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Introduction
Over the course of Projects 4a and 4b, you will implement MicroCaml — a dynamically- typed version of OCaml with a subset of its features. Because MicroCaml is dynamically typed, it is not type checked at compile time; like Ruby, type checking will take place when the program runs. As part of your implementation of MicroCaml, you will also implement parts of mutop (μtop or Microtop), a version of utop for MicroCaml.
In Project 4a, you will implement a lexer and parser for MicroCaml. Your lexer function will convert an input string of MicroCaml into a list of tokens, and your parser function will consume these tokens to produce an abstract symbol tree (AST), either for a MicroCaml expression, or for a mutop directive. In Project 4b, you will implement an interpreter to actually execute the produced AST.
Here is an example call to the lexer and parser on a MicroCaml mutop directive (as a string):

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parse_mutop (tokenize “def b = let x = true in x;;”)
This will return the AST as the following OCaml value (which we will explain in due course):
Def (“b”, Let (“x”, false, Value (Bool true), ID “x”))
Ground Rules
In your code, you may use any OCaml modules and features we have taught in this class except imperative OCaml features like references, mutable records, and arrays. This means that you’ll have to adjust the approach of the match_tok etc. functions given
in lecture; these functions will take and return a token list, rather than modify a global list in place; we provide helper functions in the project to do this. Also, we have changed the definition of lookahead from the one given in lecture; the version presented here returns an option type, which is critical for your parser to work properly.
Testing & Submitting
You can submit through gradescope-submit from the project directory and the project will be automatically submitted.
You can also manually submit to Gradescope. You may only submit
the lexer.ml and parser.ml files. To test locally, run dune runtest -f.
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. 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, to make debugging easier. We are not requiring intelligent messages that pinpoint an error to help a programmer debug, but as you do this project you might find you see where you could add those.
You can run your lexer or parser directly on a MicroCaml program by running dune exec bin/interface.bc lex [filename] or dune exec bin/interface.bc parse [filename] where the [filename] argument is required.
To test from the toplevel, run dune utop src. The necessary functions and types will automatically be imported for you.
You can write your own tests which only test the parser by feeding it a custom token list. For example, to see how the expression let x = true in x would be parsed, you can construct the token list manually (e.g. in utop):
parse_expr [Tok_Let; Tok_ID “x”; Tok_Equal; Tok_Bool true; Tok_In; Tok_ID “x”];;
This way, you can work on the parser even if your lexer is not complete yet.
Part 1: The Lexer (aka Scanner or Tokenizer)
Your parser will take as input a list of tokens; this list is produced by the lexer (also called a scanner) as a result of processing the input string. Lexing is readily implemented by use of regular expressions, as demonstrated in lecture slides 3-5. 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 helpful.
Your lexer must be written in lexer.ml. You will need to implement the following function:
• Type: string -> token list
• Description: Converts MicroCaml syntax (given as a string) to a corresponding
token list.
• Examples:
• tokenize “1 + 2” = [Tok_Int 1; Tok_Add; Tok_Int 2] •
• tokenize “1 (-1)” = [Tok_Int 1; Tok_Int (-1)] •
• tokenize “;;” = [Tok_DoubleSemi] •
• tokenize “+ – let def” = [Tok_Add; Tok_Sub; Tok_Let; Tok_Def] •
• tokenize “let rec ex = fun x -> x || true;;” =

[Tok_Let; Tok_Rec; Tok_ID “ex”; Tok_Equal; Tok_Fun; Tok_ID “x”; Tok_Arrow; Tok_ID “x”; Tok_Or; Tok_Bool true; Tok_DoubleSemi]
The token type is defined in tokenTypes.ml. Notes:
• The lexer input is case sensitive.
• Tokens can be separated by arbitrary amounts of whitespace, which your lexer should discard. Spaces, tabs (‘ ’) and newlines (‘
’) are all considered
whitespace.
• When excaping characters with within Ocaml strings/regexp you must
use \ to escape from the string and regexp.
• If the beginning of a string could match multiple tokens, the longest match should be preferred, for example:
o “let0” should not be lexed as Tok_Let followed by Tok_Int 0, but as Tok_ID(“let0”), since it is an identifier.
o “330dlet” should be tokenized as [Tok_Int 330; Tok_ID “dlet”]. Arbitrary amounts of whitespace also includes no whitespace.
o “(-1)” should not be lexed as [Tok_LParen; Tok_Sub; Tok_Int(1); Tok_LParen] but as Tok_Int(-1). (This is further explained below)
• There is no “end” token (like Tok_END the lecture slides 3-5) — when you reach the end of the input, you are done lexing.
Most tokens only exist in one form (for example, the only way for Tok_Concat to appear in the program is as ^ and the only way for Tok_Let to appear in the program is as let). 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”.
o Regular Expression: true|false
• Tok_Int of int: Valid ints may be positive or negative and consist of 1 or more digits. Negative integers must be surrounded by parentheses (without extra whitespace) to differentiate from subtraction (examples below). You may find the functions int_of_string and String.sub useful in lexing this token type.
o Regular Expression: [0-9]+ OR (-[0-9]+) o Examples of int parenthesization:
▪ tokenize “x -1” = [Tok_ID “x”; Tok_Sub; Tok_Int 1] ▪ tokenize “x (-1)” = [Tok_ID “x”; Tok_Int (-1)]

