CS G262 Assignment #3.
Assigned:  Thursday, 16 September 2004
Due:       Thursday, 30 September 2004, by 6pm

Implement a static semantic analysis phase for quirk21 as
specified below.  This phase shall define a top-level
procedure named checkTypes that takes two arguments, an
abstract syntax tree for a complete quirk21 program and
an initial type environment, and returns an attributed
abstract syntax tree after checking the original tree for
errors using the scope and type rules of quirk21.

If checkTypes detects an error, it should call the top-level
errMsg procedure with a string that describes the error, and
should then call the top-level exit procedure with an argument
of 1.  (Neither of these procedures is part of R5RS Scheme.
The errMsg will be provided by the instructor, and the exit
procedure is provided by most implementations of Scheme.  If
your implementation does not define an exit procedure, please
define your own for testing.)

If the program contains no errors, then checkTypes returns an
abstract syntax tree that is just like the original except:

    If the operation of a UnOpExp or BinOpExp node is an
    overloaded operation, then the transformed abstract
    syntax tree shall contain a type-specific version of
    that operation in place of the overloaded operation.

    If a BinOpExp node calls for mixed-mode arithmetic on
    arguments of type INT/FLOAT or FLOAT/INT, the transformed
    abstract syntax shall contain an additional UnOpExp node,
    with the coerce_int_to_float operation, inserted between
    the abstract syntax tree of type INT and the BinOpExp
    node.

    For each FieldExp node, checkTypes must use astUpdate1
    on the node to store the offset of the field that is
    referenced by the node.

The instructor is providing Scheme code to parse a quirk21
program.  The code is in

    /course/csg262/Assignments/Assignment3

Please do not modify the instructor's code, as that would
make it harder to combine your semantic analysis phase with a
code generator written by someone else.  If you find a bug in
the instructor's code, please report it, and work around it
as best you can until the instructor fixes it.

Instructions for submitting your code will be provided later.