HW 4, due 5/29/2017 (230 pts)

Hints

See the following for hints on different parts of this assignment

  1. bool-parser
  2. arith-parser-interpreter
  3. arith-parser-interpreter

Overview

The overall objective of this assignment is to fully understand the notion of scoping, binding, environments and closures, by implementing an interpreter for a subset of ML.

In addition, in this assignment you will be building a lexer and a parser.

No individual function requires more than 15-25 lines, so if you’re answer is longer, you can be sure that you need to rethink your solution.

The template for the assignment, as well as several test cases is available as a single zip file hw4.zip that you need to download and unzip.

The files contain several skeleton ML functions, with missing bodies, i.e. expressions, which currently contain the text failwith "to be written". Your task is to replace the text in those files with the the appropriate ML code for each of those expressions.

Note: All the solutions can be done using the purely functional fragment of OCaml, using constructs covered in class, and most require the use of recursion. Solutions using imperative features such as references, will receive no credit. Feel free to use any library functions that you want. It is a good idea to start this assignment very early as it is longer than the first three assignments (and will clarify your understanding of ML in time for the midterm!)

Assignment Testing and Evaluation

Your functions/programs must compile and/or run on a ACS Linux machine (e.g. ieng6.ucsd.edu) as this is where your solutions will be checked. While you may develop your code on any system, ensure that your code runs as expected on an ACS machine prior to submission. You should test your code in the directories from which the zip files (see below) will be created, as this will approximate the environment used for grading the assignment.

Most of the points, will be awarded automatically, by evaluating your functions against a given test suite. test.ml contains a very small suite of tests which gives you a flavor of of these tests. At any stage, by typing at the UNIX shell :

> ./nanoml.top test.ml > log

you will get a report on how your code stacks up against the simple tests.

If the code you submit does not compile with make, there will be no nanoml.top, and you will get 0 for the assignment.

The last line of the file log must contain the word ``Compiled" otherwise you get a zero for the whole assignment.

If for some problem, you cannot get the code to compile, leave it as is with the failwith ... with your partial solution enclosed below as a comment.

The second last line of the log file will contain your overall score, and the other lines will give you a readout for each test. You are encouraged to try to understand the code in test.ml, and subsequently devise your own tests and add them to test.ml, but you will not be graded on this.

Submission Instructions

Go into the UNIX shell and type:

> make turnin

This will run turnin which will provide you with a confirmation of the submission process.

Data Structures and Overview

In this assignment, you will build an interpreter for a subset of ML. The following data types (in nano.ml) are used to represent the different elements of the language.

Binary Operators

NanoML uses the following binary operators encoded within the interpreter as Ocaml values of type binop.

type binop =
  Plus
| Minus
| Mul
| Div
| Eq
| Ne
| Lt
| Le
| And
| Or          
| Cons

Expressions

As with ML, all Nano-ML programs correspond to expressions each of which will be represented within your interpreter by Ocaml values of type expr.

type expr =   
    Const of int             
  | True                      
  | False                       
  | NilExpr                     
  | Var of string               
  | Bin of expr * binop * expr    
  | If  of expr * expr * expr     
  | Let of string * expr * expr     
  | App of expr * expr              
  | Fun of string * expr             
  | Letrec of string * expr * expr

The following lists some NanoML expressions, and the Ocaml value (of type expr) used to represent the expression inside your interpreter.

  1. Let-bindings
let x = 3 in x + x

is represented by

Let ("x", Const 3, Bin (Var "x", Plus, Var "x"))
  1. Function definitions
fun x -> x + 1

is represented by

Fun ("x", Bin (Var "x", Plus, Const 1))
  1. Function applications (“calls”)
f x

is represented by

App (Var "f", Var "x")
  1. Recursive functions
let rec f = fun x -> f x in f 5

is represented by

Letrec ("f", Fun ("x", App (Var "f", Var "x"), App (Var "f", Const 5)))

Values

Finally, we will represent NanoML **values* using the following Ocaml datatype

type value =  
  Int of int                 
| Bool of bool                
| Closure of env * string option * string * expr
| Nil                           
| Pair of value * value          

and env = (string * value) list

Intuitively, the NanoML integer value 4 and boolean value true are represented by the Ocaml values Int 4 and Bool true. The more interesting case is for closures that correspond to function values (Lecture 7

  • Closure (env, Some "f", "x", e) represents a function named "f" (for recursive functions) with argument "x" and body-expression e that was defined in an environment env.

