UP Lecture 14

Compilers
Error Handling II
(202403)

Transcript

1. jgs Compilers Lecture 14: Error Handling II Dr. Javier Gonzalez-Sanchez jgonzalezs@up.edu.mx
2. jgs Error Handling Version 2
3. Dr. Javier Gonzalez-Sanchez | Compilers | 3 jgs Error Handling a) Panic One error, then Stop a) Simple Either ignore or increase the current token. a) First and Follow Each rule is responsible for validating the start and end.
4. Dr. Javier Gonzalez-Sanchez | Compilers | 4 jgs Error Recovery At this point, Error recovery is about ignoring tokens. § How do we know which tokens should be ignored and how many?
5. Dr. Javier Gonzalez-Sanchez | Compilers | 5 jgs Parser | Error Recovery Rule FIRST set FOLLOW set PROGRAM { EOF BODY FIRST (PRINT) U FIRST (ASIGNMENT) U FIRST(VARIABLE) U FIRST (WHILE) U FIRST(IF) U FIRST (RETURN) } PRINT print ; ASSIGNMENT identifier ; VARIABLE int, float, boolean, void, char, string ; WHILE while } U FIRST(BODY) IF if } U FIRST(BODY) RETURN return ; EXPRESSION FIRST(X) ), ; X FIRST(Y) | U FOLLOW(EXPRESSION) Y ! U FIRST(R) & U FOLLOW(X) R FIRST(E) FOLLOW(Y) E FIRST (A) !=, ==, >, < U FOLLOW(R) A FIRST (B) -, + U FOLLOW(E) B - U FIRST (C) *, /, U FOLLOW(A) C integer, octal, hexadecimal, binary, true, false, string, char, float, identifier, ( FOLLOW(B)
6. jgs Calculating the First Set
7. Dr. Javier Gonzalez-Sanchez | Compilers | 7 jgs Calculate the FIRST set loop
8. Dr. Javier Gonzalez-Sanchez | Compilers | 8 jgs Calculate the FIRST set 1.FIRST(X) = {X} if X is a terminal 2. FIRST(ε) = {ε}. note that this is not covered by the first rule because ε is not a terminal. 3.If A → Xα, add FIRST(X) − {ε} to FIRST(A) 4.If A → A1 A2 A3 ...Ai Ai+1 ... Ak and ε ∈ FIRST (A1 ) and ε ∈ FIRST (A2 ) and . . . and ε ∈ FIRST (Ai ), then add FIRST (Ai+1 ) − {ε} to FIRST (A). 5.If A → A1 A2 A3 ...Ak and ε ∈ FIRST(A1 ) and ε ∈ FIRST(A2 ) and... and ε ∈ FIRST(Ak ), then add ε to FIRST(A).
9. Dr. Javier Gonzalez-Sanchez | Compilers | 9 jgs FIRST set Definition FIRST (a) is the set of tokens that can begin the construction a. Example <E> → <A> {+ <A>} <A> → <B> {* <B>} <B> → -<C> | <C> <C> → integer FIRST(E) = {-, integer} FIRST(A) = {-, integer} FIRST(B) = {-, integer} FIRST(C) = {integer}
10. Dr. Javier Gonzalez-Sanchez | Compilers | 10 jgs FIRST set Define FIRST (BODY) FIRST(BODY) = FIRST (PRINT) U FIRST (ASSIGNMENT) U FIRST(VARIABLE) U FIRST(WHILE) U FIRST(IF) U FIRST(RETURN)
11. Dr. Javier Gonzalez-Sanchez | Compilers | 11 jgs FIRST set Define FIRST (C)
12. Dr. Javier Gonzalez-Sanchez | Compilers | 12 jgs FIRST set Define FIRST (A) Define FIRST (B)
13. Dr. Javier Gonzalez-Sanchez | Compilers | 13 jgs FIRST set <S> → <A><B><C> <S> → <F> <A> → <E><F>d <A> → a <B> → a<B>b <B> → ε <C> → c<C> <C> → d <E> → e<E> <E> → <F> <F> → <F>f <F> → ε
14. Dr. Javier Gonzalez-Sanchez | Compilers | 14 jgs FIRST set S → ABC S → F A → EFd A → a B → aBb B → ε C → cC C → d E → eE E → F F → Ff F → ε rule FIRST set - evolution S ø {a, ε} {a, ε, e, f} {a, ε, e, f, d} A ø {a} {a, e} {a, e, f, d} B ø {a, ε} C ø {c, d} E ø {e} {e, ε} {e, ε, f} F ø {ε} {ε, f}
15. Dr. Javier Gonzalez-Sanchez | Compilers | 15 jgs FIRST set | Exercise <X> → <A> | <A> a <A> → <B> | <B> b <B> → <C><D><E> | c d e | <C> c <D> d <E> e <C> → one <D> → two <E> → three
16. Dr. Javier Gonzalez-Sanchez | Compilers | 16 jgs FIRST set | Exercise <X> → <A> <X> → <A> a <A> → <B> <A> → <B> b <B> → <C><D><E> <B> → c d e <B> → <C> c <D> d <E> e <C> → one <D> → two <E> → three OPTION 1: FIRST(X) = {c, one} FIRST(A) = {c, one} FIRST(B) = {c, one} FIRST(C) = {one} FIRST(D) = {two} FIRST(E) = {three} OPTION 2: FIRST(X) = {c, one, ε} FIRST(A) = {b, c, one} FIRST(B) = {c, one} FIRST(C) = {one} FIRST(D) = {two} FIRST(E) = {three}
17. Dr. Javier Gonzalez-Sanchez | Compilers | 17 jgs FIRST set | Exercise <X> → <A> <X> → <A> a <A> → <B> <A> → <B> b <B> → <C><D><E> <B> → c d e <B> → <C> c <D> d <E> e <C> → one <C> → ε <D> → two <D> → ε <E> → three FIRST(X) = {c, one, three, two} FIRST(A) = {c, one, three, two} FIRST(B) = {c, one, three, two} FIRST(C) = {one, ε} FIRST(D) = {two, ε} FIRST(E) = {three}
18. Dr. Javier Gonzalez-Sanchez | Compilers | 50 jgs Homework Working on your Parser (final version)
19. Dr. Javier Gonzalez-Sanchez | Compilers | 51 jgs Questions
20. jgs Compilers Javier Gonzalez-Sanchez, Ph.D. jgonzalezs@up.edu.mx Spring 2024 Copyright. These slides can only be used as study material for the Compilers course at Universidad Panamericana. They cannot be distributed or used for another purpose.