Project Overview

You may do this assignment in OCaml, Python or Ruby. You must use each language at least once (over the course of PA2 - PA5); you will use one language (presumably your favorite) twice.

You may work in a team of two people for this assignment. You may work in a team for any or all subsequent programming assignments. You do not need to keep the same teammate. The course staff are not responsible for finding you a willing teammate. However, you must still satisfy the language breadth requirement (i.e., you must be graded on at least one OCaml program, at least one Ruby program, and at least one Python program).

Goal

You will also write additional code to unserialize the AST produced by the parser stage and to serialize the class map and implementation map produced by your semantic analysis.

The Specification

1. A program that takes a single command-line argument (e.g., file.cl-ast). That argument will be an ASCII text Cool abstract syntax tree file (as described in PA3). Your program must either indicate that there is an error in the input (e.g., a type error) or emit file.cl-type, a serialized Cool abstract syntax tree, class map, and implementation map. If your program is called checker, invoking checker file.cl-ast should yield the same output as cool --type file.cl. Your program will consist of a number of OCaml files, a number of Python files, or a number of Ruby files.
2. A plain ASCII text file called readme.txt describing your design decisions and choice of test cases. See the grading rubric. A few paragraphs should suffice.
3. Testcases good.cl, bad1.cl, bad2.cl and bad3.cl. The first should pass the semantic analysis stage. The remaining three should yield semantic analysis errors.

Error Reporting

class Main inherits IO {
  main() : Object {
    out_string("Hello, world.\n" + 16777216) -- adding string + int !?
  } ;
} ;

Example error report output:

ERROR: 3: Type-Check: arithmetic on String Int instead of Ints

• Errors related to parameter-less method main in class Main: always line 0
• Inheritance cycle: always line 0
• Other inheritance type problem: inherited type identifier location
• self or SELF_TYPE used in wrong place: self (resp. SELF_TYPE) identifier (resp. type) location
• Redefining a feature: (second) feature location
• Redefining a formal or class: (second) identifier location
• Other attribute problems: attribute location
• Redefining a method and changing types: (second) type location
• Other problems with redefining a method: method location
• Method body type does not conform: method name identifier location
• Attribute initializer does not conform: attribute name identifier location

• Errors with types of arguments to relational/arithmetic operations: location of relational/arithmetic operation expression
• Errors with types of while / if subexpression(s): location of (enclosing) while or if expression (not the location of the conditional expression)
• Errors with case expression (e.g., lub): location of case expression
• Errors with conformance in let: location of let expression (not location of initializer)
• Errors in blocks: location of (beginning of) block expression
• Errors in actual arguments: location of method invocation expression (not the location of any particular actual argument)
• Assignment does not comform: assignment expression location (not right-hand-side location)
• Unknown identifier: location of identifier
• Unknown method: location of method name identifier
• Unknown type: location of type

The .cl-type File Format

A .cl-type file consists of three sections:

1. The class map.
2. The implementation map.
3. The AST (just copy it from your input).

Simply output the three sections in order, one after the other.

We will now describe exactly what to output for the class and implementation maps. The general idea and notation (one string per line, recursive descent) are the same as in PA3.

• The Class Map
  • Output class_map \n.
  • Output the number of classes and then \n.
  • Output each class in turn (in ascending alphabetical order):
    • Output the name of the class and then \n.
    • Output the number of attributes and then \n.
    • Output each attribute in turn (in order of appearance, with inherited attributes from a superclass coming first):
      • Output no_initializer \n and then the attribute name \n and then the type name \n.
      • or Output initializer \n and then the attribute name \n and then the type name \n and then the initializer expression.
• The Implementation Map
  • Output implementation_map \n.
  • Output the number of classes and then \n.
  • Output each class in turn (in ascending alphabetical order):
    • Output the name of the class and then \n.
    • Output the number of methods for that class and then \n.
    • Output each method in turn (in ascending alphabetical order):
      • Output the method name and then \n.
      • Output the number of formals and then \n.
      • Output each formal's name only:
        • Output the name and then \n
      • Output the method body expression.

• New Expression Kind:

  We add a new kind of expression, internal, for internal method calls. Internal method calls are those that are handled by the run-time interpreter -- you might think of them as part of the standard library. You output internal expression as follows: 0 \n internal \n Class.method \n

  The valid kinds of internal expressions (i.e., the values for Class.method) are:

  • Object.abort Object.type_name Object.copy IO.out_string IO.out_int IO.in_string IO.in_int String.length String.concat String.substr

  They are defined here (for Object), here (for IO), and here (for String).

