Introduction

In class we looked at depython, some types for representing python abstract syntax trees.

In python you can introspect code using the ast module.

import ast

ast.dump(ast.parse("123"))

The result is

Module(body=[Expr(value=Num(n=123))])

A more complicated example:

ast.dump(ast.parse("2 + 3"))

Results in

Module(body=[Expr(value=BinOp(left=Num(n=2), op=Add(), right=Num(n=3)))])

Types

We can define OCaml types similar to the above representations of python ast

(* depython - data types for representing python code 
   Copyright 2022 Humberto Ortiz-Zuazaga <humberto.ortiz@upr.edu>
   See LICENSE file for details.
*)

type binop = Add | Sub | Mul | Div

type expr =
 | BinOp of binop * expr * expr 
 | Num of int                   
 | Name of string               

type stm =
 | Expr of expr
 | Print of expr
 | Assign of string * expr

type prog = Module of stm list

let p1 = Module [Assign ("a", BinOp (Add, Num 5, Num 4))];;

let rec interp e =
  (* unfinished *)
  match e with
    | Num x -> x
    | BinOp (Add, e1, e2) -> interp e1 + interp e2

I have made a github repository with the depython data types.

Homework

Implement a function to compute the depth of the largest expression in a depython prog. Be careful to check everywhere there may be an expression.

Examples

Some example depython programs and their depth:

Module [Expr (Num 33)]

Depth 0.

Module [Print (BinOp (Sub, Num 112, Num 23))]

Depth 1.

Module [Assign ("a", Num 4);
        Expr (BinOp (Add, Num 8, 
                          BinOp (Mul , Name "a", Name "a")))]

Depth 2.

depython - OCaml types for a subset of python

Introduction

Types

Homework

Examples