CS241

Inference Rules for Types

Type checking & is int, else is error, if is when it matches the type Result is a pointer

Multiplication, division and %: both needs to be type int, recurse to check

Addition and Subtraction:

• Addition
  • not the same types
  • both can be int
• Subtraction
  • not allowed to do int - pointer
  • allowed to do pointer - pointer: to know where in the array for instance

Procedures:

• all types in the declarations tau1, tau2, … tau_n needs to match the types in the procedures
• find procedure name in , check the number of parameters = arguments and types

First we traverse the tree is checking the denominator, then we apply the numerator.

Inference Rules For Well-Typed

• Expression have a type
• Statements: we ask a boolean questions, is it well-typed or not.
• Comparisons: check both sides they need to be the same type

Statements:

• Symbol table already took care of what it should’ve taken care of.
• Procedures: well-typed: statment lies must be well-typed and must return type int.
  • control statements: if, while

Assignments

• LeftHandSide: has the same form as a general , but have a different function: store data lvalue is a non-terminal
• comes back to haunt us in code generation.
• Now only need to check left and right hand side having the same type.

Brush up on Recursion.

An Example

Consider:

int foo(int a){
	int x = 0;
	return x + a;
}
int wain(int *a, int b*{
	return foo(b) + a;
}

• Need to check s is well-typed.
• Return expression is type int.
• Return is type int
• When it recurse, we find out it’s additive, check addition rules for types.
  • Check rhs and lhs, recurse + for type checking x.
  • Is x ID in our declarations, does it have type int? yes
  • Do same thing on recurse a. return int.

Type-Checking Recommendations…

• A5
• A6 turn parse tree into a typed tree (intermediate file)