  • Closure (env, None, "x", e) represents an anonymous (hence, non-recursive) function with argument "x" and body-expression e that was defined in an environment env.

The environment itself is a value of type env which is a phone-book represented as a list of string * value pairs that maps each bound name to its value.

Problem 1: NanoML Interpreter (nano.ml)

NOTE: For this problem, you will use ocaml-top to edit and test the file nano.ml.

In this problem, you will implement an interpreter for NanoML.

(a) 25 points

First consider the (restricted subsets of) types described below:

type binop = Plus | Minus | Mul | Div

type expr  = Const of int 		
           | Var of string                
           | Bin of expr * binop * expr    

type value = Int of int		

type env   = (string * value) list

Here, binop, and expr are used to represent simple NanoML expressions.

  • An expression is either an integer constant, a variable, or a binary operator applied to two sub-expressions.

  • A value is an integer, and an environment is a list of pairs of variable names and values.

Use listAssoc to write an Ocaml function

val lookup: 'a * ('a * 'b) list -> 'b

that finds the most recent binding for a variable (i.e. the first from the left) in the list representing the environment. If no such value is found, you should raise an exception:

raise (MLFailure "not found")

Next, use lookup to write an Ocaml function

val eval : env * expr -> value

that when called with the pair (evn, e) evaluates the NanoML expression e in the environment evn (i.e. uses evn for the values of the free variables in e) and raises an exception MLFailure ("variable not bound: x") if the expression contains a free variable "x" that is not bound in evn.

Once you have implemented this functionality and recompiled, you should get the following behavior:

# let evn = [("z1",Int 0);("x",Int 1);("y",Int 2);("z",Int 3);("z1",Int 4)];;
val evn : (string * Nano.value) list =
  [("z1", Int 0); ("x", Int 1); ("y", Int 2); ("z", Int 3); ("z1", Int 4)]

# let e1 = Bin(Bin(Var "x",Plus,Var "y"), Minus, Bin(Var "z",Plus,Var "z1"));;
val e1 : Nano.expr =
  Bin (Bin (Var "x", Plus, Var "y"), Minus, Bin (Var "z", Plus, Var "z1"))

# eval (evn, e1);;
- : Nano.value = Int 0

# eval (evn, Var "p");;
Exception: Nano.MLFailure "variable not bound: p".

(b) 20 points

Next, add support for the binary operators

type binop = Plus | Minus | Mul | Div
           | Eq | Ne | Lt | Le | And | Or

This will require using the new value type Bool

type value = Int of int		
           | Bool of bool

The operators Eq and Ne should work if both operands are Int values, or if both operands are Bool values.

The operators Lt and Le are only defined for Int values, and && and || are only defined for Bool values. For all other (invalid) arguments, a MLFailure exception should be raised with an appropriate error message.

Next, implement the evaluation of If expressions.

To evaluate If(p,t,f) your evaluator should first evaluate p and if it evaluates to the true value (as a Bool) then the expression t should be evaluated and its value returned as the value of the entire If expression. Instead, if p evaluates to the false value, then f should be evaluated and that result should be returned. If p does not evaluate to a Bool value, then your evaluator should raise an MLFailure exception carrying an appropriate error message.

