What happens? SYNTAX: Symbol expected : found B SYNTAX: Type name expected found B SYNTAX: Symbol expected ; found B comma is used to decide if several ids in id_list() id_list() is A; back in var_dec() colon expected type name expected semicolon expected exit var_dec() & call var_dec() NOW re synchronisation on B, C : integer; A is of unknown type (semantic error see the symbol table) Other solution(s)? 4DFR - PL - Errors

5 Missing begin testp.pas program testp (input, output); var A, B, C: integer; begin missing A := B + C * 2 end. SEMANTIC: ID already declared: A SYNTAX: Symbol expected : found := SYNTAX: Type name expected found := SYNTAX: Symbol expected ; found := SYNTAX: Symbol expected begin found := SYNTAX: ID expected found := SEMANTIC: Assign types: error := integer wp A (in the assign ) treep var_dec_list() A is id; treep var_dec() wp := (in the assign ) finally treep assign_stat() and the parse continues but with 1 semantic error since the id in assign_stat() (id := <expr>) has been skipped treated as an id in id_list() instead! 5DFR - PL - Errors

6 Missing symbol testr.pas program testr (input, output); var A, B, C: integer; begin A = B + C * 2 end. SYNTAX: Symbol expected := found = SYNTAX: Operand Expected SEMANTIC: Assign types: integer := error SYNTAX: Symbol expected end found = SYNTAX: Symbol expected. found = SYNTAX: Extra symbols after end of parse! = B + C * 2 end. treep assign_stat() treep expr() treep term() treep factor() treep operand() treep assign_stat() treep stat() treep reached end of tree BUT wp = (not $) 6DFR - PL - Errors

7 Missing symbol A similar sequence to testr.pas takes place in tests.pas testu.pas testv.pas testw.pas 7DFR - PL - Errors

8 General Points on Syntax Errors The most common syntax error is emitted by the match function expected X found Y Syntax errors may only be detected when terminal symbols are processed AND there are no empty rules in the grammar See the examples below 8DFR - PL - Errors

9 Top Down Parser: Example Original grammar left recursive line expr { ; expr}* ; '\n expr expr + term term term term * factor factor factor '(' expr ')' DIGIT * = 0 or more occurrences New version right recursive line expr { ; expr} ; '\n expr term R1 R1 e '+' term R1 term factor R2 R2 e '*' factor R2 factor '(' expr ')' DIGIT (e = empty) This is an alternative conversion to a right recursive grammar, taken from The Dragon Book (1986) 9DFR - CC - Parser Code

10 No Syntax Error detection possible Non terminal symbols only void expr() { term(); R1(); } Empty rule in grammar void R1() { if (lookahead == '+') { match('+'); term(); R1(); } /* else empty case */ } DFR - PL - Errors 10

11 Syntax Error detection possible void factor() { if (DEBUG) printf("\n *** In factor *** "); // factor '(' expr ')' DIGIT if (lookahead == '( ) { match('('); expr(); match(')'); return; } else if (lookahead == DIGIT) { match(lookahead); return; } else error("*** UNEXPECTED SYMBOL *** (in procedure factor)"); }; DFR - CC - Parser Code 11

12 Error Handling: SEMANTIC Identifier doubly declared (in var_part() ) Id is already in the symbol table Identifier NOT declared (in stat_part() ) Id is NOT in the symbol table Type errors In assign() id and expression not of same type In expr() arguments to operators not of same type (checked via the operator table) DFR - PL - Errors 12

13 Note: significance of KEYWORDS program program_header() var var_part() begin stat_part() In the case of statements id assignment statement if if statement (else else part) while while statement (etc: for; do; case; switch; repeat; ) Data Code DFR - PL - Errors 13

14 Note the significance of separators <id_list> id <id_list>, id <id_list> id {, id } * <id_list> id id, <id_list> (BNF LR) (EBNF) (BNF RR) LR = Left Recursive, RR = Right Recursive In <stat_list> the ; has a similar role to, In <expr> the + has a similar role to, In <term> the * has a similar role to, DFR - PL - Errors 14

15 Note the significance of separators In <var_dec_list> <var_dec> <var_dec_list> <var_dec> there is no separator The recursion is decided by Id which signals the beginning of a <var_dec> This works except in the testp.pas case (missing begin) Question: how would you fix this problem using the grammar? DFR - PL - Errors 15

16 Summary The 2 main assumptions in error handling Missing symbol in w (do nothing) Extra symbol in w (skip symbol) Error recovery means re synchronising the 2 pointers wp and the treep Syntax errors usually from match Expected X found Y Semantic errors Symbol table: X doubly declared (var_part) / X not declared (stat_part) Type checking: A := <expr> and in <expr> DFR - PL - Errors 16