CSE 431S Type Checking. Washington University Spring 2013

Transcription

1 CSE 431S Type Checking Washington University Spring 2013

2 Type Checking When are types checked? Statically at compile time Compiler does type checking during compilation Ideally eliminate runtime checks Dynamically at runtime Compiler generates code to do type checking at runtime JavaScript vs. Java Java still does a large amount of runtime type checking We ll focus on static typing for basic types

3 Types of Types Primitive types int, char, float, double, long, boolean Compound types array, struct, union, tuple, pointer? Object types OO language types Functions

4 What has a type? Variables (e.g. int x ) Functions Expressions (e.g. x + y ) + int float char int int float? float float float? char???

5 Compare + int float char int int float? float float float? char??? = int float char int int int int float float float float char char char char

6 Checking Expression Types z = x+y What is the type of x+y?

7 Checking Expression Types What if the variables are declared as: int x = 2; short y = 3; Is this allowed? What is the result (type and value)? What does the compiler need to do?

8 Expression Types For every operator we need to know allowed types of operands resulting type implicit coercion changes the representation, not the data short to long implicit conversion may change the data int to float explicit cast may lose information float to int, int to short

9 Which nodes have types? = x + y 3.14

10 A Visit from the Type Checking Visitor Depth first visit of nodes, perhaps using recursion: // pre-work foreach (child in childnodes) { child.accept(this); } // post-work

11 Type Checking Visitor 1 = 10 2 x y Pre-visit Post-visit

12 Type Checking Visit Why do work in post-visit? need to know types of children to validate types don t know types of children until they have been type checked

13 What are the types? =? x int +? y int 3.14 float

14 Determining Types make sure type is allowed (int + float) assign the resultant type to the operator (float) generate any necessary coercion(s) or conversion(s) most hardware has (int + int) and (float + float) but not (int + float)

15 Adding Coercion =? x int + float int 2 float float 3.14 float y int

16 But wait! Can you assign a float to an int? Most languages do not allow it How do we tell the compiler to cast? x = (int)(y+3.14) Most compilers won t automatically add float to int conversions compiler will throw a type error Does the assignment node get a type? s = t = 5

17 Explicit Casting = int x int float 2 int int + float int 2 float float 3.14 float y int

18 Visitor public class TypeChecker extends Visitor { public void visit(plusnode node) { visitchildren(node); node.type = getresultanttype(lhs, rhs); Node lc = getcoercionnode(lhs, node); if (lc!= null) node.insertleftchild(lc); Node rc = getcoercionnode(rhs, node); if (rc!= null) node.insertrightchild(rc); } }

19 Resultant Type How would you implement getresultanttype( )?

20 Implementation Every built in operation needs a visit method Other places have type requirements if, for, and while require boolean in Java

21 What is a struct? Are these equivalent? struct s1 { int x; }; struct s2 { int y; }; Compound Types

22 Function Call Type Checking A function can be viewed as a conversion of its parameter types to its return type short foo(int x, float y); A compiler should check parameters are of the correct type (or coercing if necessary) The resultant type of the function node is the return type At function call, assume that the returned value is the correct type Check the returned value when checking function body

23 Object Types

24 Object Types

25 Object Types

26 Object Type Checking Type hierarchy Java has Object as root All other class types are descendants of Object An object type is comprised of a parent class and any interfaces it implements Need to check assignment (that object type assigned to object variable is compatible (e.g. subtype)) lhs has to be equal or parent type of rhs otherwise, explicit downcast is required function or method call is legal for a type could be in a parent hierarchy or interface function overloading