Once you have implemented this functionality, you should get the following behavior:

# let evn =[("z1",Int 0);("x",Int 1);("y",Int 2);("z",Int 3);("z1",Int 4)];;
val evn : (string * Nano.value) list =
  [("z1", Int 0); ("x", Int 1); ("y", Int 2); ("z", Int 3); ("z1", Int 4)]

# let e1 =If(Bin(Var "z1",Lt,Var "x"),Bin(Var "y",Ne,Var "z"),False);;
val e1 : Nano.expr =
  If (Bin (Var "z1", Lt, Var "x"), Bin (Var "y", Ne, Var "z"), False)

# eval (evn,e1);;
- : Nano.value = Bool true

# let e2 =If(Bin(Var "z1",Eq,Var "x"),Bin(Var "y",Le,Var "z"),Bin(Var "z",Le,Var "y"));;
val e2 : Nano.expr =
  If (Bin (Var "z1", Eq, Var "x"), Bin (Var "y", Le, Var "z"),
   Bin (Var "z", Le, Var "y"))

# eval (evn,e2);;
- : Nano.value = Bool false

(c) 25 points

Now consider the extended the types as shown below which includes the let-in expressions which introduce local bindings exactly as in OCaml.

type expr = ...
  | Let of string * expr * expr   
  | Letrec of string * expr * expr

The expression Let (x, <e1> , <e2>) should be evaluated as the ML expression let x = <e1> in <e2>. Similarly, Letrec (x, <e1>, <e2>) should be evaluated as let rec x = <e1> in <e2>. (Since at this point, we do not support functions, Let and Letrec should do the same thing.

Once you have implemented this functionality and recompiled, you should get the following behavior:

# let e1 = Bin(Var "x",Plus,Var "y");;
val e1 : Nano.expr = Bin (Var "x", Plus, Var "y")

# let e2 = Let("x",Const 1,Let("y",Const 2,e1));;
val e2 : Nano.expr =
  Let ("x", Const 1, Let ("y", Const 2, Bin (Var "x", Plus, Var "y")))

# eval ([],e2);;
- : Nano.value = Int 3

# let e3 = Let("x",Const 1,Let("y",Const 2,Let("z",e1,Let("x",Bin(Var "x",Plus,Var "z"),e1))));;
val e3 : Nano.expr =
	  Let ("x", Const 1,
	   Let ("y", Const 2,
	    Let ("z", Bin (Var "x", Plus, Var "y"),
	     Let ("x", Bin (Var "x", Plus, Var "z"), Bin (Var "x", Plus, Var "y")))))

# eval ([],e3);;
- : Nano.value = Int 6

(d) 25 points

Next, extend the evaluator so it includes the expressions corresponding to function definitions and applications.

type expr = ...
		| App of expr * expr
		| Fun of string * expr

Naturally, to handle functions, you will need to extend the set of values yielded by your evaluator to include closures.

type value = ...
		   | Closure of env * string option * string * expr

Recall that App(<e1>, <e2>) corresponds to the ML expression <e1> <e2> (i.e. applying the argument <e2> to the function <e1>), and Fun ("x", <e>) corresponds to the ML function defined fun x -> <e>.

For now, assume the functions are not recursive. However, functions do have values represented by the Closure (evn, n, x, e) where evn is the environment at the point where that function was declared, and n, x , and e are name, formal and body expression of the function.

  • If the function is anonymous or declared in a let-binding the name component of the closure tuple should be None.

  • If the function is declared in a let rec binding statement, then the name of the function should be Some f (where f is the name of the function, i.e. the variable bound in the let rec function).

Extend your implementation of eval by adding the appropriate cases for the new type constructors. Once you have implemented this functionality and recompiled, you should get the following behavior:

# eval ([],Fun ("x",Bin(Var "x",Plus,Var "x")));;
- : Nano.value = Closure ([], None, "x", Bin (Var "x", Plus, Var "x"))

# eval ([],App(Fun ("x",Bin(Var "x",Plus,Var "x")),Const 3));;
- : Nano.value = Int 6

# let e3=Let("h",Fun("y",Bin(Var "x", Plus, Var "y")),App(Var "f",Var "h"));;
val e3 : Nano.expr =
  Let ("h", Fun ("y", Bin (Var "x", Plus, Var "y")), App (Var "f", Var "h"))

# let e2 = Let("x",Const 100,e3);;
val e2 : Nano.expr =
  Let ("x", Const 100,
   Let ("h", Fun ("y", Bin (Var "x", Plus, Var "y")), App (Var "f", Var "h")))

