Type Checking (Example) int x, y; float z; z = x + y; /* + - - PowerPoint PPT Presentation

Type Checking (Example)

int x, y; float z; z = x + y; /* + takes ints, int assignable to real */ x = z + y; /* + takes float/int, but float not assignable to int */

Compiler prints “foo.c, line 20: float not assignable to int”

Type Checking – From the Inside

• “Big idea” in compilation is Syntax Directed

Translation

Expr ::= Expr Op Expr {: /* check code here */ :}

• Elegance of the front-end is that syntax-action

idea is explicit in code

• Your job is to fill in inside the curly brackets ;)

What is a Type?

• Set of values (1, 2, 3, ...)
• Set of values with operations on them

(+, /, *, ...)

• What values can be used with what
• perations? (cannot / by 0)
• Set of values, with operations, and rules for

their combination

• Really, though type is behavior, static type is

static behavior

Static Soundness “Impossible”

class ASCIITable { // Example public static void main(String args[]) throws IOException { String asciiTab[256] = { ... "a", "b", "c" ... }; int charNum = System.in.read(); String c = asciiTab[charNum]; // charNum in bounds? System.out.println(c); } }

Composite Types (Constructed)

• Basic or atomic types are things like integer,

real, boolean

• Simplicity of these types isn’t up to complex

programming, hence composite types

• Declarations:

int a[10]; float f(int a, int b); struct { int a; int b; }; int *px;

• even some executable code

f(1, x); /* arg list to func. call */

What are the types?

[1] int i, x, y[20]; [2] float z; [3] int f(int a, int b); [4] z = (x + y[i]) * f(i, x); [5] x = z + y[i];

What are the values of y’s type? Ops? Rules?

Type Structure Guides SW Design

class ArrayType : CompositeType { // pointers to children private Type elementType; private RangeType range; // accessors public … }

What you can do now

• Check #2: Detect an illegal assignment -- that is,

an assignment of the form: A := Expr where A is either a type, a procedure, or a constant (i.e., anything other than a variable). This is an elaboration of a check from an earlier

• assignment. The old message should be replaced

with this one. How might you go about this? Where start? When are you done?

What you can do now

• Some lingo:

STO = Symbol Table Object

• An STO will often be a variable with a type. For

expressions, return a VarSTO that is a “fake” variable with a type, because later it will be a var: x := y + w * z t1 := w * z t2 := y + t1 x := t2

• For now, just think of passing a variable as an

“easy” way to pass a type So what should you do to STO?

What you can do now

• Create a unique instance of each basic type and

make it easily accessible

TypeSpec ::= INTEGER {: RESULT = TypeValues.IntType; :} | REAL ...

• A public static class field is essentially a global

class TypeValues { public static IntegerType IntType = new IntegerType(); … }

What class hierarchy might we have for basic types?