put_in.rb

def put_in_f(hash, keys, value)
  if keys.empty?
    value
  else
    car, cdr = [keys[0], keys[1..]]
    hash.merge(car => put_in_f(hash[car], cdr, value))
  end
end

def extract(binding, ast)
  case ast.type
  when :SCOPE
    extract(binding, ast.children[2])
  when :CALL
    selv, methot, args = ast.children
    if methot == :[]
      keys, varname = extract(binding, selv)
      [[*keys, args.children[0].children[0]], varname]
    else
      raise NotImplementedError
    end
  when :DVAR
    [[], binding.local_variable_get(ast.children[0])]
  when :GVAR
    [[], eval(ast.children[0].to_s)]
  when :IVAR
    [[], binding.receiver.instance_variable_get(ast.children[0])]
  when :FCALL, :VCALL
    [[], binding.eval(ast.children[0].to_s)]
  else
    raise NotImplementedError, "ast.type #{ast.type} is not supported yet"
  end
end

def put_in(b, value)
  ast = RubyVM::AbstractSyntaxTree.of(b)
  keys, hash = extract(b.binding, ast)
  put_in_f(hash, keys, 150)
end

users = {
  ujihisa: {
    email: 'a@a.a',
    wallet: {
      money: 120,
    },
  },
}
users2 = put_in(-> { users[:ujihisa][:wallet][:money] }, 150)
# this put_in behaves like
#   users.merge(ujihisa: users[:ujihisa].merge(wallet: users[:ujihisa][:wallet].merge(money: 150)))
# to create an updated version of the given hash
p users
p users2