That's it for the output specification. Here's some example input:

class Main inherits IO {
  my_attribute : Int <- 5 ; 
  main() : Object { 
    out_string("Hello, world.\n") -- adding string + int !? 
  } ;
} ; 

Example .cl-type class map output -- with comments:

 
class_map
6                       -- number of classes
Bool                    -- note: includes predefined base classes
0
IO
0
Int
0
Main
1                       -- our Main has 1 attribute
initializer             
my_attribute            -- named "my_attribute"
Int                     -- with type Int
2                       --  initializer expression line number
integer                 --  initializer expression kind
5                       --  what integer constant is it? 
Object
0
String
0

Example .cl-type implementation map output -- with comments:

implementation_map
6                       -- 6 classes
Bool                    -- first is Bool
3                       --  it has 3 methods 
abort                   --  first is abort
0                       --  abort has 0 formal arguments
0                       --   abort is "defined" on line 0 
internal                --   kind of body expression 
Object.abort            --   what kind of internal method is it? 
copy                    --  second is copy
0                       --   copy has 0 formal arguments 
0                       --   copy is "defined" on line 0 
internal                --   kind of body expression
Object.copy             --   what kind of internal method is it? 
        -- many lines skipped
Main                    -- another class is Main
8                       --  it has 8 methods
        -- many lines skipped
main                    --  one method is "main"
0                       --  it has 0 formal arguments
4                       --  the body expression starts on line 4 
self_dispatch           --   the body is a self_dispatch 
        -- many lines skipped

Writing the rote code to output a .cl-type text file given an AST may take a bit of time but it should not be difficult; our reference implementation does it in 35 lines and cleaves closely to the structure given above. Reading in the AST is similarly straightforward; our reference implementation does it in 171 lines.

Commentary

• $ cool --parse file.cl  
  $ cool --out reference --type file.cl 
  $ my-checker file.cl-ast
  $ diff -b -B -E -w file.cl-type reference.cl-type 
  

You should implement all of the typing rules in the Reference Manual. There are also a number of other rules and corner cases you have to check (e.g., no class can inherit from Int, you cannot redefine a class, you cannot have an attribute named self, etc.). They are sprinkled throughout the manual. Check everything you possibly can.

Because PA4 involves spitting out parts of the AST in the same format as PA3, there is a slight synergy to doing PA3 and PA4 in the same language. It's not major, though, and the vast majority of your effort in PA4 will be centered on type checking (rather than output formatting).

What To Turn In For PA4

1. readme.txt -- your README file
2. good.cl -- a positive testcase
3. bad1.cl -- a negative testcase
4. bad2.cl -- a negative testcase
5. bad3.cl -- a negative testcase
6. source_files -- your implementation

• team.txt -- an optional file listing your other team member (see below -- if you are not working in a team, do not include this file)

Submit the file using Toolkit (as with PA1-PA3).

What To Turn In For WA4

For WA4 you should turn in (electronically, via Toolkit) an early version of PA4 that does the following:

• Reads in the .cl-ast file given as a command-line argument.
  • You do not need to use a parser generator to read in the .cl-ast file -- its format was specifically chosen to make it easy to read with just some mutually-recursive procedures. It should take you (much) less than 150 lines to read in the .cl-ast file.
• Does every bit of typechecking and semantic analysis possible without typechecking expressions.
• Prints out error messages as normal.
• Outputs only the class map to .cl-type if there are no errors.
  • You can use the --class-map command-line argument to get the reference interpreter to spit out the class map after typechecking (for comparison).

• Check to see if a class inherits from Int (etc.).
• Check to see if a class inherits from an undeclared class.
• Check for cycles in the class hierarchy.
• Check for duplicate method or attribute definitions in the same class.
• Check for a child class that redefines a parent method but changes the parameters.
• Check for a missing method main in class Main.
• Check for self and SELF_TYPE mistakes in classes and methods.
• This list is not exhaustive -- read the Cool Reference Manual carefully and find everything you might check for without typechecking expressions.
• Basically, you'll look at classes, methods and attibutes (but not method bodies).

You must turn in a zip file containing these files:

1. source_files -- your implementation

• team.txt -- an optional file listing your other team member (see below -- if you are not working in a team, do not include this file)
• This is the only written assigment that allows explicit teams.

Working In Pairs

Students on a team are expected to participate equally in the effort and to be thoroughly familiar with all aspects of the joint work. Both members bear full responsibility for the completion of assignments. Partners turn in one solution for each programming assignment; each member receives the same grade for the assignment. If a partnership is not going well, the teaching assistants will help to negotiate new partnerships. Teams may not be dissolved in the middle of an assignment.

If you are working in a team, exactly one team member should submit a PA4 zipfile. That submission should include the file team.txt, a one-line flat ASCII text file that contains exactly and only the email address of your teammate. Don't include the @virgnia.edu bit. Example: If ph4u and wrw6y are working together, ph4u would submit ph4u-pa4.zip with a team.txt file that contains the word wrw6y. Then ph4u and wrw6y will both receive the same grade for that submission.

Autograding

In each case we will then compare your output to the correct answer:

• diff -b -B -E -w testcase.cl-type correct-answer.cl-type

For error messages and negative testcases we will compare your output but not the particular error message. Basically, your semantic analyzer need only correctly identify that there is an error on line X. You do not have to faithfully duplicate our English error messages. Many people choose to (because it makes testing easier) -- but it's not required.

We will perform the autograding on some unspecified test system. It is likely to be Solaris/UltraSPARC, Cygwin/x86 or Linux/x86. However, your submissions must officialy be platform-independent (not that hard with a scripting language). You cannot depend on running on any particular platform.

There is more to your grade than autograder results. See the Programming Assignment page for a point breakdown.