Homework 7: Parsing
In this homework, you’ll implement a top-down predictive parser for an alternative syntax for our language. This syntax is called ML (Berkeley), because it bears a vague resemblance to ML-family languages like OCaml. We’ll usually refer to it as MLB.
Here’s an example of an MLB program:
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function add_up(a, b, c) = a+b=c
let x = if
add_up(read_num(), read_num(), read_num())
Unlike the previous assignments, you won’t be modifying either the interpreter or the compiler. We’ve provided an AST-based version of the HW6 compiler and interpreter as well as functions to produce the AST from s-expressions. You’ll write a parser that produces the same AST but that instead reads in MLB- formatted source code.
Here’s a grammar (like the ones we discussed in class) for MLB:
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|
| FUNCTION ID LPAREN
| ID
| COMMA ID
| IF
| LET ID EQ
Programming Help
|
| SEMICOLON
|
| EQ
| LT
|
| PLUS
| MINUS
| ID LPAREN
| LPAREN
|
| COMMA
Tests will not be graded
Except for the specific example we require in Task 1.1, we will NOT be grading your tests for this homework.
However, it will still be that case that when you submit your implementation to Gradescope (to the assignment hw7 ), your suite of examples in the examples/ directory will be run against the reference interpreter and compiler. If the reference implementation fails on any of your examples, Gradescope will show you how its output differed from the expected output of your example (if you wrote a .out file for it).
You can do this as many times as you want. We encourage you to use this option to develop a good set of examples before you start working on your parser!
Support for .mlb files
We’ve extended the testing framework to support programs in the new syntax. You can write MLB-syntax
examples either by:
Putting .mlb files in the examples directory
Writing a tab-separated examples/mlb-examples.tsv file. This file is tab-separated instead of comma- separated because, unlike our Lisp-like syntax, MLB uses commas pretty extensively.
Note that in general, the interpreter and the compiler will give the same result on all of your programs! You’ll probably want to write .out files (or include expected output in the .tsv file) to make sure your parser is actually working. These work exactly the same as with .lisp files.
On this homework more than on previous ones, it may be useful to run your functions in an OCaml shell. You can do that by running dune utop from the hw7 directory, then entering, for example:
1. Writing an MLB program (1 subtask)
To start, complete the following task to familiarize yourself with the MLB syntax. Note that this task WILL be graded!
Task 1.1: In the file examples/task1.mlb , write a valid program in the MLB syntax that defines a sort function that sorts a list of integers.
Hint: Recall that we represent lists as nested pair s with a nil at the end. For example, the list [1, 2] would be written as pair(1, pair(2, nil)) in the MLB syntax.
Hint: It may be helpful to define a sorted_insert helper function that takes in an integer n and a sorted list xs and returns xs with n inserted into it at a location that maintains the sorted order.
2. Parsing MLB syntax (1 subtask)
Now that you are familiar with MLB syntax, it’s time to write a parser for the language!
We provide you with a tokenizer in mlb_syntax/tokenizer.ml ; it should not be necessary to change it, but you are free to do so if you wish. Instead, you’ll be working on the parser in the file
mlb_syntax/parser.ml .
The AST you’ll produce is defined in ast/ast.ml ; it’s quite similar to the AST we defined in class. A few hints
for mapping the MLB grammar to the AST:
The
The
1 >openMlb_syntax;; 2 >parse”1+3″;;
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the identifier false , Nil on the identifier nil , and Var id on other identifiers.
The second ID case in the
Task 2.1: In the file mlb_syntax/parser.ml , finish the implementation of the parse function by implementing parse_defns and parse_expr , which parse MLB definitions and expressions respectively.
Hint: We recommend writing a top-down parser like the ones we developed in class:
Write one function per non-terminal (with the exception of primed cases—for instance, you can handle infix1′ inside the function for infix1 ).
Return a value (usually, but not always, an expression) and a list of tokens from each function. Decide which production rule to use by examining the front of the token list.
You may wish to review handparser.ml and handparser2.ml from class, along with the class sessions covering how they work. Make sure you remember how you’re using the returned token list!
Hint: The MLB grammar does not have any left-recursion or left-ambiguity except for in
Hint: You shouldn’t need to change the top-level parse_program function, but you’ll need to fill in the bodies of parse_defns and parse_expr and add additional non-terminal parsing functions.
Hint: We’ve provided one other helper function: consume_token . This function checks to see that the head of a token list is what you want it to be, returning the tail of the list if it is and raising an error otherwise.
Extra credit: associativity (1 extra credit subtask worth 10%)
With the grammar specified above, the MLB expression 2 + 3 + 4 will parse to something like (in s- expression syntax) (+ 2 (+ 3 4) .
This is a little different from what we’d usually expect: addition is generally defined to left-associative. Most languages parse that same expression to (+ (+ 2 3) 4) .
For addition, this doesn’t really matter—since it’s associative, those expressions evaluate to the same thing. This can lead to weird behavior on subtraction, though. The expression
should probably evaluate to 5 , but if you implement the grammar as specified above it will instead evaluate
to 9 (i.e., 10 – (3 – 2) ).
Extra Credit Task: If you finish your parser early, try to fix this! Specifically, modify your parser so that +
and – associate to the left.
Hint: There’s more than one way to achieve this! One way to get started would be to take a look at the
similar to get a list of terms, then transform the list into an AST of the correct shape?
This task is worth 10% additional credit on this homework assignment.
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