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warn, error and print functions in Lua

Lua has some useful built-in functions for debugging and error handling: print, warn, and error. Here are some tricks to using them.

print

Never underestimate the power of print function in debugging!

print is one of the simplest ways to debug a program and print the intermediate results. The function prints a list of tab-separated arguments as strings to stdout and adds the line-ending symbol to the end of the output.

Consider:

print() --> prints empty line
print("") --> prints empty line
print("Hello") --> Hello
print("Hello", " ", "World", 42) --> Hello  World   42 (tab separated)

Complex data structures, such as tables, cannot be printed using the print function. For the complex data structures, it prints the address in the computer’s memory. To print out a table, you need to use loops, 3rd party libraries, or your homemade solutions. Lua is supposed to be as minimal as possible, and there is no magic function like console.log() in JavaScript.

Consider:

local t = {name = "Jonh", age = 25}
print(t) --> table: 0x557cfeb01d70

local fn = function() return 1 + 1 end
print(fn) --> function: 0x557cfeb034a0

-- Simple one-level table print
for k, v in pairs(t) do
    print(k, v)
end
-- Outputs (order changes):
--  "name"  "John"
--  "age"   25

warn

Lua 5.4 introduces the warn function, which allows the user to output warning messages to stderr when warning output has been turned on.

Enable and disable warnings

You can enable or disable warnings inside your program’s code with @on or @off arguments.

Consider:

warn("Clean the room") --> empty output
warn("@on") --> Warnings are enabled
warn("Clean the room again") --> Lua warning: Clean the room again
warn("@off") -> Warnings are disabled
warn("Once again clean room") --> empty output

Another option to enable warnings’ emission in run a program with -W flag.

Consider test.lua:

-- Your amazing code here ...
warn("Clean the room")
-- Also your amazing code here ...

Shell/Bash:

lua -W test.lua
Lua warning: Clean the room

warn multiple arguments

All arguments should be strings and all string arguments are concatenated into one string.

Consider:

warn("@on")
warn("Foo", "Bar", "Xyzzy") --> Lua warning: FooBarXyzzy

error

error function raises the error and, by default, it stops the program execution. The function accepts 2 arguments: the message as a string and an error level number (0, 1, 2). The level argument specifies how to get the error position.

If error function is called in the main program, it prints the error and stops the program execution.

Examples:

Level 0:

error("Something is wrong", 0)

--[[
Something is wrong
stack traceback:
  [C]: in function 'error'
  stdin:1: in main chunk
  [C]: in ?
--]]

Level 1 (default):

error("Something is wrong")
--[[
stdin:1: Something is wrong
stack traceback:
  [C]: in function 'error'
  stdin:1: in main chunk
  [C]: in ?
--]]

Level 2:

f = function() error("Error from f()", 2) end
g = function() f() end
g()
--[[
stdin:1: Error from f()
stack traceback:
[C]: in function 'error'
stdin:1: in function 'f'
stdin:1: in function 'g'
(...tail calls...)
[C]: in ?
--]]

Catch errors with protected call

In Lua language, you can use pcall() function to catch errors. This is called a protected call. pcall() returns two values: the first is a boolean status which determines whether the call was successful or not, and the second value is a return string of an error message. When using protected call the program catches the error and continues to execute.

Failed call:

local cube = function(a)
  return a * a * "a"
end
local success, result = pcall(cube, 10)
print(success, result)
--> false test.lua:2: attempt to mul a 'number' with a 'string'

Artificially failed call:

local err = function()
  error("Sonething went wrong")
end
--> false test.lua:2: Sonething went wrong

Successful case:

local cube = function(a)
  return a * a * a
end
local success, result = pcall(cube, 10)
print(success, result)
--> true 1000

The common case might be like in this example:

local cube = function(a)
  return a * a * a
end
local success, result = pcall(cube, 10)

if (success) then --> 1000
  print(result)
else
  error(result) --> error message
end

There is also a function xpcall(). It is similar to pcall(), but also accepts the error handler, which might be defined as a common error handler in your program or project.

Consider:

local error_hander = function(e)
  return "Error handler says: " .. e
end
local cube = function(a)
  return a * a * a
end
local success, result = xpcall(cube, error_hander, "a")

if (success) then --> 1000
  print(result)
else
  error(result)
end

--[[ output
lua: test.lua:12: Error handler says: test.lua:5: attempt to mul a 'string' with a 'string'
stack traceback:
  [C]: in function 'error'
  test.lua:12: in main chunk
  [C]: in ?
--]]

References

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