xpcall

ok, result = xpcall (f, err)

Calls function f with err as the custom error handler.

If an error occurs in f it is caught and the error-handler 'err' is called. Then xpcall returns false, and whatever the error handler returned.

If there is no error in f, then xpcall returns true, followed by the function results from f.

Note that the supplied error function is called before the stack is unwound (in case of error) so this is a good time to find what functions were on the stack leading up to the error. In the example below we use the debug.traceback function, which shows a stack trace as at the time of the error.

function f ()
  return "a" + 2  -- will cause error
end -- f

function err (x)
  print ("err called", x)
  return "oh no!"
end -- err

print (xpcall (f, err))

 -->
 
err called [string "Immediate"]:2: attempt to perform arithmetic on a string value
false oh no!

function f2 ()
  return 2 + 2
end -- f

print (xpcall (f, err))  --> true 4

function f ()
return "a" + 2
end -- f

print (xpcall (f, debug.traceback))

 -->

false   stdin:2: attempt to perform arithmetic on a string value
stack traceback:
        stdin:2: in function `f'
        [C]: in function `xpcall'
        stdin:1: in main chunk
        [C]: ?