• Tok_String of string: Valid string will always be surrounded by “” and should accept any character except quotes within them (as well as nothing). You have to “sanitize” the matched string to remove surrounding escaped quotes.
o Regular Expression: ”[^”]*” o Examples:
▪ tokenize “330” = [Tok_Int 330]
▪ tokenize “”330”” = [Tok_String “330”]
▪ tokenize “”””” (* InvalidInputException *)
• Tok_ID of string: Valid IDs must start with a letter and can be followed by any
number of letters or numbers. Note: Keywords may be substrings of IDs.
o Regular Expression: [a-zA-Z][a-zA-Z0-9]* o Valid examples:
▪ “a1b2c3DEF6” ▪ “fun1”
▪ “ifthenelse”
MicroCaml syntax with its corresponding token is shown below, excluding the four literal token types specified above.
Token Name Lexical Representation
Tok_LParen (
Tok_RParen )
Tok_Equal =
Tok_NotEqual <>
Tok_Greater >
Tok_Less < Tok_GreaterEqual >=
Tok_LessEqual <= Tok_And && Tok_Not not Tok_Then then Token Name Lexical Representation Tok_Else else Tok_Mult * Tok_Concat ^ Tok_Let let Tok_Def def Tok_Rec rec Tok_Fun fun Tok_Arrow ->
Tok_DoubleSemi ;;
• Your lexing code will feed the tokens into your parser, so a broken lexer can cause you to fail tests related to parsing.
• In grammars given below, the syntax matching tokens (lexical representation) is used instead of the token name. For example, the grammars below will
use ( instead of Tok_LParen.
Part 2: Parsing MicroCaml Expressions
In this part, you will implement parse_expr, which takes a stream of tokens and outputs as AST for the input expression of type expr. Put all of your parser code in parser.ml in accordance with the signature found in parser.mli.
We present a quick overview of parse_expr first, then the definition of AST types it
should return, and finally the grammar it should parse.
parse_expr
• Type: token list -> token list * expr
• Description: Takes a list of tokens and returns an AST representing the MicroCaml expression corresponding to the given tokens, along with any tokens left in the token list.

• Exceptions: Raise InvalidInputException if the input fails to parse i.e does not match the MicroCaml expression grammar.
• Examples (more below):
• parse_expr [Tok_Int(1); Tok_Add; Tok_Int(2)] = ([], Binop (Add, Value (Int
1), Value (Int 2)))
• parse_expr [Tok_Int(1)] = ([], Value (Int 1)) •
• parse_expr [Tok_Let; Tok_ID(“x”); Tok_Equal; Tok_Bool(true); Tok_In;
Tok_ID(“x”)] =
• ([], Let (“x”, false, Value (Bool true), ID “x”)) •
parse_expr [Tok_DoubleSemi] (* raises InvalidInputException *)
You will likely want to implement your parser using the
the lookahead and match_tok functions that we have provided; more about them is at the end of this README.
AST and Grammar for parse_expr
Below is the AST type expr, which is returned by parse_expr. Note that
the environment and Closure of environment * var * expr parts are only relevant to
Project 4b, so you can ignore them for now.
type op = Add | Sub | Mult | Div | Concat | Greater | Less | GreaterEqual | LessEqual | Equal | NotEqual | Or | And
type var = string
type value =
| Int of int
| Bool of bool
| String of string
| Closure of environment * var * expr (* not used in P4A *)
and environment = (var * value) list (* not used in P4A *)
and expr =
| Value of value
| ID of var
| Fun of var * expr (* an anonymous function: var is the parameter and expr is the
| Not of expr
| Binop of op * expr * expr
| If of expr * expr * expr
| FunctionCall of expr * expr
| Let of var * bool * expr * expr (* bool determines whether var is recursive *)
The CFG below describes the language of MicroCaml expressions. This CFG is right- recursive, 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)).) In the given CFG note that 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, Tok_String and Tok_ID) will be printed as italicized code.
• Expr -> LetExpr | IfExpr | FunctionExpr | OrExpr
• LetExpr -> let Recursion Tok_ID = Expr in Expr
o Recursion -> rec | ε
• FunctionExpr -> fun Tok_ID -> Expr
• IfExpr -> if Expr then Expr else Expr
• OrExpr -> AndExpr || OrExpr | AndExpr
• AndExpr -> EqualityExpr && AndExpr | EqualityExpr
• EqualityExpr -> RelationalExpr EqualityOperator EqualityExpr | RelationalExpr
o EqualityOperator -> = | <>
• RelationalExpr -> AdditiveExpr RelationalOperator RelationalExpr | AdditiveExpr
o RelationalOperator -> < | > | <= | >=
• AdditiveExpr -> MultiplicativeExpr AdditiveOperator AdditiveExpr | MultiplicativeExpr
o AdditiveOperator -> + | –
• MultiplicativeExpr -> ConcatExpr MultiplicativeOperator MultiplicativeExpr | ConcatExpr
o MultiplicativeOperator -> * | /
• ConcatExpr -> UnaryExpr ^ ConcatExpr | UnaryExpr
• UnaryExpr -> not UnaryExpr | FunctionCallExpr
• FunctionCallExpr -> PrimaryExpr PrimaryExpr | PrimaryExpr
• PrimaryExpr -> Tok_Int | Tok_Bool | Tok_String | Tok_ID | ( Expr )
Notice that this grammar is not actually quite compatible with recursive descent
parsing. In particular, the first sets of the productions of many of the non-terminals overlap. For example:
• OrExpr -> AndExpr || OrExpr | AndExpr
defines two productions for nonterminal OrExpr, separated by |. Notice that both productions starting with AndExpr, so we can’t use the lookahead (via FIRST sets) to determine which one to take. This is clear when we rewrite the two productions thus:
• OrExpr -> AndExpr || OrExpr
• OrExpr -> AndExpr