# let e1 = Let("f",Fun("g",Let("x",Const 0,App(Var "g",Const 2))),e2);;
val e1 : Nano.expr =
  Let ("f", Fun ("g", Let ("x", Const 0, App (Var "g", Const 2))),
   Let ("x", Const 100,
    Let ("h", Fun ("y", Bin (Var "x", Plus, Var "y")),
     App (Var "f", Var "h"))))

# eval ([],e1);;
- : Nano.value = Int 102

# eval ([],Letrec("f",Fun("x",Const 0),Var "f"));;
- : Nano.value = Closure ([], Some "f", "x", Const 0)

(e) 30 points

Make the above work for recursively defined functions (declared with let rec). Once you have implemented this functionality and recompiled, you should get the following behavior:

# eval ([],Letrec("fac",Fun("n",If(Bin(Var "n",Eq,Const 0),Const 1,Bin(Var "n",Mul,App(Var "fac",Bin(Var "n",Minus,Const 1))))),App(Var "fac",Const 10)));;
- : Nano.value = Int 3628800

(f) 40 points (extra credit)

Extend your program to support operations on lists.

type binop = ...
           | Cons

type expr = ...
          | NilExpr

type value = ...
           | Nil
           | Pair of value * value

In addition to the changes to the data types, add support for two functions "hd" and "tl" which do what the corresponding ML functions do. Once you have implemented this functionality and recompiled, you should get the following behavior:

# eval ([],Bin(Const 1,Cons,Bin(Const 2,Cons,NilExpr)));;
- : Nano.value = Pair (Int 1, Pair (Int 2, Nil))

# eval ([],App(Var "hd",Bin(Const 1,Cons,Bin(Const 2,Cons,NilExpr))));;
- : Nano.value = Int 1

# eval ([],App(Var "tl",Bin(Const 1,Cons,Bin(Const 2,Cons,NilExpr))));;
- : Nano.value = Pair (Int 2, Nil)

Problem 2: NanoML Lexer (nanoLex.mll) and Parser (nanoParse.mly)

NOTE: From now on, you are done with ocaml-top. For this problem your favorite editor to modify nanoLex.mll and nanoParse.mly, recompile at the UNIX shell using make and test by running nanoml.top.

The goal of this problem is to write a lexer and parser for NanoML using (Ocaml)Lex and (Ocaml)Yacc. Google those terms for more information about them! In each subproblem, we will increase the complexity of the expressions parsed by your implementation.

(a) 15 points

We will begin by making our parser recognize some of the simplest NanoML expressions: constants and variables.

Begin with nanoParse.mly and define tokens TRUE, FALSE, and Id. Note that a token Num is already defined. An Id token should have a single string argument, which holds the name of the variable (identifier) represented by the token.

Next, add rules to nanoLex.mll. A Num constant is a sequence of one or more digits. An Id is a letter (capital or lowercase) followed by zero or more letters or digits. The strings "true" and "false" should return the corresponding tokens TRUE and FALSE.

Finally, add a rule to nanoLex.mll to ignore whitespace which includes space, newline \n, carriage return \r, and tab \t.

Once you have implemented this functionality, you should get the following behavior. Note: $ is the unix-shell prompt, # is a nanoml.top prompt.

$ make
...

You should now hopefully have in your directory a file called nanoml.top, which is an Ocaml shell that is pre-loaded with the modules corresponding to your NanoML implementation.

$ ./nanoml.top
NanoML

$ ./nanoml.top
        Objective Caml version ...

# NanoLex.token (Lexing.from_string "true");;
- : NanoParse.token = NanoParse.TRUE

# Main.token_list_of_string "true false 12345 foo bar baz";;
- : NanoParse.token list = [NanoParse.TRUE; NanoParse.FALSE; NanoParse.Num 12345; NanoParse.Id "foo"; NanoParse.Id "bar"; NanoParse.Id "baz"]

Now return to nanoParse.mly. Add rules to the parser so that true, false, integers, and identifiers are parsed into expressions of type Nano.expr as described above and shown in nano.ml.

