ITEC 460 - Chapter 7 Notes

7.1 Semantic Errors - Overview

Example: Functions and variables must be defined/declared

Example: No type errors (ops only applied to valid values)

CFG can not generate all and only programs without type errors

How to check: not with embedded actions!

How to check: Visitor Pattern!

Focus on compile time checks (where do Java and Ada do runtime checks?)

7.1 Semantic Errors

Type errors

Structured Variable Errors

Type Errors - Examples:

Arithmetic Operators: generate code for coercion?

Polymorphism?

Assignment compatibility

Name or structural compatibility

Tests: boolean or true/false values

Function and Method Errors

Call must match signature
Parameters: Assignment compatibility
Parameters: Subtypes??? Co-contra variant???

Access Violation Errors:

Classes
Nested routines

7.2 Environments

Need table(s) of definitions
Problem: nested scopes
Solution 1: list of key, value pairs (eg name, type)
Solution 2: Hash table w/ lists of key, value pairs
Example: Figures 7.3, 4, 6, 7
Use KeyStack to simulate popping environments (Fig 7.7)
Solution 3: BST of key, value pairs

7.3 Overall Structure

Variable Environment - all local and parameter variables

Type Environment - all built-in and class types

Built-in types: int, boolean, void
Array Types: entry for base type
Class types: each has its own variable environment for its fields

Function (prototype) environment

Functions must be defined (or prototyped) before use [is this documented anywhere]

7.3 Type Checking - Categories of Types

Built-in types (aka ...)
Enumerated types and subranges (primitive/subtype?)
Structured types: collection of other types: array, record, class, struct, variant records, unions, sets
Pointer types (and references)
Function types: First class objects? C, Ada, Java, Haskell

7.3 SimpleJava - Built-in Types

Builtin types include: INTEGER, BOOLEAN, VOID
Type Environment: each builtin has entry in environment
Internal representation entry contains pointer to type object
Variable entries also contain pointer to type object
Type checking is simply pointer checking
Example: Figure 7.9, declare x,y,a,b; x = y; x = a;

7.3 SimpleJava - Class Types

New kind of type: CLASS
Add entry to Type environment for the class
Internal representation value is a new variable environment
Example: Figures 7.10,11, and a.x = c

7.3 SimpleJava - Array Types

New kind of type: ARRAY
Add entry to Type environment for the array type
Internal representation value is an ARRAY type object
ARRAY type object has pointer to base type
Example: Figures 7.12,13
Example: int a[]

Is array type (int[]) in type table
Is base type (int) in type table

No: error
Yes: Add new array type to table

Add variable to variable environment

7.4 Semantic Analysis

Traverse AST with a Visitor and ...

Add declarations to table
Check other code for errors

7.4 Variable and Array Declarations

int x; - check if int exists and add x to variable environment
MyClass y; - ...
int a[]; - ... (how is this different?)

May have to add several base types

                int A[][][];
                int B[]
                int C[][];

7.4 SimpleJava - Function Prototypes

New Environment: Functions
New Kind of Entry: FUNCTION
FUNCTION contains: return type and param list
Example: Fig 7.14

7.4 Class Definitions

Create new variable environment
Add fields to the new environment
Create new entry in type environment
Example: Figure 7.15

            class MyClass{
                int integerval;
                int Array[];
                boolean boolval;
            }

7.4 Function Definition

Compare type and number of formal parameters with prototype

If no prototype exists, add one to table
Begin a new scope in the variable environment
Add formal parameters to the variable environment
Visit function body (with modified variable environment)
End current scope (removing ...)

7.4 Expressions

Evaluate operand types
Determine return type
Examples: x+y, x++, x < y; foo(x)
Recursion is your friend - arbitrary complexity

7.4 Variables and Simple Variables

Check if defined in current scope
Determine (ie return) type

Simple variables - simple

7.4 Array Variables

Check type of index: must be ...
Check type of base variable: must be ...
Return type of base variable (from type environment entry array variable)
Examples pages 176 and 177

7.4 Instance Variables

Example: base.field
Variable base must be class variable
look up field in variable environment for base
Return type for field
base can have arbitrary complexity - so use ...

7.4 Statements

Look at the parts
Example: If Statements
Example: Assignment Statements

Topic

Last modified on