When my parser is handling OrExpr, which production should it use? From the above, you cannot tell. The solution is: You need to refactor the grammar, as shown
in lecture slides 35-37.
To illustrate parse_expr in action, we show several examples of input and their output AST.
Example 1: Basic math
(1 + 2 + 3) / 3
Output (after lexing and parsing):
Binop (Div,
Binop (Add, Value (Int 1), Binop (Add, Value (Int 2), Value (Int 3))), Value (Int 3))
In other words, if we run parse_expr (tokenize “(1 + 2 + 3) / 3”) it will return the AST above.
Example 2: let expressions Input:
let x = 2 * 3 / 5 + 4 in x – 5
Output (after lexing and parsing):
Let (“x”, false,
Binop (Add,
Binop (Mult, Value (Int 2), Binop (Div, Value (Int 3), Value (Int 5))),
Value (Int 4)),
Binop (Sub, ID “x”, Value (Int 5)))
Example 3: if then … else … Input:
let x = 3 in if not true then x > 3 else x < 3 Output (after lexing and parsing): Let ("x", false, Value (Int 3), If (Not (Value (Bool true)), Binop (Greater, ID "x", Value (Int 3)), Binop (Less, ID "x", Value (Int 3)))) Example 4: Anonymous functions let rec f = fun x -> x ^ 1 in f 1
Output (after lexing and parsing):
Let (“f”, true, Fun (“x”, Binop (Concat, ID “x”, Value (Int 1))), FunctionCall (ID “f”, Value (Int 1)))
Keep in mind that the parser is not responsible for finding type errors. This is the job of the interpreter (Project 4b). For example, while the AST for 1 1 should be parsed
as FunctionCall (Value (Int 1), Value (Int 1)); if it is executed by the interpreter, it will at that time be flagged as a type error.
Example 5: Recursive anonymous functions
Notice how the AST for let expressions uses a bool flag to determine whether a function is recursive or not. When a recursive anonymous function let rec f = fun x – > … in … is defined, f will bind to fun x -> … when evaluating the function. The interpreter will be responsible for handling this. In Project 4b, you will also handle the cases where rec is used without anonymous functions as well as attempting recursion without using rec.
For now, let’s create an infinite recursive loop for fun.
let rec f = fun x -> f (x*x) in f 2
Output (after lexing and parsing):
Let (“f”, true,
Fun (“x”, FunctionCall (ID “f”, Binop (Mult, ID “x”, ID “x”))), FunctionCall (ID “f”, Value (Int 2))))
Example 6: Currying
We will ONLY be currying to create multivariable functions as well as passing multiple
arguments to them. Here is an example:
let f = fun x -> fun y -> x + y in (f 1) 2
Output (after lexing and parsing):
Let (“f”, false,

