Documentation

Declaration of a type checking errors

Environment method entry. Contains method parameters and types. The MethodEntry is created during the generateEnvironment function, which creates an Environment (symbol's table). After the MethodEntry has been created, it can be queried via helper functions, e.g., findMethod ty m.

Environment field entry. Contains class' fields parameters and types.

Environment class entry. Contains fields parameters and methods.

Environment. The Env is used during type checking, and is updated as the type checker runs. Most likely, one uses the Reader monad to hide details of how the environment is updated, via the common local function.

Conditionally update the environment to track if we are in a constructor method.

Helper function to lookup a class given a Name and an Env. Usually it relies on the Reader monad, so that passing the Env can be omitted. For example:

findClass :: Type p1 -> TypecheckM ClassEntry
findClass (ClassType c) = do
  cls <- asks $ lookupClass c
  case cls of
    Just cdef -> return cdef
    Nothing -> tcError $ UnknownClassError c
findClass ty = tcError $ NonClassTypeError ty

In this function (findClass), the Reader function asks will inject the Reader monad as the last argument. More details in the paper.

Look up a variable by its Name in the Env, returning an option type indicating whether the variable was found or not.

Find a class declaration by its Type

Find a method declaration by its Type and method name m

Find a field declaration by its Type (ty) and field name f

Find a variable in the environment by its name x

Environment generation from a parsed AST program.

Add a variable name and its type to the environment Env.

Add a list of parameters, Param, to the environment.

The type checking monad. The type checking monad is the stacking of the Reader and Exception monads.

Main entry point of the type checker. This function type checks an AST returning either an error or a well-typed program. For instance, assuming the following made up language: > > class C > val f: Foo >

it should be parsed to generate the following AST:

testClass1 =
 ClassDef {cname = "C"
          ,fields = [FieldDef {fmod = Val, fname = "f", ftype = ClassType "Foo"}]
          ,methods = []}

To type check the AST, run the tcProgram combinator as follows:

tcProgram testClass1

The type class defines how to type check an AST node.

This combinator is used whenever a certain type is expected. This function is quite important. Here follows an example:

doTypecheck MethodDef {mname, mparams, mbody, mtype} = do
  (mparams', mtype') <- forkM typecheck mparams <&>
                         typecheck mtype
  -- extend environment with method parameters and typecheck body
  mbody' <- local (addParameters mparams') $ hasType mbody mtype'

in the last line we are type checking a method declaration, and it is statically known what should be the return type of the function body. In these cases, one should use the hasType combinator.

Class definition for didactic purposes. This AST represents the following class, which is named C, contains an immutable field f of type Foo:

class C:
  val f: Foo

This class is ill-typed, as there is no declaration of Foo anywhere. To check how to type checker catches this error, run:

tcProgram (Program [testClass1])

Test program with a class, field, method, and variable access. The class Bar does not exist in the environment. The variable access is unbound.

This program is the AST equivalent of the following syntax:

class D
  val g: Bar
  def m(): Int
    x

Test program with a two classes, field, method, and variable access. The class declaration are duplicated.

This program is the AST equivalent of the following syntax:

class D
  val g: Bar
  def m(): Int
    x

class D
  val g: Bar
  def m(): Int
    x