CS 444/544 Compilers
Spring 2017


Cool (Classroom Object-Oriented Language) is a language designed at Stanford by Prof. Alex Aiken for use in a compilers course. We will be developing a compiler from Cool to LLVM IR.

A long version of the Cool manual is available. Here, I summarize the manual according to our needs.


Several Cool examples exist in the examples folder. Cool source files have extension .cl.


Multiple classes may be defined in the same file. Class definitions have the form:

  class <Name> [ inherits <Name> ] {

The notation [...] denotes an optional construct. Class names begin with an uppercase letter.


The body of a class definition consists of a list of feature definitions. A feature is either an attribute (i.e. a field) or a method. All attributes have scope local to the class (i.e. they are private), and all methods have global scope (i.e. they are public). Thus, the only way to provide access to object state in Cool is through methods.

Feature names must begin with a lowercase letter. No method name may be defined multiple times in a class, and no attribute name may be defined multiple times in a class, but a method and an attribute may have the same name.

Methods are executed according to the dynamic dispatch (i.e. late binding) principle, as in Java (i.e. virtual methods in C++).

The special variable self refers to the object on which a method is executed (just like this in Java and C++).

The expression new C creates a new instance of the class C.


A child class inherits all the attributes of its parent. Methods can be redefined in a child class. It is illegal to redefine attribute names.

There is a distinguished class Object. If a class definition does not specify a parent class, then the class inherits from Object by default. A class may inherit only from a single class (i.e. there is single inheritance).

In addition to Object, Cool has four other basic classes: Int, String, Bool, and IO.


A type declaration has the form x:C, where x is a variable and C is a type. The use of types and type-checking in Cool is similar to Java, except for SELF_TYPE.

The type SELF_TYPE is used to refer to the type of the self variable. This is useful in classes that will be inherited by other classes, because it allows the programmer to avoid specifying a fixed final type at the time the class is written. For example, consider the program below.

class Silly {
    copy() : SELF_TYPE { self };
class Sally inherits Silly { };

class Main {
    x : Sally <- (new Sally).copy();

    main() : Sally { x };

Because SELF_TYPE is used in the definition of the copy method, we know that the result of copy is the same as the type of the self variable. Thus, it follows that (new Sally).copy() has type Sally, which conforms to the declaration of attribute x.

Note that the meaning of SELF_TYPE is not fixed, but depends on the class in which it is used. In general, SELF_TYPE may refer to the class C in which it appears, or any child (or grandchild) class of C.

Finally, SELF_TYPE may be used in the following places:

  • new SELF_TYPE,
  • as the return type of a method,
  • as the declared type of a let variable,
  • or as the declared type of an attribute.

No other uses of SELF_TYPE are permitted.

Cool has a solid type system. We will not implement a type checker for Cool in this course.


An attribute definition has the form

<id> : <type> [ <- <expr> ];

The expression is optional initialization that is executed when a new object is created. If no initialization is supplied, then the default initialization is used (see below). When a new object of a class is created, all of the inherited and local attributes must be initialized. Inherited attributes are initialized first in inheritance order beginning with the attributes of the greatest ancestor class. Within a given class, attributes are initialized in the order they appear in the source text. Attributes are local to the class in which they are defined or inherited. Inherited attributes cannot be redefined.


All variables in Cool are initialized to contain values of the appropriate type. The special value void is used as the default initialization for variables where no initialization is supplied by the user. (void is used where one would use NULL in C or null in Java; Cool does not have anything equivalent to C’s or Java’s void type.) Note that there is no name for void in Cool; the only way to create a void value is to declare a variable of some class other than Int, String, or Bool and allow the default initialization to occur, or to store the result of a while loop.

There is a special form isvoid expr that tests whether a value is void. In addition, void values may be tested for equality. A void value may be passed as an argument, assigned to a variable, or otherwise used in any context where any value is legitimate, except that a method call or case on void generates a runtime error.

Variables of the basic classes Int, Bool, and String are initialized specially (discussed later below).


