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
Sunday Nov 24 02:42:56 2024 - Workspace.Script:3: 'end' expected (to close 'while' at line 1) near '<eof>'
Let's break the message down in parts:
- "Wed Dec 09 12:34:56 2009" - 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
Sunday Nov 24 02:42:56 2024 - Workspace.Script:7: attempt to perform arithmetic on global 'a' (a function value) Wed Dec 09 12:34:56 2009 - 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
Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: attempt to index global 'a' (a function value) Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: attempt to perform arithmetic on local 'a' (a string value) Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: attempt to concatenate upvalue 'a' (a nil value) Sunday Nov 24 02:42:56 2024 - 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
Sunday Nov 24 02:42:56 2024 - 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)
Sunday Nov 24 02:42:56 2024 - Workspace.Script:4: '<eof>' expected near 'end'
The end on line 4 is extra.
Sunday Nov 24 02:42:56 2024 - Workspace.Script:1: 'then' expected near '='
This is caused by using '=' (assignment) in an if instead of '==' (comparsion)
Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: '=' expected near 'if' Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: unexpected symbol near 'if' Sunday Nov 24 02:42:56 2024 - Workspace.Script:2: function arguments expected near 'if'
Errors on the beginning of a line are often because an incomplete previous line. In this case, the problems are:
Game.GetService -- missing parentheses () Game.GetService( -- missing close parenthesis ) Game:GetService -- missing parentheses () or "function arguments"
Sunday Nov 24 02:42:56 2024 - Workspace.Script:3: '=' expected near '=='
This is caused by using '==' (comparison) instead of '=' assignment.
Sunday Nov 24 02:42:56 2024 - 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" )
Sunday Nov 24 02:42:56 2024 - 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
Sunday Nov 24 02:42:56 2024 - Workspace.Script:1: bad argument #2 to '?' (string expected, got function)
By indexing an object with the wrong type:
print( Workspace.Part[print] )
Sunday Nov 24 02:42:56 2024 - Part1 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?
Unusual Error Messages
Sunday Nov 24 02:42:56 2024 - maximum event re-entrancy depth exceeded
This is caused by too many events triggering each other. A simple example is this script:
local bv = Instance.new("BoolValue")
bv.Parent = script
bv.Value = false
bv.Changed:connect( function() bv.Value = not bv.Value end )
bv.Value = true
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.
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.
