Project Overview

You may do this assignment in OCaml, Python, JavaScript, Haskell or Ruby. You must use be graded on a different language for each of PA2 - PA5.

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 a different language for each of PA2 - PA5).

Goal

You will also write additional code to unserialize the AST produced by the parser stage and to serialize the class map, implementation map, parent map, and annotated AST 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, implementation map, and parent 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 conform: 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

Semantic checks are unordered — if a program contains two or more errors, you may indicate whichever you like. You can infer from this that all of our test cases will contain at most one error.

The .cl-type File Format

A .cl-type file consists of four sections:

1. The class map.
2. The implementation map.
3. The parent map.
4. The annotated AST.

Simply output the four 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 order of apperance, with inherited or overridden methods from a superclass coming first):
      • 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
      • If this method is inherited from a parent class and not overriden, output the name of the ultimate parent class that defined the method body expression and then \n. Otherwise, output the name of the current class and then \n.
      • Output the method body expression.
• The Parent Map
  • Output parent_map \n.
  • Output the number of parent-child inheritance relations and then \n. This number is equal to the number of classes minus one (since Object has no parent).
  • Output each child class in turn (in ascending alphabetical order):
    • Output the name of the child class and then \n.
    • Output the name of the child class's parent and then \n.
• The Annotated AST
  • With two exceptions, the annotated AST format is identical to the normal AST from PA3.
  • The first change involves expressions. To output an Expression:
    1. Output the line number of the expression and then a newline (as in PA3).
    2. Output the name of type associated with the expression and then a newline. For example, the expression 3+x is associated with the type Int. This is new to PA4. It should be done for PA4, but not for PA4c.
    3. Output the name of the expression and then a newline and then any subparts (as in PA3).
  • The second change is a new kind of expression, internal, used to represent the bodies of predefined methods. Internal expressions are those that are handled by the run-time system -- you might think of them as part of the standard library. You output Internal Expressions 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).

  Note that you must output information about all classes and methods defined in the program as well as all base classes (and their methods). Do not just print out "classes actually used" or "methods actually called" or something like that. Output all classes and methods -- no optimizations or shortcuts!

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") 
  } ;
} ; 

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
Int	-- initializer expression type (see above: this is an expression annotated with a type) -- do not emit these expression type annotations for the PA4c Checkpoint!
integer	-- initializer expression kind
5	-- which interger constant is it?
Object
0
String
0

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

implementation_map
6	six classes
Bool	first is Bool
3	- it has three methods
abort	- first is abort()
0	-- abort has 0 formal arguments
Object	-- name of parent class from which Bool inherits abort()
0	-- abort's body expression starts o line 0
Object	-- abort's body expression has type Object
internal	-- abort's body is an internal kind of expression (i.e., a system call; see above)
Object.abort	-- extra detail on abort's body expression
copy	- second of Bool's three methods is copy()
0	-- copy has 0 formal arguments
Object	-- name of parent class from which Bool inherits copy()
0	-- copy's body expression starts on line 0
SELF_TYPE	-- copy's body expression has type SELF_TYPE
internal	-- copy's body is an internal kind of expression (i.e., a system call; see above)
Object.copy	-- extra detail on copy's body expression
... many lines skipped
Main	another class is Main
8	- it has 8 methods
... many lines skipped ...
main	- one of Main's methods is main()
0	-- main has 0 formal arguments
Main	-- the name of the class where Main.main() is defined
4	-- the body expression of Main.main starts on line 4
SELF_TYPE	-- the body expression of Main.main has type SELF_TYPE
self_dispatch	-- the body of Main.main() is a self_dispatch kind of expression
... many lines skipped ...

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

parent_map
5	there are five classes with parents (Object is the sixth class)
Bool	Bool's parent ...
Object	... is Object.
IO	IO's parent ...
Object	... is Object.
Int	Int's parent ...
Object	... is Object.
Main	Main's parent ...
IO	... is IO.
String	String's parent. ..
Object	... is Object.

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 a similar format to 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 PA4t

The goal of PA4t is to leave you with a high-quality test suite of Cool programs that you can use to evaluate your own PA4 type checker. Writing a type checker requires you to consider many corner cases when reading the formal and informal typing rules in the Cool Reference Manual. While you you can check for correct "positive" behavior by comparing your typechecker's output to the reference compiler's output on existing "good" Cool programs, it is comparatively harder to check for "negative" behavior (i.e., correctly reporting ill-typed Cool programs).

If you fail to construct a rich test suite of syntactically-valid but semantically-invalid programs you will face a frustrating series of "you fail held-out negative test x" reports for PA4 proper, which can turn into unproductive guessing games. Because students often report that this is frustrating (even though it is, shall we say, infinitely more realistic than making all of the post-deployment tests visible in advance), the PA4t preliminary testing exercise provides a structured means to help you get started with the construction of a rich test suite.

The course staff have produced 20 variants of the reference compiler, each with a secret intentionally-introduced defect related to type-checking. A high-quality test suite is one that reveals each introduced defect by showing a difference between the behavior of the true reference compiler and the corresponding buggy version. You desire a high-quality test suite to help you gain confidence in your own PA4 submission.

For PA4t, you must produce syntactically valid Cool programs (test cases). There are 20 separate held-out seeded type-checker bugs waiting on the grading server. For each bug, if one of your tests causes the reference and the buggy version to produce difference output (that is, either a different .cl-type file or a different error report), you win: that test has revealed that bug. For full credit your tests must reveal at least 15 of the 20 unknown defects.

The secret defects that we have injected into the reference compiler correspond to common defects made by students in PA4. Thus, if you make a rich test suite for PA4t that reveals many defects, you can use it on your own PA4 submission to reveal and fix your own bugs!

For PA4t you should turn in (electronically):

• A zip file containing a set of .cl files: Cool typechecker testcases.
  • Each testcase you submit must be syntactically valid (i.e., must pass cool --parse).
  • Each testcase you submit may be semantically valid or semantically invalid, your choice (i.e., it can pass or fail cool --type).
  • No testcase should be named bug... or ref... because the testing server uses those prefices internally. If you submit a test case with such a name it will be ignored. (Limited course resources can either be spent making better lectures, grading your assignments, etc., or ironing out wrinkles such as these from grading scripts. We have chosen to focus on pedagogy.)

• 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)

For PA4c you should turn in (electronically) 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.
  • This you should not annotate types in initializer expressions in the class map.
• 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 compiler 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, including
   • main.rb or
   • main.py or
   • main.hs or
   • main.js or
   • main.ml

• 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)

• Q: What's the exact list of errors I have to check for in PA4c?
• A1: No such list is provided! Part of the assignment is thinking up all possible checks that do not involve expressions.
• A2: Use the test cases you made for PA4t.

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 -- including
   • main.rb or
   • main.py or
   • main.js or
   • main.hs or
   • main.ml

• 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)

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.

Your submission may not create any temporary files. Your submission may not read or write any files beyond its input and output. We may test your submission in a special "jail" or "sandbox".