Once you have implemented this functionality, you should get the following behavior (recall $ is the unix shell and # the Ocaml prompt.)

$ make

(* hopefully this succeeds without errors. *)

$ ./nanoml.top
NanoML

$ ./nanoml.top
        Objective Caml version ...

# NanoParse.exp NanoLex.token (Lexing.from_string "true");;
- : Nano.expr = Nano.True

# NanoParse.exp NanoLex.token (Lexing.from_string "false");;
- : Nano.expr = Nano.False

# NanoParse.exp NanoLex.token (Lexing.from_string "   \n123");;
- : Nano.expr = Nano.Const 123

# NanoParse.exp NanoLex.token (Lexing.from_string "\rfoo");;
- : Nano.expr = Nano.Var "foo"

(b) 15 points

Add the following tokens to the lexer and parser.

String Token
let LET
rec REC
= EQ
in IN
fun FUN
-> ARROW
if IF
then THEN
else ELSE

These should be parsed as in real ML to Nano.Let, Nano.Letrec, Nano.Fun, and Nano.If expressions (of type Nano.expr).
That is,

  • a let expression should have the form let <id> = <expr> in <expr>,
  • a letrec expression should have the form let rec <id> = <expr> in <expr>
  • a function expression should have the form fun <id> -> <expr> and
  • an if expression should be `if then else “.

Here denotes any identifier from part (a), and denotes any expression from part (a), or any let / letrec / fun / if expression.

Once you have implemented this functionality and recompiled (by typing make at the unix prompt) you should get the following behavior at the ./nanoml.top prompt:

# Main.token_list_of_string "let rec foo = fun x -> if y then z else w in foo";;
- : NanoParse.token list =
	[NanoParse.LET; NanoParse.REC; NanoParse.Id "foo"; NanoParse.EQ;
	 NanoParse.FUN; NanoParse.Id "x"; NanoParse.ARROW; NanoParse.IF;
	 NanoParse.Id "y"; NanoParse.THEN; NanoParse.Id "z"; NanoParse.ELSE;
	 NanoParse.Id "w"; NanoParse.IN; NanoParse.Id "foo"]

# Main.string_to_expr "let rec foo = fun x -> if y then z else w in foo";;
- : Nano.expr =	Nano.Letrec ("foo", Nano.Fun ("x", Nano.If (Nano.Var "y", Nano.Var "z", Nano.Var "w")), Nano.Var "foo")

(c) 15 points

Add the following tokens to the lexer and parser.

String Token
+ PLUS
- MINUS
* MUL
/ DIV
< LT
<= LE
!= NE
&& AND
|| OR

Add all of these as binary operators to your parser.
Each should result in a Nano.Bin expression with the corresponding Nano.binop. The arguments to these binary operators may be any expressions.
(You don’t need to worry about types. "3+true||7" is allowed as far as the parser is concerned.)

Once you have implemented this functionality and recompiled, you should get the following behavior at the ./nanoml.top prompt:

# Main.token_list_of_string "+ - /*|| < <= = && !=";;
- : NanoParse.token list =
[NanoParse.PLUS; NanoParse.MINUS; NanoParse.DIV;
 NanoParse.MUL; NanoParse.OR; NanoParse.LT;
 NanoParse.LE; NanoParse.EQ; NanoParse.AND; NanoParse.NE]

# Main.string_to_expr "x + y";;
- : Nano.expr = Nano.Bin (Nano.Var "x", Nano.Plus, Nano.Var "y")

# Main.string_to_expr "if x <= 4 then a || b else a && b";;
- : Nano.expr =
	Nano.If (Nano.Bin (Nano.Var "x", Nano.Le, Nano.Const 4),
	 Nano.Bin (Nano.Var "a", Nano.Or, Nano.Var "b"),
	 Nano.Bin (Nano.Var "a", Nano.And, Nano.Var "b"))

# Main.string_to_expr "if 4 <= z then 1-z else 4*z";;
- : Nano.expr =
	Nano.If (Nano.Bin (Nano.Const 4, Nano.Le, Nano.Var "z"),
	 Nano.Bin (Nano.Const 1, Nano.Minus, Nano.Var "z"),
	 Nano.Bin (Nano.Const 4, Nano.Mul, Nano.Var "z"))

# Main.string_to_expr "let a = 6 / 2 in a!=11";;
- : Nano.expr =
	Nano.Let ("a", Nano.Bin (Nano.Const 6, Nano.Div, Nano.Const 2),
	Nano.Bin (Nano.Var "a", Nano.Ne, Nano.Const 11))

(d) 10 points

Add the following tokens to the lexer and parser.

