SimpleC language overview
Lecture 6

What is the utility of the visitor pattern for compilers?

• A list of declarations or definitions
• Declarations may be variables or functions
  • Distinguished by its type

• Symbol name followed by type
• Only one declaration per name per scope
  • Unique names in global scope and function scope
• Function local names may shadow global scope
  • Reference to name first checks nearest scope

• Function definitions
• Declare parameter and return types
• Body contains declarations first, then statements
• Definitions may define already-declared function symbols

• primitive: int, bool, string
• compound: function (type, type, …) -> return_type
  • Corresponds to C function pointer when used as a value

• x = 1 + y
  • First computes RHS
  • Then stores resulting value in memory associated with x
  • x must be declared already
  • Types of x and RHS must match

• f(1);
  • An expression by itself
  • Useful for function calls where return value is irrelevant
    • (Function calls are expressions in this language)

int x;
x*y;
typedef int x;
x*y;

• return 0;
  • Resumes control to caller of function
  • Replacing function call with value of return expression
  • Type of return expression must match function return type

• while (condition) statement
  • condition expression must evaluate to a bool
  • while takes only a single statement
    • (compound statement allow multiple statements)
  • Checks condition for truth, then executes statement
  • Executes statement until condition is no longer true

• if (condition) statement or if (condition) statement else statement
  • Evaluates condition first, when positive, execute if-branch statements
  • When negative skips if-branch (and optionally goes to else-branch if there)
  • Produces a Void type, i.e., has no resulting value

• { statement* }
• Executes multiple statements in sequence
• Useful for control-flow statements to execute many statements

• f(1)
  • Transfers control to function's statements
    • Calling function's local state is frozen
  • Replaces call with return value of function

• x
  • Replaces value with current value associated with the symbol

• add +, subtract -, multiply *, divide /
  • Only operates on int types
  • Passes through semantics to C
    • Overflows possible

• greater than > (or equal to >=), less than < (or equal to <=), equals ==, not equals !=
  • Left and right operators must match in type
  • Resulting value is a bool
  • Passes through semantics to C

• and &&, or ||, not !
  • Operands are bool
  • Produces a bool
  • Passes through semantics to C

• numbers
• strings