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Type checking compiler construction Chapter #6
- 2. Lexical Analyzer • Tokens Syntax Analyzer • Context free grammar • Parse tree(Parser)
- 3. Semantic Analyzer • Before we don’t have rules and attributes in CFG • Then we add some semantic rule with productions Rule Syntax Directed Definition • Each grammar symbol associate with some attributes that gives information • CFG + Semantic Rules = Syntax directed definition
- 4. • Do not specify the order in which semantic actions should be executed. Translation Scheme • explicitly specify the ordering of the semantic actions
- 5. TYPES OF ATTRIBUTES • Synthesized • Inherited Synthesized These attributes get value from child node attributes Inherited These attributes get value from parent node or from siblings
- 6. What Semantic Analyzer do ? Scope resolution • Variables should be declare at least once before usage Array Bound Checking • When boundaries of an Array exceed Type Checking • Check type • Eg: int x = 15.5;
- 7. Type Checking • Type checking is the process of verifying that each operation executed in a program respects the type system of the language. • This generally means that all operands in any expression are of appropriate types and number. • Much of what we do in the semantic analysis phase is type checking.
- 8. How to design a Type Checker? When designing a type checker for a compiler, here’s the process: • Identify the types that are available in the language • Identify the language constructs that have types associated with them • Identify the semantic rules for the language • A language is considered strongly- typed if each and every type error is detected during compilation.
- 9. Type Checking Preventions • Application of a function to wrong number of arguments • Application of integer functions to floats • Use of undeclared variables in expressions • Functions that do not return values, • Division by zero • Array indices out of bounds
- 10. Two Types of Type Checking Static Type Checking Dynamic Type Checking Static Type Checking : Check on Compile time 1- Type Check • 2+2.5 = Error 2- Flow of Control • Flow of control stop at somewhere
- 11. 3- Uniqueness Check • Int a = 2; a should be unique 4- Name Related Check • Calling add() Definition add() Example: For example, if a and b are of type int and we assign very large values to them, a * b may not be in the acceptable range of ints, or an attempt to compute the ratio between two integers may raise a division by zero. These kinds of type errors usually cannot be detected at compiler time.
- 12. Dynamic Type Checking : Check on Runtime • Common dynamically typed languages are : JavaScript, Php and Python etc. • Most of the languages used both. • Static or Dynamic doesn’t mean Weak or strong
- 13. Type System • Type system is a collection of rules applied on Type expression • Designing of type checker vary from language to language E.G : 2+2 = 4 • Each expression has a type associated • Basic types: Boolean, Int , Char
- 14. • Constructed types : Pointer, Array , Structures Type Expression • A basic type is a type expression • A type name is a type expression • A type expression can be formed by applying the array type constructor to a number and a type expression. • A record is a data structure with named field • A type expression can be formed by using the type constructor for function types
- 15. • If s and t are type expressions, then their Cartesian product s*t is a type expression • Type expressions may contain variables whose values are type expressions Error Recovery • Type checker found an error • They report the location and nature of an error • Ideal type checker recover the error and move on to detect more
- 16. Type Checking of Statements S -> id := E {S.type := if id.type = E.type then void else error} S -> if E then S1 {S.type := if E.type = boolean then S1.type else error} S -> while E do S1 {S.type := if E.type = boolean then S1.type} S -> S1; S2 {S.type := if S1.type = void ∧
- 17. Equivalence of Type Expression Structural Equivalence Name for Type Expression Structural Equivalence: • Two expression have same basic type • Formed by applying same constructor type Name for Type Expression: • In some languages type expression can given some name
- 19. Type Conversion • Expression X + I (The X is real and I is int type) • First compiler check both of the operand are of same type or not • Usually convert int type into real type and perform real operand operation How to Convert • X + I (int convert into real) • X I intoreal real +
- 20. • Intoreal operation convert I intoreal • Or real+ operation perform real addition in between both real operands Coercions • Convert one type to another automatically called implicit or coercions like ASCII • It is limited in many language in case of no information loss • Integer convert into real but no vice versa because of some memory loss • When programmer write something to convert called Explicit
- 21. Over Loading Functions • The same name is used for several different operations over several different types. • Type checker is used to detect the error while creating overloading functions Polymorphic Functions • A piece of code that can be executed with arguments of different types
- 22. Thank you