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[shubhamnarlawar77]

• Mini Interpreter
  Write a small C program that is representative of a miniature
  interpreter. The program
  should take as input from file - a list, evaluate it and output the
  result.

For eg:
(= a 1)
(= b 2)
(+ (* a b) 3)

The result of evaluation should be 5.
Note that the lists can be arbitrarily nested:
(+ (* a (- c  d)) (* k x))
The program should gracefully handle nested lists.

Each element of this "list" is either an integer, a variable name, an
operator like +, -
or a list itself, that is, it's definition is recursive.

More formally:
list: element
    |  element list

element:  number | variable | operator | '( list ')'

The above specification is called Context Free Grammar, which is a
mathematical notation of specifying language grammars [2].

For operators just consider the standard arithmetic operators: +, -, *, /,
Bonus points for multi-name variables like "foo", but single-letter
variables are also fine to
begin with.

Write the program in two parts:
a) The first part should correctly parse the input and build the
data-structure to represent the evaluation. This data structure is
called abstract syntax tree or AST for short [2].
b) In the second part, traverse the AST and evaluate the expression.
Note that a single AST should represent single expression.

For example: (= a (+ 2 3))
can be represented as following tree:

                 =
            a            +
                      2       3

The first phase should build this tree, and the second phase should
then do the tree-walk and evaluate the expression.
The notation of "lists" is more commonly known as s-expression [3] and
is the basis of functional languages like LISP.  Make the program
robust. It should be able to deal
with incorrect syntax like
(+ 1) and gracefully exit with an error message.