PA4 — The Semantic Analyzer

You may complete this assignment in OCaml, Python, JavaScript, Haskell or Ruby.

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.

Goal

For this assignment you will write a semantic analyzer. Among other things, this involves traversing the abstract syntax tree and the class hierarchy. You will reject all Cool programs that do not comply with the Cool type system.

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.

Specification

You must create three artifacts:

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, a number of JavaScript files, a number of Haskell 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

To report an error, write the string

to standard output and terminate the program. You may write whatever you want in the message, but it should be fairly indicative. Example erroneous input:

Example error report output:

Line Number Error Reporting

The typing rules do not directly specify the line numbers on which errors are to be reported. As of v1.11, the Cool reference compiler uses these guidelines (possibly surprising ones are italicized):

• 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

Remember that you do not have to match the English prose of the reference compiler's error messages at all. You just have to get the line number right.

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

If there are no errors in file.cl-ast your program should create file.cl-type and serialize the class map, implementation map, parent map, and annotated AST to it.

The class and implementation maps are described in the Cool Reference Manual.

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 appearance, with inherited or overridden methods from a superclass coming first; internal methods are defined to appear 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
      • 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 (including the type annotation, as above) as follows:
    • 0 \n type \n internal \n Class.method \n
    The valid kinds of internal expressions (i.e., the values for Class.method) are:
    • IO.in_int IO.in_string IO.out_int IO.out_string Object.abort Object.copy Object.type_name String.concat String.length String.substr
    They are formally defined in the Cool Reference Manual.

    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!

Detailed .cl-type Example

Now that we've formally defined the output specification, we can present a worked example. Here's the example input we will consider:

Resulting .cl-type class map output with comments:

.cl-type class map	comment
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 integer constant is it?
Object
0
String
0

Resulting .cl-type implementation map output with comments:

.cl-type implementation map	comment
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 on 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 ...

Finally, the resulting .cl-type parent map output with comments:

.cl-type parent map	comment
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

You can do basic testing as follows:

You should implement all of the typing rules in the Cool 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.

PA4t — Creating PA4 Tests

PA4t is a preliminary testing exercise that introduces a form of test-driven development or mutation testing into our software development process and requires you to construct a high-quality test suite.

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!

Video Guides

A number of Video Guides are provided to help you get started on this assignment on your own. The Video Guides are walkthroughs in which the instructor manually completes and narrates, in real time, the first part of this assignment — including a submission to the grading server. They include coding, testing and debugging elements.

If you are still stuck, you can post on the forum, approach the TAs, or approach the professor. The use of online instructional content outside of class weakly approximates a flipped classroom model. Click on a video guide to begin, at which point you can watch it fullscreen or via Youtube if desired.

What to Turn In For PA4t

For PA4t you should turn in (electronically):

• A zip file containing a set of up to 99 .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.)
  • If you submit more than 99 tests, some will be ignored.

Your zip file may also contain:

• team.txt -- an optional file listing your other team member

Hint: because you can find "positive" bugs in your typechecker more easily (e.g., by running your typechecker on the correct Cool programs from cool-examples.zip), the PA4t exercise is strongly biased toward "negative" bugs (i.e., the secret buggy typecheckers usually fail to report certain semantic errors).

PA4c — Checkpoint

PA4c is a checkpoint for PA4. The typechecker is a large project (and a large part of your grade), so it behooves you to start it early.

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.
  • Thus 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).

Thus you should build the class hierarchy and check everything related to that. For example:

• 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).

What to Turn In For PA4c

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

Your zip file may also contain:

• team.txt -- an optional file listing your other team member

What to Turn In For PA4

You must turn in a zip file containing these files:

1. readme.txt — your README file
2. good.cl — a novel positive testcase
3. bad1.cl — a novel negative testcase
4. bad2.cl — a novel negative testcase
5. bad3.cl — a novel negative testcase
6. source_files — including
   • main.rb or
   • main.py or
   • main.js or
   • main.hs or
   • main.ml

Your zip file may also contain:

• team.txt — an optional file listing your other team member

Grading Rubric

PA4 Grading (out of 100 points):

• 65 points — for autograder tests
  • Each missed test removes points, to a minimum of 0/65, even if there are more tests than total points.
• 5 points — for a correct PA4t submission
• 5 points — for a correct PA4c submission
• 1 point — for a correct team.txt file
  • 1 — file contains a single word: the uva email address of your partner (e.g., wrw6y)
  • 1 — file is not present at all, and you are working alone
  • 0 — file is present but contains something other than the single word of your partner's email address
• 8 points — for a clear description in your README
  • 8 — thorough discussion of design decisions (e.g., handling of the class hierarchy, case and new and dispatch) and choice of test cases; a few paragraphs of coherent English sentences should be fine
  • 4 — vague or hard to understand; omits important details
  • 0 — little to no effort, or submitted an RTF/DOC/PDF file instead of plain TXT
• 8 points — for valid and novel good.cl, bad1.cl, bad2.cl and bad3.cl files
  • 8 — wide range of test cases added, stressing most Cool features and three error conditions, novel files
  • 4 — added some tests, but the scope not sufficiently broad
  • 0 — little to no effort, or course files resubmitted as tests
• 8 point — for code cleanliness
  • 8 — code is mostly clean and well-commented
  • 4 — code is sloppy and/or poorly commented in places
  • 0 — little to no effort to organize and document code