Today in CS362: Type Checking

Missing
  Ananta, Daren, Erik, Mark, Yaw

Admin
  Lab tomorrow: Type checking in Pascal
  Questions on project, phase 2?
  Sorry for rough notes in the outline
  Pascal user manual and report.  Put on "Compilers Shelf"

Overview
  What is a type?
  What is type checking?
  Why type check?
  When should we type check?
  What kinds of things have types?
  Where should we type check?
  What are types, revisited?
  Type equivalence basics

What is a type?
  A primitive data structure, containing only one
   element of information?  NOT QUITE
  What kinds of things have elements/members?
   * Classes
   * Lists
   * Sets
  The data perspective: A type is a set of values
  The use perspective: A type is a set of restrictions
    on when values can be used

If types are sets, we have standard ways of combining
sets to make new types
(0) List the elements (enumeration types, in some
    languages)
(1) Union
(2) Intersection
(3) Pairwise product
    If A,B are sets, AxB is a set
    Elements of AxB are (a,b) s.t. a in A, b in B
(4) Kleene Star
empty set union A union AxA union AxAxA union AxAxAxA ...
vector or list

Why have types?
* "So we don't screw up" - Make sure that we perform
  the right sort of operations on data
* Organization: Clarifies code
* Adds a level of abstraction
  + Abstraction is good
* Allows us to convert values when appropriate

Type checking:
* Assign types to objects (explicitly and implicitly)
* Verify that objects are used in a type-safe fashion

When should you type check?
* Compile time
  + Catches lots of errors early
  + Shows errors to developers
  + Probably less overhead
  + May have to make more explicit
* Run time
  + Allows more freedom of input
  + "More elegant"
  + Frees programmers: You can't always know types in advance
  + Allows cool run-time type coercion

In CSC362 we'll do compile-time type checking
* Standard for Pascal
* Good first example of traversing parse trees
* Probably easier than generating code to do run-time
  type check.

What kinds of things have types in Pascal programs?
  variables have types 
  functions have types
  any terminal? or selected terminals (constants)
What kinds of types can they have?
  built-in primitive types (integer, real, char)
  arrays, records
  strings (in some versions of Pascal)
How do we determine types?
  Build a symbol table for variables, functions, procs
  Rely on the lexer to assign types to literals
  Write rules to compute the types of expressions
Where should we check for valid types?
  Any time you try to apply an operation
  +,*,/,-: Make sure both parameters are numbers
  Procedure calls: parameters are appropriate types
  Assignment statements:
    "Return" statements
  Array indices
  case statements and for loops
    enumerated types
When are two types equivalent?

Example:

E0 -> id '[' E1 ']'
  if (id.type != array) 
    crashandburn("What makes you think you can index non arrays?")
  if (E1.type != the argument type of the array) 
    crashandburn("You indexed an array with the wrong type")
  E0.type = check the result type of the array

type
  arraytype : array[type0] of type1

  type0 can be
    subrange type  val1 ... val2
    any enumerated type

  weekdays : (monday,tuesday,wednesday,thursday,friday);
  classesperday : array[weekdays] of integer;

What's the value of day or x after each of these loops?

for day := monday to friday do
  begin
  end;

for x := 1 to 5 do
  begin
  end;

in C

for (x = 1; x <= 5; x++) {
}