Lua Errors

Reading Lua Error Messages

Most errors are shown in Output Window in Roblox Studio or Tools. Understanding them can make debugging efforts much more focused.

Compile Errors

Wed Apr 28 21:10:21 2010 - Workspace.Script:3: 'end' expected (to close 'while' at line 1) near '<eof>'

Let's break the message down in parts:

• "Wed Apr 28 21:10:21 2010" - time of the message
• "Workspace.Script" - Full name of the script
• "3" - the line with the error
• "'end' expected (to close 'while' at line 1) near '<eof>'" - the error message

Important: Output window doesn't always show these errors (see Debugging)

Runtime Errors

Wed Apr 28 21:11:56 2010 - Workspace.Script:7: attempt to perform arithmetic on global 'a' (a function value)
Wed Apr 28 21:11:56 2010 - Workspace.Script, line 7 - global b Workspace.Script, line 2 - global a Workspace.Script, line 14 stack end 

The first part is the same as a compile time error. The second part is called a stack trace.

• "Workspace.Script, line 7" - the source of the error (inside function b)
• "- global b Workspace.Script, line 2" - call to function b (from function a)
• "- global a Workspace.Script, line 14" - call to function a (from main script)
• "stack end" - end of the stack trace

Advanced note: Stack trace doesn't include calls to functions that use a tail call.

Basic Lua Errors

Thu May 06 20:15:26 2010 - Workspace.Script:2: attempt to index global 'a' (a function value)
Thu May 06 20:15:55 2010 - Workspace.Script:2: attempt to perform arithmetic on local 'a' (a string value)
Thu May 06 20:17:01 2010 - Workspace.Script:2: attempt to concatenate upvalue 'a' (a nil value)
Thu May 06 20:17:03 2010 - Workspace.Script:2: attempt to call field 'a' (a number value)

Many Lua "operators" give similar error messages:

• "Workspace.Script:2:" - The source of the error
• "attempt to ___" - The error is caused by the given operation on the wrong type of variable

Operation Name	Operator	Allowed Types
index	. or [___]	table, string
perform arithmetic	various	number, string*
concatenate	..	string, number
call	(___)	function

• • Note: String can only be used in arithmetic if it can be converted to a number.
  • Advanced Note: Metatables can be used to allow tables or userdata perform any of these operations.
• "___ 'a'" - The given type of a variable named 'a' (a will be the actual name in the code)
  • NOTE: that a field name itself can be a variable in which case the message says '?'
• "(a ___ value)" - Gives the actual type of the variable

Confusing Error Messages

Wed Apr 28 21:10:30 2010 - Workspace.Script:3: 'end' expected (to close 'while' at line 1) near '<eof>'

The while on line 1 is missing an end. You need an end for each of the following:

• function
• do
• if (but not elseif)

Wed Apr 28 21:10:30 2010 - Workspace.Script:4: '<eof>' expected near 'end'

The end on line 4 is extra.

Wed Apr 28 21:34:08 2010 - Workspace.Script:1: 'then' expected near '='

This is caused by using '=' (assignment) in an if instead of '==' (comparsion)

Wed Apr 28 21:36:33 2010 - Workspace.Script:2: '=' expected near 'if'
Wed Apr 28 21:38:42 2010 - Workspace.Script:2: unexpected symbol near 'if'

Errors on the beginning of a line are often because an incomplete previous line.

Wed Apr 28 21:36:33 2010 - Workspace.Script:3: '=' expected near '=='

This is caused by using '==' (comparison) instead of '=' assignment.

Thu May 06 20:49:19 2010 - Workspace.Script:1: malformed number near '1..'

This is caused by attempting to concatenate a number without putting a space since dots can be part of a number:

print( 1.." tests passed" )

instead of

print( 1 .." tests passed" )

Thu May 06 20:55:43 2010 - Workspace.Script:1: bad argument #3 to '?' (Object expected, got function)

This error was caused by assigning the wrong type of value to a property:

Workspace.Part.Parent = print

• Advanced Note: The message comes from the __newindex(obj,ndx,val) function in the Instance metatable.

Similarly, you can get

Thu May 06 20:59:49 2010 - Workspace.Script:1: bad argument #2 to '?' (string expected, got function)

By indexing an object with the wrong type:

print( Workspace.Part[print] )

Thu May 06 21:02:18 2010 - Part is not a valid member of Workspace

Aside from the obvious, this message also occurs if you attempt to set a child directly:

print( Workspace.Part1 ) -- ok
Workspace.Part1 = Instance.new("Part") -- error

You need to set the Name and Parent properties instead:

local part = Instance.new("Part")
part.Name = "Part1"
part.Parent = Workspace

• Advanced Note: Why does this not have a source line?

Tricky Mistakes

Floating point calculations can be surprising; it's safer to use inequalities. Or use an integer for controlling the loop.

n = 0
while true do
  n = n + 0.1
  if( n == 0.9 ) then break end -- never happens
end

Just like the number 1/3 would be 0.3333... (repeating) in decimal. The number 1/10 is 0.00011001100... (repeating) in binary. Exact binary values are powers of 2, like 1/2, 1/4, 1/8, 1/16 etc. And multiples of those.

Since a computer must stop at a certain number of digits, the (repeating) idea is lost. If you add up 3 * 1/3, you get 0.999 instead of 1 or 10 * 1/10 in the computer is very close to 1 but not exactly. (In fact, it's so close to 1 that if you print it, it will say "1", but if you compare it with == or subtract it from one you'll see a slight difference).

part.Transparency = 0.1
if( part.Transparency == 0.1 ) then -- false
--
end

In this case, Transparency property of Roblox stores less digits than Lua uses.

Example

Lua says 1/3 is 0.333333, but Transparency only holds 3 digits so it is set to 0.333.

When Lua compares 0.333 to 0.333333 they are not equal.