A method definition has the form

<id>(<id> : <type>,...,<id> : <type>): <type> { <expr> };

There may be zero or more formal parameters. The identifiers used in the formal parameter list must be distinct. When a method is invoked, the formal parameters are bound to the actual arguments and the expression is evaluated; the resulting value is the meaning of the method invocation (i.e. the method’s return value). A formal parameter hides any definition of an attribute of the same name.


Expressions are the largest syntactic category in Cool.


The simplest expressions are constants. The boolean constants are true and false. Integer constants are unsigned numbers such as 0, 123, and 007. String constants are sequences of characters enclosed in double quotes, such as "This is a string." String constants may be at most 1024 characters long.

The constants belong to the basic classes Bool, Int, and String. The value of a constant is an object of the appropriate basic class.


The names of local variables, formal parameters of methods, self, and class attributes are all expressions. The identifier self may be referenced, but it is an error to assign to self or to bind self in a let, a case, or as a formal parameter. It is also illegal to have attributes named self. Local variables and formal parameters have lexical scope. The binding of an identifier reference is the innermost scope that contains a declaration for that identifier, or to the attribute of the same name if there is no other declaration. The exception to this rule is the identifier self, which is implicitly bound in every class.


An assignment has the form

<id> <- <expr>

The value of an assignment is the value of the expression.


There are three forms of dispatch (i.e. method call) in Cool. The three forms differ only in how the called method is selected. The most commonly used form of dispatch is


Consider the dispatch e0.f(e1, . . . , en). To evaluate this expression, the arguments are evaluated in left-to-right order, from e1 to en. Next, e0 is evaluated and its class C noted (if e0 is void, a runtime error is generated). Finally, the method f in class C is invoked, with the value of e0 bound to self in the body of f and the actual arguments bound to the formals as usual.

The other forms of dispatch are:


The first form is shorthand for self.<id>(<expr>,...,<expr>).

The second form provides a way of accessing methods of parent classes that have been hidden by redefinitions in child classes. Instead of using the class of the leftmost expression to determine the method, the method of the class explicitly specified is used. For example, e@B.f() invokes the method f in class B on the object that is the value of e.


A conditional has the form

if <expr> then <expr> else <expr> fi

The semantics of conditionals is standard. The predicate (i.e. the condition) is evaluated first. If the predicate is true, the then branch is evaluated. If the predicate is false, the else branch is evaluated. The value of an if-expression is the value of the evaluated branch.


A loop has the form

while <expr> loop <expr> pool

The predicate is evaluated before each iteration of the loop. If the predicate is false, the loop terminates and void is returned. If the predicate is true, the body of the loop is evaluated and the process repeats.


A block has the form

{ <expr>; ... <expr>; }

The expressions are evaluated in left-to-right order. Every block has at least one expression; the value of a block is the value of the last expression.

An occasional source of confusion in Cool is the use of semi-colons (“;”). Semi-colons are used as terminators in lists of expressions (e.g. the block syntax above) and not as expression separators.


A let expression has the form

let <id1> : <type1> [ <- <expr1> ], ..., <idn> : <typen> [ <- <exprn> ] in <expr>

The optional expressions are initialization; the other expression is the body. A let is evaluated as follows. First <expr1> is evaluated and the result is bound to <id1>. Then <expr2> is evaluated and the result is bound to <id2>, and so on, until all of the variables in the let are initialized. (If the initialization of <idk> is omitted, the default initialization of type <typek> is used.) Next, the body of the let (i.e. <expr>) is evaluated. The value of the let is the value of the body.

The let identifiers <id1>,…,<idn> are visible in the body of the let. Furthermore, identifiers <id1>,…,<idk> are visible in the initialization of <idm> for any m > k.

If an identifier is defined multiple times in a let, later bindings hide earlier ones. Identifiers introduced by let also hide any definitions for the same names in containing scopes. Every let expression must introduce at least one identifier.


