Type Checking - Lecture Slides | CMSC 430, Study notes of Computer Science

Download Type Checking - Lecture Slides | CMSC 430 and more Study notes Computer Science in PDF only on Docsity! 1 Type Checking CS430 2 Roadmap (Where are we?) Last lecture • Context-sensitive analysis → Motivation → Attribute grammars → Ad hoc Syntax-directed translation This lecture • Type checking → Type systems → Using syntax directed translation • Symbol tables → Lexical scoping → Implementation
Stack
Threaded stack CS430 3 Types Type: A set of values and meaningful operations on them Types provide semantic “sanity checks” (consistency checks) and determine efficient implementations for data objects Types help identify → errors, if an operator is applied to an incompatible operand
dereferencing of a non-pointer
adding a function to something
incorrect number of parameters to a procedure
… → which operation to use for overloaded names and operators, or what type coercion to use (e.g.: 3.0 + 1) → identification of polymorphic functions CS430 4 Type Systems Type system: Each language construct (operator, expression, statement, …) is associated with a type expression. The type system is a collection of rules for assigning type expressions to these constructs. Type expressions for → basic types
integer, char, real, boolean, typeError → constructed types // T is a type expression
array(lb…ub, T) // array of T
pointer(T) // pointer to T
T1 X T2 // tuple of T1, T2
T1 → T2 // function w/ arg T1 returning T2 CS430 5 Type Checker • A type checker implements a type system. It computes or “constructs” type expressions for each language construct • Static type checking → Detects type errors at compile time → No run time overhead → Not always possible (e.g., A[i]) • Dynamic type checking → Performed at run time → More flexible, allows prototyping → Run-time overhead to maintain & check tags CS430 6 Types Inference Rules • Specifies the type of an expression • Example → If operands of addition are of type integer, result is of type integer → Result of unary & operator is pointer to type of operand • Denotational semantics of type inference rule E e1 : integer E e2 : integer E (e1 + e2): integer where E is a type environment that maps constants and variables to their type expressions. • Question → How to specify rules that allow type coercion (type widening) from integers to reals in arithmetic expressions? 3.0 + 1 or 1 + 3.0 2 CS430 7 Type Equivalence Structural -- type equivalence: type names are expanded Name -- type equivalence: type names are not expanded Example: type A is array(1..10) of integer; type B is array(1..10) of integer; a : A; b : B; c, d: array(1..10) of integer; e: array(1..10) of integer; Answer: structural equivalence: name equivalence: (a, b, c, d, e) (a), (b), (c, d, e) CS430 8 Syntax Directed Translation Scheme (in CUP) Revisit our type inference rule for “+”. exp ::= exp:e1 PLUS exp:e2 {: if (e1 == sym.INT && e2 == sym.INT ) RESULT = sym.INT ; else { RESULT = typeError; System.out.println(“Error: illegal operand types”); } :} • The definition of type expression as Java types (static final int fields in class sym) should be done in mycc.cup. • The assignment of type expression Java types to terminals and nonterminals of the grammar is done in mycc.cup. CS430 9 Syntax Directed Translation Scheme (in Yacc) Revisit our type inference rule for “+”. exp : exp ‘+’ exp { if ($1 == integer && $3 == integer) $$ = integer; else { $$ = typeError; printf(“Error: illegal operand types\n”); } } • The definition of type expression as C types (structs) should be done in attr.h . attr.c may contain helper functions. • The assignment of type expression C types to terminals and nonterminals of the grammar is done in parse.y. CS430 10 Type Checker Example CS430 11 Type Checker Example (cont.) • Handling declarations CS430 12 Type Checker Example (cont.) • Handling expressions