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Lab 3 (second part) - Type checker

In this lab we will write a type checker for our tiger compiler. The type checker is responsible for the following tasks:

  • assigning types to expressions and declarations lacking them;
  • checking types consistency.

For example, after running the typer, the expression

let var a := 3
    var b := a
in
  let var c := a + b in print_int(c) end
end

will be typed as void and will be displayed as

function main(): int =
  (
    let
      var a: int := 3
      var b: int := a
    in
      let
        var c: int := (a + b)
      in
        print_int(c)
      end
    end;
    0
  )

Note that main always return an int: its return value is used by the operating system as the program exit code (0 meaning that everything worked fine – indeed if we reach the end of main, we had no error).

Setup

You will work in the same directory as the first part of lab 3. You do not need to retrieve and unpack an archive for this part.

You do not need to do any particular setup.

Adding the type checker file and command-line argument

The type checker is a visitor similar to the binder. It must be located in src/ast/type_checker.hh (and src/ast/type_checker.cc if needed, which will probably the case) in order to be picked up by the automated grader.

You must add a --type/-t command to the driver which will run your AST tree through the type checker. Once you have added this command like argument, the --help output should look like:

$ src/driver/dtiger --help
Options:
  -h [ --help ]         describe arguments
  --dump-ast            dump the parsed AST
  -b [ --bind ]         run the binder on the parsed AST
  -t [ --type ]         run the type checker on the parsed AST
  --trace-parser        enable parser traces
  --trace-lexer         enable lexer traces
  -v [ --verbose ]      be verbose
  --input-file arg      input Tiger file

The binder must be run before the type checker even if the user does not explicitly give --bind on the command line, as it makes no sense to type check the tree if it has not been decorated.

Also, in order to see the results of the type checker, the dump of the AST (if requested) must take place after the type checker has run.

For interoperability purpose with later labs, the class of the type checker must be TypeChecker and be declared in namespace ast::type_checker.

Implementing the type checker

The type checker must perform the following operations:

  • IntegerLiteral and StringLiteral nodes are given respectively the t_int and t_string types.
  • Sequence nodes have the same type as their last expression if they have one, otherwise they are t_void.
  • IfThenElse nodes must have type-compatible branches, whose type become their type.
  • Let expressions have the same type as their last expression if they have one, otherwise they are t_void.
  • VarDecl nodes without an explicit type (in the type_name field) take their type from their expression. This type cannot be t_void since a variable is either an integer or a string.
  • VarDecl nodes with an explicit type given in the source must check that the expression (if any) is compatible with this type, which will become the type of the node.
  • BinaryOperator nodes must check that their arguments have types compatible with the operation and with each other, and set the resulting type.
  • Identifier nodes inherit the type of their declaration.
  • Assign nodes must check the validity of the assignment and be t_void themselves.
  • WhileLoop nodes must have integer conditions and a body of type t_void, and are themselves t_void.
  • ForLoop nodes must have integer bounds, an integral index, and a body of type t_void, and are themselves t_void.
  • Break nodes are t_void.
  • FunDecl nodes have either an explicit type (in the type_name field) which matches the type of their expression, or no explicit type in which case their expression and themselves must be t_void.
  • FunCall nodes inherit the type of their declaration. The type checker also ensures that number of arguments matches the number of parameters, and that they all have the right type.

Note that in some cases you might analyze a FunCall targeting a FunDecl which has not yet been analyzed. It may happen in two cases:

  • you are calling a primitive function, and primitives are not located in the tree;
  • you are calling a function defined after the current one in the same declarative block (mutually recursive functions).

In those cases, you might note that the target has not been analyzed because its type is still t_undef. When this happens, you should recurse and analyze the declaration of your target.

It also mean that before analyzing a FunDecl you must make sure that it has not been analyzed yet, as it may have been analyzed during the processing of a FunCall in the second case described above. In this case, the visit of the FunDecl should not have any effect since it has been done already.

Note on automated test results

Since by default the binder accepts anything without checking the types consistency, some type checker tests may appear to pass very early. This is due to the fact that some constructs are valid and will not be rejected. However, as you add more type checking, some tests that were passing may start to fail because you are rejecting valid constructs.

Optional: implementing the escaper

This part will not be tested automatically, but must be implemented if you want to be able to use your own code in the next lab.

You need to build yet another visitor, the escaper, which will run right after the binder. Its role is, for every VarDecl which escapes (this flag has already been set by the binder), to add it to the current function get_escaping_decls() vector.

This is necessary so that when we build the frames for the functions, we can put the escaping variables in the frame structure (since they might be accessed from outside the function and need to be stored in memory), while other variables may be stored separately and put into registers as they will never be read or modified outside the function by nested functions.

So your work is simple:

  • When visiting a FunDecl node, save the current function, set the current function to the one you are visiting, recurse, then restore the current function.
  • When visiting a VarDecl node, if it escapes, add it to the list of escaping declarations for the current functions (and recurse into its expression if any).
  • For every other node, recurse in case you encounter a FunDecl or VarDecl somewhere lower in the tree.

For interoperability purpose with later labs, the class of the escaper must be Escaper and be declared in namespace ast::escaper.