String Token
( LPAREN
) RPAREN

Add rules to your parser to allow parenthesized expressions.
In addition, add a rule to your parser for function application.
Recall that function application is simply "<expr> <expr>" which corresponds to calling the (function corresponding to the) left expression with the (argument corresponding to the) right expression.

Once you have implemented this functionality and recompiled, you should get the following behavior at the ./nanoml.top prompt:

# Main.token_list_of_string "() (  )";;
- : NanoParse.token list =
	[NanoParse.LPAREN; NanoParse.RPAREN; NanoParse.LPAREN; NanoParse.RPAREN]

# Main.string_to_expr "f x";;
- : Nano.expr = Nano.App (Nano.Var "f", Nano.Var "x")

# Main.string_to_expr "(fun x -> x+x) (3*3)";;
- : Nano.expr =
    Nano.App (Nano.Fun ("x", Nano.Bin (Nano.Var "x", Nano.Plus, Nano.Var "x")),
	Nano.Bin (Nano.Const 3, Nano.Mul, Nano.Const 3))

# Main.string_to_expr "(((add3 (x)) y) z)";;
- : Nano.expr =
	Nano.App (Nano.App (Nano.App (Nano.Var "add3", Nano.Var "x"), Nano.Var "y"),
	 Nano.Var "z")

# Main.filename_to_expr "tests/t1.ml";;
- : Nano.expr =
	Nano.Bin (Nano.Bin (Nano.Const 2, Nano.Plus, Nano.Const 3), Nano.Mul,
	Nano.Bin (Nano.Const 4, Nano.Plus, Nano.Const 5))

# Main.filename_to_expr "tests/t2.ml";;
- : Nano.expr =
	Nano.Let ("z1", Nano.Const 4,
	 Nano.Let ("z", Nano.Const 3,
	  Nano.Let ("y", Nano.Const 2,
	   Nano.Let ("x", Nano.Const 1,
	    Nano.Let ("z1", Nano.Const 0,
	     Nano.Bin (Nano.Bin (Nano.Var "x", Nano.Plus, Nano.Var "y"), Nano.Minus,
	      Nano.Bin (Nano.Var "z", Nano.Plus, Nano.Var "z1")))))))

(d) 35 points

Restructure your parser to give binary operators the following precedence and associativity. This will likely require that you add additional rules to your parser, or see how to add precedence and associativity to OcamlYacc.

  • (Highest) Fun Application
  • *, /
  • +, -
  • =, !=, <, <=
  • &&
  • (Lowest) ||

Precedence Function application having higher precedence than multiplications, and multiplication higher than addition means that "1+f x*3" should be parsed as if it were "1+((f x)*3)".

Associativity All binary operators are *left associative** meaning that "1-2-3-4" should be parsed as if it were "((1-2)-3)-4", and "f x y z" should be parsed as if it were "((f x) y) z".

Once you have implemented this functionality and recompiled, you should get the following behavior at the ./nanoml.top prompt:

# Main.string_to_expr "1-2-3-4";;
- : Nano.expr =
	Nano.Bin
	 (Nano.Bin (Nano.Bin (Nano.Const 1, Nano.Minus, Nano.Const 2),
	  Nano.Minus,
	   Nano.Const 3),
	 Nano.Minus, Nano.Const 4)

