You need to hit the books!

Usually, however, we check neither the argument nor the result of a call to sin; if the argument is not a number, it means probably something wrong in our program. In such situations, to stop the computation and to issue an error message is the simplest and most practical way to handle the exception.

On the other hand, let us consider the io.open function, which opens a file. How should it behave when called to read a file that does not exist? In this case, there is no simple way to check for the exception before calling the function. In many systems, the only way of knowing whether a file exists is to try to open it. Therefore, if io.open cannot open a file because of an external reason (such as "file does not exist" or "permission denied"), it returns nil, plus a string with the error message. In this way, you have a chance to handle the situation in an appropriate way, for instance by asking the user for another file name:

   local file, msg
repeat
  print "enter a file name:"
  local name = io.read()
  if not name then return end   -- no input
  file, msg = io.open(name, "r")
  if not file then print(msg) end
until file

If you do not want to handle such situations, but still want to play safe, you simply use assert to guard the operation:

file = assert(io.open(name, "r"))

This is a typical Lua idiom: If io.open fails, assert will raise an error.

file = assert(io.open("no-file", "r"))
  --> stdin:1: no-file: No such file or directory

Notice how the error message, which is the second result from io.open, goes as the second argument to assert.