Description

I have provided a pretty printer for depython on github.

Your task is to modify our lexer and parser to recognize a small language with infix arithmetic as described in Lecture 4.

‹expr›: 
           NUMBER
           add1 ( ‹expr› )
           sub1 ( ‹expr› )
           ( <expr> )
           <expr> + <expr>
           <expr> - <expr>
           <expr> * <expr>

Your parser should produce an abstract syntax as shown in Lecture 3 and 4 as well:

type prim2 =
  | Plus
  | Minus
  | Times

type expr =
  | Num of int64
  | Add1 of expr
  | Sub1 of expr
  | EPrim2 of prim2 * expr * expr

Example code

There's sample code in github. To run it you will need menhir and dune installed. I used opam to install mine.

You can parse a sample depython program using:

$ dune build
$ dune exec ./pretty.exe 4410.dp
Num 4410

After modifying the lexer and parser you should be able to parse a program like the examples in lecture 4.

$ cat 234.dp
(2 + 3) + 4

The current parser can't understand this program:

$ dune exec ./pretty.exe 234.dp
Fatal error: exception Dune__exe__Parser.MenhirBasics.Error

After you complete the assignment, the output should be:

EPrim2 (Plus, EPrim2 (Plus, Num 2L, Num 3L), Num 4L) 

Deliverables

You should turn in your modified lexer.mll and parser.mly. You shouldn't need to make changes to other files.