# Main.string_to_expr "1+a&&b||c+d*e-f/g x";;
- : Nano.expr =
	Nano.Bin
	 (Nano.Bin (Nano.Bin (Nano.Const 1, Nano.Plus, Nano.Var "a"), Nano.And,
	   Nano.Var "b"),
	 Nano.Or,
	 Nano.Bin
	  (Nano.Bin (Nano.Var "c", Nano.Plus,
	    Nano.Bin (Nano.Var "d", Nano.Mul, Nano.Var "e")),
	  Nano.Minus,
	  Nano.Bin (Nano.Var "f", Nano.Div, Nano.App (Nano.Var "g", Nano.Var "x"))))

# Main.filename_to_expr "tests/t13.ml";;
- : Nano.expr =
	Nano.Let ("f",
	 Nano.Fun ("x",
	  Nano.Fun ("y",
	   Nano.Fun ("a",
	    Nano.Bin (Nano.App (Nano.Var "a", Nano.Var "x"), Nano.Mul, Nano.Var "y")))),
	 Nano.Let ("g",
	  Nano.Fun ("x",
	   Nano.Bin (Nano.Var "x", Nano.Plus,
	    Nano.Bin (Nano.Const 1, Nano.Mul, Nano.Const 3))),
	  Nano.App (Nano.App (Nano.App (Nano.Var "f", Nano.Const 7), Nano.Const 8),
	   Nano.Var "g")))

(e) 15 points

Add the following tokens to the lexer and parser.

<tr><th>String</th><th>Token</th></tr>
<tr><td><tt>[</tt></td><td><tt>LBRAC</tt></td></tr>
<tr><td><tt>]</tt></td><td><tt>RBRAC</tt></td></tr>
<tr><td><tt>;</tt></td><td><tt>SEMI</tt></td></tr>
<tr><td><tt>::</tt></td><td><tt>COLONCOLON</tt></td></tr>

Add rules to your lexer and parser to support parsing lists. "[a;b;c;d;e;f;g]" should be parsed as if it were "a::b::c::d::e::f::g::[]". The :: operator should have higher priority than the comparison functions (=, <= etc.), and lower priority than + and -. In addition, :: should be right associative. "[]" should be parsed as NilExpr, and :: should be treated as any other binary operator.

Once you have implemented this functionality and recompiled, you should get the following behavior at the ./nanoml.top prompt:

# Main.string_to_expr "1::3::5::[]";;
- : Nano.expr =
	Nano.Bin (Nano.Const 1, Nano.Cons,
	 Nano.Bin (Nano.Const 3, Nano.Cons,
	  Nano.Bin (Nano.Const 5, Nano.Cons, Nano.NilExpr)))

# Main.string_to_expr "[1;3;5]";;
- : Nano.expr =
	Nano.Bin (Nano.Const 1, Nano.Cons,
	 Nano.Bin (Nano.Const 3, Nano.Cons,
	  Nano.Bin (Nano.Const 5, Nano.Cons, Nano.NilExpr)))

# Main.string_to_expr "1::3::5::[] = [1;3;5]";;
- : Nano.expr =
	Nano.Bin
	 (Nano.Bin (Nano.Const 1, Nano.Cons,
	   Nano.Bin (Nano.Const 3, Nano.Cons,
	    Nano.Bin (Nano.Const 5, Nano.Cons, Nano.NilExpr))),
	 Nano.Eq,
	 Nano.Bin (Nano.Const 1, Nano.Cons,
	  Nano.Bin (Nano.Const 3, Nano.Cons,
	   Nano.Bin (Nano.Const 5, Nano.Cons, Nano.NilExpr))))

Problem 3: NanoML Executable

Once you have completed the first two parts, you should end up with an executable nanoml.byte. You should be able to test it as follows from the unix shell prompt:

$ ./nanoml.byte tests/t1.ml
...
out: 45

$./nanoml.byte tests/t2.ml
...
out: 0

$ ./nanoml.byte tests/t3.ml
...
out: 2

and so forth, for all the files in tests. To get the expected (i.e. “correct”) value for the other tests, run them with Ocaml:

# #use "tests/t1.ml";;
- : int = 45

and so forth. “tests/t14.ml” requires that you have completed both extra credit parts.

We strongly encourage you to make sure that your nanoml.byte produces exactly the same answer as the “real” Ocaml for each of the files in tests/