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Project 3A: SmallC Parser

CMSC 330, Spring 2018

Due April 14th at 11:59 PM (Late April 15th at 11:59 PM)

P/R/S: 48/ 52/ 0

Ground Rules and Extra Info

This is NOT a pair project.

In your code, you may use any OCaml modules and features we have taught in this class (If you come asking for help using something we have not taught we will direct you to use methods taught in this class). You may even choose to use imperative OCaml, but are not required to.


In this project, you will implement the lexer and parser for SmallC, which was the input language of the interpreter you wrote in project 3b. This parser will be capable of parsing expressions, statements, and full programs, thus making it a complete replacement for the parser we supplied for the interpreter project. The parser will operate on a flat token list assembled by your lexer and create a correct stmt and/or expr corresponding to the input. When you're done, you will have written the complete pipeline to turn a text file into a running SmallC program!

The only requirement for error handling is that input that cannot be lexed/parsed according to the provided rules should raise an InvalidInputException. We recommend using relevant error messages when raising these exceptions, as it will make debugging easier.

All tests will be run on direct calls to your code, comparing your return values to the expected return values. Any other output (e.g., for your own debugging) will be ignored. You are free and encouraged to have additional output.

Project Files

To begin this project, you will need to commit any uncommitted changes to your local branch and pull updates from the git repository. Click here for directions on working with the Git repository. The following are the relevant files:

  • OCaml files you should edit
    • lexer.ml: This file is the sole place you will implement lexer code for the first half of this project.
    • parser.ml: This file is the sole place you will implement parser code for the second half of this project.
  • Provided OCaml Files (No need to edit, changes will be overwritten!)
    • interface.ml: This driver can be used to output your lexer/parser results on standard input or supplied files. It's a lot like the smallc.ml file in project 3b, but for the frontend (parser/lexer) rather than the backend (interpreter).
    • public.ml and public_inputs/: The public test driver file and the SmallC input files to go with it, respectively.
    • smallCTypes.ml: This file contains all type definitions used in this project.
    • utils.ml and testUtils.ml: These files contain functions that we have written for you and for us that aid in testing and debugging. The small part of utils.ml that concerns you in implementing this project is called out very explicitly when it is needed later in the document, and otherwise you should not need to look at either of these files.
  • Submission Scripts and Other Files
    • submit.rb: Execute this script to submit your project to the submit server.
    • submit.jar and .submit: Don't worry about these files, but make sure you have them.
    • Makefile: This is used to build the public tests and other project-specific targets by simply running the command make, just as in 216.

Compilation, Tests, and Running

In order to compile your project, simply run the make command and our Makefile will handle the compilation process for you. After compiling your code, two executable files will be created:

  • public.byte
  • interface.byte

The public tests can be run by simply running public.byte (i.e. ./public.byte in the terminal; think of this just like with a.out in C).

You can run your lexer or parser directly on a SmallC program by running ./interface.byte lex [filename] or ./interface.byte parse [filename] where the [filename] argument is optional. This driver, provided by us, reads in a program from standard input (or from a file, if a second argument is provided) and performs the requested action (i.e. lex or parse) and prints information about the result of running the SmallC lexer and parser that you will write.

Note that you don't need to touch interface.ml yourself, as it only functions as an entry point for your code and is structured independent of your exact implementation.

The Lexer

Before your parser can process input, the raw file must be transformed into logical units called tokens. This process is readily handled by use of regular expressions. Information about OCaml's regular expressions library can be found in the Str module documentation. You aren't required to use it, but you may find it very helpful. Note that a lexer is the same as a scanner, which is discussed in the lecture slides.

Your lexer must be written in lexer.ml. You will need to implement the function tokenize : string -> token list which takes as input the program as a string and outputs the associated token list. The token type is implemented in smallCTypes.ml.

Your lexer must meet these general requirements:

  • Tokens can be separated by arbitrary amounts of whitespace, which your lexer should discard. Spaces, tabs ('\t') and newlines ('\n') are all considered whitespace.
  • The lexer should be case sensitive.
  • Lexer input should be terminated by the EOF token, meaning that the shortest possible output from the lexer is [EOF].
  • If the beginning of a string could be multiple things, the longest match should be preferred, for example:
    • "while0" should not be lexed as Tok_While, but as Tok_ID("while0"), since it is an identifier

Most tokens only exist in one form (for example, the only way for Tok_Pow to appear in the program is as ^ and the only way for Tok_While to appear in the program is as while). However, a few tokens have more complex rules. The regular expressions for these more complex rules are provided here:

  • Tok_Bool of bool: The value will be set to true on the input string "true" and false on the input string "false".
    • Regular Expression: true|false
  • Tok_Int of int: Valid ints may be positive or negative and consist of 1 or more digits. You may find the function int_of_string useful in lexing this token type.
    • Regular Expression: -?[0-9]+
  • Tok_ID of string: Valid IDs must start with a letter and can be followed by any number of letters or numbers. Note that keywords may be contained within IDs and they should be counted as IDs unless they perfectly match a keyword!
    • Regular Expression: [a-zA-Z][a-zA-Z0-9]*
    • Valid examples:
      • "a"
      • "ABC"
      • "a1b2c3DEF6"
      • "while1"
      • "ifelsewhile"

In grammars given later in this project description, we use the lexical representation of tokens instead of the token name; e.g. we write ( instead of Tok_LParen. This table shows all mappings of tokens to their lexical representations, save for the three variant tokens specified above:

Token Name (in OCaml) Lexical Representation (in grammars below)
Tok_LParen (
Tok_RParen )
Tok_LBrace {
Tok_RBrace }
Tok_Equal ==
Tok_NotEqual !=
Tok_Assign =
Tok_Greater >
Tok_Less <
Tok_GreaterEqual >=
Tok_LessEqual <=
Tok_Or ||
Tok_And &&
Tok_Not !
Tok_Semi ;
Tok_Int_Type int
Tok_Bool_Type bool
Tok_Print printf
Tok_Main main
Tok_If if
Tok_Else else
Tok_While while
Tok_Add +
Tok_Sub -
Tok_Mult *
Tok_Div /
Tok_Pow ^

Your lexing code will feed the tokens into your parser, so a broken lexer will render the parser useless. Test your lexer thoroughly before moving on to the parser!

The Parser

Once the program has been transformed from a string of raw characters into more manageable tokens, you're ready to parse. The parser must be implemented in parser.ml in accordance with the signatures for parse_expr, parse_stmt and parse_main found in parser.mli. parser.ml is the only file you will write code in. The functions should be left in the order they are provided, as a good implementation will rely heavily on earlier functions.

We provide an ambiguous CFG for the language that you must convert to be right-recursive and right-associative, so it can be parsed by recursive descent. (By right associative, we are referring to binary infix operators—so something like 1 + 2 + 3 will parse as Add(Int(1), Add(Int(2), Int(3))), essentially implying parentheses in the form (1 + (2 + 3)).) As convention, in the given CFG all non-terminals are capitalized, all syntax literals (terminals) are formatted as non-italicized code and will come in to the parser as tokens from your lexer. Variant token types (i.e. Tok_Bool, Tok_Int, and Tok_ID) will be printed as italicized code.

Parser Part 1: parse_expr

Expressions are a self-contained subset of the SmallC grammar. As such, implementing them first will allow us to build the rest of the language on top of them later. Recall the expr type from project 3b:

type expr =
  | ID of string
  | Int of int
  | Bool of bool
  | Add of expr * expr
  | Sub of expr * expr
  | Mult of expr * expr
  | Div of expr * expr
  | Pow of  expr * expr
  | Greater of expr * expr
  | Less of expr * expr
  | GreaterEqual of expr * expr
  | LessEqual of expr * expr
  | Equal of expr * expr
  | NotEqual of expr * expr
  | Or of expr * expr
  | And of expr * expr
  | Not of expr

The function parse_expr : token list -> token list * expr takes a list of tokens and returns a tuple of the remaining tokens and the expr that was parsed. Examples in class used a more imperative style with a global reference, but the parse_expr and parse_stmt functions in this project use a purely functional style where remaining tokens are returned along with the produced AST types. How you choose to use this part of the return value is up to you, but it must satisfy the same property of finally returning all remaining tokens regardless of your design decisions around it.

The (ambiguous) CFG of expressions, from which you should produce a value of expr AST type, is as follows:

  • Expr -> OrExpr
  • OrExpr -> OrExpr || OrExpr | AndExpr
  • AndExpr -> AndExpr && AndExpr | EqualityExpr
  • EqualityExpr -> EqualityExpr EqualityOperator EqualityExpr | RelationalExpr
    • EqualityOperator -> == | !=
  • RelationalExpr -> RelationalExpr RelationalOperator RelationalExpr | AdditiveExpr
    • RelationalOperator -> < | > | <= | >=
  • AdditiveExpr -> AdditiveExpr AdditiveOperator AdditiveExpr | MultiplicativeExpr
    • AdditiveOperator -> + | -
  • MultiplicativeExpr -> MultiplicativeExpr MultiplicativeOperator MultiplicativeExpr | PowerExpr
    • MultiplicativeOperator -> * | /
  • PowerExpr -> PowerExpr ^ PowerExpr | UnaryExpr
  • UnaryExpr -> ! UnaryExpr | PrimaryExpr
  • PrimaryExpr -> Tok_Int | Tok_Bool | Tok_ID | ( Expr )

The transformation of the above ambiguous grammar into a parsable, non-ambiguous, grammar can be found in the addendum. We encourage you to do the transformation yourself and utilize the addendum to check your work and ensure correctness before coding.