A case expression has the form

case <expr0> of
  <id1> : <type1> => <expr1>;
  <idn> : <typen> => <exprn>;

Case expressions provide runtime type tests on objects. First, expr0 is evaluated and its dynamic type C noted (if expr0 evaluates to void, a run-time error is produced). Next, from among the branches the branch with the type <typek> that is closest to C in the inheritance hierarchy is chosen. The identifier <idk> is bound to the value of <expr0> and the expression <exprk> is evaluated. The result of the case is the value of <exprk>. If no branch can be selected for evaluation, a run-time error is generated. Every case expression must have at least one branch.

The case expression has no special construct for a “default” or “otherwise” branch. The same affect is achieved by including a branch

  x : Object => ...

because Object is at the top of the inheritance hierarchy.

The case expression provides programmers a way to insert explicit runtime type checks in situations where static types inferred by the type checker are too conservative. A typical situation is that a programmer writes an expression e and type checking infers that e has static type P. However, the programmer may know that, in fact, the dynamic type of e is always C for some C that is a (grand)child of P. This information can be captured using a case expression:

  case e of x : C => ...

In the branch, the variable x is bound to the value of e but has the more specific static type C.


A new expression has the form

new <type>

The value is a fresh object of the appropriate class. If the type is SELF_TYPE, then the value is a fresh object of the class of self in the current scope.


The expression

isvoid expr

evaluates to true if expr is void, and evaluates to false if expr is not void.

Arithmetic and Comparison Operations

Cool has four binary arithmetic operations: +, -, *, /. The syntax is

expr1 <op> expr2

To evaluate such an expression, first expr1 is evaluated and then expr2. Cool has only integer division.

Cool has three comparison operations: <, <=, =. For < and <= the rules are exactly the same as for the binary arithmetic operations, except that the result is a Bool.

The comparison = is a special case. If either <expr1> or <expr2> has static type Int, Bool, or String, then the other must have the same static type. Any other types, including SELF_TYPE, may be freely compared. On non-basic objects, equality simply checks for pointer equality (i.e., whether the memory addresses of the objects are the same). Equality is defined for void.

In principle, there is nothing wrong with permitting equality tests between, for example, Bool and Int. However, such a test must always be false and almost certainly indicates some sort of programming error. The Cool type checking rules catch such errors at compile-time instead of waiting until runtime. So, we shall not about such cases when implementing our compiler.

Finally, there is one arithmetic and one logical unary operator. The expression ~<expr> is the integer complement of <expr> (i.e. negation). The expression not <expr> is the boolean complement of <expr>.

Basic Classes


The Object class is the root of the inheritance graph. A method with the following declaration is defined:

type_name() : String

The method type_name returns a string with the name of the class of the object.


The Int class provides integers. There are no methods special to Int. The default initialization for variables of type Int is 0 (not void). It is an error to inherit from or redefine Int.


The Bool class provides true and false. The default initialization for variables of type Bool is false (not void). It is an error to inherit from or redefine Bool.


The String class provides strings. The following methods are defined:

length() : Int
concat(s : String) : String
substr(i : Int, l : Int) : String

The method length returns the length of self. The method concat returns a string formed by concatenating s after self. The method substr returns the substring of self beginning at position i with length l; string positions are numbered beginning at 0.

The default initialization for variables of type String is "" (not void). It is an error to inherit from or redefine String.


The IO class provides the following methods for performing simple input and output operations:

out_string(x : String) : SELF_TYPE
out_int(x : Int) : SELF_TYPE
in_string() : String
in_int() : Int

The methods out_string and out_int print their argument and return self. The method in_string reads a string from the standard input, up to but not including a newline character. The method in_int reads a single integer. Any characters following the integer, up to and including the next newline, are discarded by in_int.

A class can make use of the methods in the IO class by inheriting from IO. It is an error to redefine the IO class.

Main Class

Every program must have a class Main. Furthermore, the Main class must have a method main that takes no formal parameters. The main method must be defined in class Main (not inherited from another class). A program is executed by evaluating (new Main).main().