1 / 8

Review: Why symbol table? Given an example of the attributes in a symbol table.

Review: Why symbol table? Given an example of the attributes in a symbol table. How to deal with nested scope? How to create the symbol table? Grammar: P->D D ->D; D D->id : T D->proc id; D; S T ->integer | real | array[num] of T | ^T. Type checking

pippa
Download Presentation

Review: Why symbol table? Given an example of the attributes in a symbol table.

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Review: • Why symbol table? Given an example of the attributes in a symbol table. • How to deal with nested scope? • How to create the symbol table? • Grammar: P->D D ->D; D D->id : T D->proc id; D; S T ->integer | real | array[num] of T | ^T

  2. Type checking • make sure operations can be performed on operands. • Make sure the types of actual arguments matches the types of formal arguments. • Need a type system to do the job. • A type system is a collection of rules for assigning type expression to the various parts of a program. • The type system for a practical language can be complicated. • Type checking of expressions: P->D;E D->D;D | id : T T->char | integer | array[num] of T | ^T E->literal | num | id | E mod E | E[E] | E^

  3. P->D;E D->D;D D->id : T {enter(id.val, T.type);} T->char {T.type = char;} | integer {T.type = integer;} | array[num] of T1 {T.type = array(num.val, T1.type);} | ^T1 {T.type = pointer(T1.type);} E->literal {E.type = char;} | num {E.type = integer;} | id {E.type = lookup(id.val);} | E1 mod E2 {if E1.type == integer && E2.type ==integer then E.type = integer; else E.type =error;} | E1[E2] {if E1.type == array(s, t) && E2.type == integer then E.type = t; else E.type =error;} | E1^ {if E1.type == pointer(t) then E.type = t; else E.type =error;}

  4. Type checking for statements S -> id := E S -> if E then S1 S ->while E do S1 S->S1;S2

  5. Type checking for statements S -> id := E {if id.type == E.type then S.type = void; else S.type = error;} S -> if E then S1 {if E.type == boolean then S.type = S1.type; else S.type = error;} S ->while E do S1 { if E.type == boolean then S.type = S1.type; else S.type = error;} S->S1;S2 {if S1.type == void and S2.type == void then S.type = void; else S.type = type_error; }

  6. Type checking for functions: T->T1->T2 /* function declaration */ {T.type = T1.type ->T2.type} E->E1(E2) /* function call */ {if E1.type == t1.type->t2.type && E2.type == t1.type then T.type = t2.type; else T.type - error; }

  7. Equivalence of type expressions • Name equivalence - each type with different name is different • structural equivalence - names are replaced by the type expressions they define • Example: type link = ^cell; var next : link last : link p: ^cell • Is structural equivalence good for C++?

  8. Other things related to type. • coercion: implicit type conversion: • e.g. double x; ….x = 1; • overloading: • a function or operator can represent different operations in different contexts. • polymorphic functions: • the body of a polymorphic function can be executed with arguments of different types.

More Related