Chapter 6 Type Checking Compiler Design

1. TYPE CHECKING
Muhammad Daniyal
Salman
Saif Ali

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