flatten_expr.hs

-- this would turn the expression
--    (((anArray[(10)])))
-- into
--    anArray[10]

flatten :: Expr -> Expr
  -- base case: do nothing to constants
  flatten (Constant i) = Constant i
  -- this is the important case: we shed the Paren constructor and just 
  -- apply `flatten` to its contents
  flatten (Paren e) = flatten e
  -- all the other cases preserve their constructors and just apply 
  -- the flatten function to their children that are of type `Expr`.
  flatten (Index e i) = Index (flatten e) (flatten i)
  flatten (Call e args) = Call (flatten e) (map flatten args)
  flatten (Unary op arg) = Unary op (flatten arg)
  flatten (Binary l op r) = Binary (flatten l) op (flatten r)