Fun (“x”, Fun (“y”, Binop (Add, ID “x”, ID “y”))),
FunctionCall (FunctionCall (ID “f”, Value (Int 1)), Value (Int 2)))
Part 3: Parsing mutop directives
In this part, you will implement parse_mutop (putting your code in parser.ml) in
accordance with the signature found in parser.mli. Thus function takes a token list produced by lexing a string that is a mutop (top-level) MicroCaml directive, and returns an AST of OCaml type mutop. Your implementation of parse_mutop will reuse
your parse_expr implementation, and will not be much extra work.
We present a quick overview of the function first, then the definition of AST types it should return, and finally the grammar it should parse.
parse_mutop
• Type: token list -> token list * mutop
• Description: Takes a list of tokens and returns an AST representing the
MicroCaml expression at the mutop level corresponding to the given tokens,
along with any tokens left in the token list.
• Exceptions: Raise InvalidInputException if the input fails to parse i.e does not
match the MicroCaml definition grammar.
• Examples:
• parse_mutop [Tok_Def; Tok_ID(“x”); Tok_Equal; Tok_Bool(true); Tok_DoubleSemi]
= ([], Def (“x”, Value (Bool true)))
• parse_mutop [Tok_DoubleSemi] = ([], NoOp) •
• parse_mutop [Tok_Int(1); Tok_DoubleSemi] = ([], Expr (Value (Int 1)))) •
• parse_mutop [Tok_Let; Tok_ID “x”; Tok_Equal; Tok_Bool true; Tok_In; Tok_ID
“x”; Tok_DoubleSemi] =
([], Expr (Let (“x”, false, Value (Bool true), ID “x”)))
AST and Grammar of parse_mutop
Below is the AST type mutop, which is returned by parse_mutop, followed by the CFG that it parses for MicroCaml expressions at the mutop level. This CFG is similar (and similarly
formatted) to the CFG of parse_expr and relies its implementation of Expr. type mutop =
| Def of var * expr | Expr of expr
The CFG is as follows:
• Mutop -> DefMutop | ExprMutop | ;;
• DefMutop -> def Tok_ID = Expr ;;

• ExprMutop -> Expr ;;
Notice how a valid input for the parse_mutop must always terminate
with Tok_DoubleSemi and input of just Tok_DoubleSemi to the parser is considered valid as per the AST.
For this part, we created a new keyword def to refer to top-level MicroCaml expressions to differentiate local let. In essence, def is similar to top-level
(global) let expressions in normal (OCaml) utop. This means def will create global
definitions for variables while running mutop, in part 4b. Another key difference between def and the let expressions defined in Part 2 is that def should be implicitly recursive. (Note that def rec x = …;; is not valid as per the given AST—basically the rec is implicit).
Here are some example mutop directives. Note that parse_mutop should return a tuple of (updated token list, parsed AST), but in these examples we omit the updated token list since it should always just be an empty list.
Example 1: Global definition
def x = let a = 3 in if a <> 3 then 0 else 1;;
Output (after lexing and parsing):
Let (“a”, false, Value (Int 3),
If (Binop (NotEqual, ID “a”, Value (Int 3)), Value (Int 0), Value (Int 1))))
Example 2: Implicit recursion on f
def f = fun x -> if x > 0 then f (x-1) else “done”;;
Output (after lexing and parsing):
If (Binop (Greater, ID “x”, Value (Int 0)),
FunctionCall (ID “f”, Binop (Sub, ID “x”, Value (Int 1))), Value (String “done”))))
Example 3: Expression Input:

(fun x -> “(” ^ x ^ “)”) “parenthesis”;;
Output (after lexing and parsing):
FunctionCall (Fun (“x”,
Binop (Concat, Value (String “(“),
Binop (Concat, ID “x”, Value (String “)”)))),
Value (String “parenthesis”)))
Provided functions
To help you implement both parsers, we have provided some helper functions in
the parser.ml file. You are not required to use these, but they are recommended. match_token
• Type: token list -> token -> token list
• Description: Takes the list of tokens and a single token as arguments, and returns a new token list with the first token removed IF the first token matches the second argument.
• Exceptions: Raise InvalidInputException if the first token does not match the second argument to the function.
match_many
• Type: token list -> token list -> token list
• Description: An extension of match_token that matches a sequence of tokens
given as the second token list and returns a new token list with that matches each token in the order in which they appear in the sequence. For
example, match_many toks [Tok_Let] is equivalent to match_token toks Tok_Let. • Exceptions: Raise InvalidInputException if the tokens do not match.
lookahead_many
• Type: token list -> token option
• Description: Returns the top token in the list of tokens as an option,
returning None if the token list is empty. In constructing your parser, the lack of lookahead token (None) is fine for the epsilon case.
• Type: token list -> int -> token option
• Description: An extension of lookahead that returns token at the nth index in the
list of tokens as an option, returning None if the token list is empty at the given

index or the index is negative. For example, lookahead_many toks 0 is equivalent to lookahead toks.

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