As an example, see how the parser will break down an input mixing a few different operators with different precedence:


2 * 3 ^ 5 + 4

Output (Stylized to show order):


Parser Part 2: parse_stmt

The next step to parsing is to build statements up around your expression parser. When parsing, a sequence of statements should be terminated as a NoOp, which you will remember as a do-nothing instruction from the interpreter. Recall the stmt type:

type stmt =
  | NoOp
  | Seq of stmt * stmt
  | Declare of data_type * string
  | Assign of string * expr
  | If of expr * stmt * stmt
  | While of expr * stmt
  | Print of expr

The function parse_stmt : token list -> token list * stmt takes a token list as input and returns a tuple of the tokens remaining and the stmt that was parsed from the consumed input tokens. The stmt type isn't self contained like the expr type, and instead refers both to itself and to expr; use your parse_expr function to avoid duplicate code!

Again, we provide a grammar that is ambiguous and must be adjusted to be parsable by your recursive descent parser:

  • Stmt -> Stmt Stmt | DeclareStmt | AssignStmt | PrintStmt | IfStmt | WhileStmt
    • DeclareStmt -> BasicType ID ;
      • BasicType -> int | bool
    • AssignStmt -> ID = Expr ;
    • PrintStmt -> printf ( Expr ) ;
    • IfStmt -> if ( Expr ) { Stmt } ElseBranch
      • ElseBranch -> else { Stmt } | ε
    • WhileStmt -> while ( Expr ) { Stmt }

As with the Expression grammar, the transformation to enable the grammar to be parsable can be found in the addendum.

If we expand on our previous example, we can see how the expression parser integrates directly into the statement parser:


int x;
x = 2 * 3 ^ 5 + 4;
printf(x > 100);

Output (Stylized to show order):

Seq(Declare(Int_Type, "x"),
Seq(Print(Greater(ID("x"), Int(100))), NoOp)))

Parser Part 3: parse_main

The last and shortest step is to have your parser handle the function entry point. This is where parse_main : token list -> stmt comes in. This function behaves the exact same way as parse_stmt, except for two key semantic details:

  • parse_main will parse the function declaration for main, not just the body.
  • parse_main validates that a successful parse terminates in EOF. A parse not ending in EOF should raise an InvalidInputException in parse_main. As such, parse_main does NOT return remaining tokens, since it validates ensures that the token list is emptied by the parse.

The grammar for this parse is provided here:

  • Main ::= int main ( ) { Stmt } EOF

For this slightly modified input to the example used in the previous two sections, the exact same output would be produced:


int main() {
  int x;
  x = 2 * 3 ^ 5 + 4;
  printf(x > 100);

The output is the exact same as in the statement parser, but parse_main also trims off the function header and verifies that all tokens are consumed.

Project Submission

You should submit the files lexer.ml and parser.ml containing your solution. You may submit other files, but they will be ignored during grading. We will run your solution as individual OUnit tests just as in the provided public test file.

Be sure to follow the project description exactly! Your solution will be graded automatically, so any deviation from the specification will result in lost points.

You can submit your project in two ways:

  • Submit your files directly to the submit server as a zip file by clicking on the submit link in the column "web submission".
    Where to find the web submission link
    Then, use the submit dialog to submit your zip file containing eval.ml directly.
    Where to upload the file
    Select your file using the "Browse" button, then press the "Submit project!" button. You will need to put it in a zip file since there are two component files.
  • Submit directly by executing a the submission script on a computer with Java and network access. Included in this project are the submission scripts and related files listed under Project Files. These files should be in the directory containing your project. From there you can either execute submit.rb or run the command java -jar submit.jar directly (this is all submit.rb does).

No matter how you choose to submit your project, make sure that your submission is received by checking the submit server after submitting.

Academic Integrity

Please carefully read the academic honesty section of the course syllabus. Any evidence of impermissible cooperation on projects, use of disallowed materials or resources, or unauthorized use of computer accounts, will be submitted to the Student Honor Council, which could result in an XF for the course, or suspension or expulsion from the University. Be sure you understand what you are and what you are not permitted to do in regards to academic integrity when it comes to project assignments. These policies apply to all students, and the Student Honor Council does not consider lack of knowledge of the policies to be a defense for violating them. Full information is found in the course syllabus, which you should review before starting.