Documentation

Data declaration of available errors. Value constructors are used to create statically known errors. For example:

UnknownClassError  (Name c)

creates a UnknownClassError. This error should be created whenever there is a class whose declaration is unknown or inexistent.

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.

Generates an empty environment.

Helper function to lookup a class given a Name and an Env. For example:

typecheck env (ClassType c) = do
  _ <- lookupClass env c
  return $ ClassType c

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

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

Look up a variable by its Name in the Env, returning an exception with the type checking error, TCError, or the Type of the variable x.

Generates an environment (symbol's table) from a Program,

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

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

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

which either returns an error or the resulting typed AST.

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 mdef@(MethodDef {mparams, mbody, mtype}) = do
  -- typecheck the well-formedness of types of method parameters
  mparams' <- mapM typecheck mparams
  mtype' <- typecheck mtype

  -- extend environment with method parameters and typecheck body
  mbody' <- local (addParameters mparams) $ hasType mbody mtype'
  ...

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

Test programs of a class with a single field. This program is the AST equivalent of the following syntax:

class C
  val f: Foo

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

Test suite that runs testProgram.