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Lua tool for checking function arguments against specifications parsed from comments
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argcheck - Function argument checking for Lua

1. What?

argcheck is a Lua module that checks function arguments conform to a specification. It's more of a proof of concept at the moment than a fully-developed tool.

argcheck is brought to you by Incremental ( and is available under the MIT Licence.

2. How?

Install with either sudo make install (no need to make) or luarocks install argcheck (which doesn't actually work yet, since I haven't submitted the rock).

Use it with lua -largcheck file.lua, or, if you want a warning rather than an error on a failed argument check, lua -largwarn file.lua.

Argument specifications are parsed from comments near the function declaration in the code.

These can be quite simple comments:

-- string
-- number
function printn(s, n)
  for i = 1, n do print(s) end

printn(10, "hello")

$ lua -largcheck test/simple.lua 
lua: test/simple.lua:7: bad argument #1 to 'printn' (string expected, got number '10')

Or LuaDoc-like comments:

--- prints the string s n times
-- @tparam number n how many times to print the string
-- @tparam string s the string to print
function printn(s, n)
  for i = 1, n do print(s) end

printn(10, "hello")

$ lua -largcheck test/ldoc.lua 
lua: test/ldoc.lua:8: bad argument #1 to 'printn' (string expected, got number '10')

Note that if you name the arguments in LuaDoc comments, they don't have to be in the right order.

Or comments inline with the function arguments:

function printn(
  s,  -- string: a string
  n)  -- number: a number
  for i = 1, n do print(s) end

printn(10, "hello")

$ lua -largcheck test/simple_arg.lua 
lua: test/simple_arg.lua:7: bad argument #1 to 'printn' (string expected, got number '10')

In this case, if you want to add a description after the specification, you'll need to finish the specification with a colon. This is so you can write:

function foo(
  a) -- I'd like to comment on this parameter, but not check its type

Without the colon rule, argcheck would complain than a isn't of type I'd.

You can specify more than one constraint or optional constraints using the same syntax as [this] (

-- string|number
-- ?number
function printn(s, n)
  n = n or 2
  for i = 1, n do print(tostring(s)) end

printn({}, "hello")

$ lua -largwarn test/optional.lua 
bad argument #1 to 'printn' (string or number expected, got table 'table ...')
bad argument #2 to 'printn' (nil or number expected, got string 'hello')

Constraints can be:

  • any of the 8 Lua types (nil, boolean, number, string, function, userdata, thread, table)

  • literal strings in double or single quotes (useful with |)

  • literal numbers (again, useful with |)

  • number ranges in the form 1..3. If neither number contains a decimal point or an e or E (as in 10e-3), it's assumed you also want an integer

  • Names of functions in an internal table you can't change yet. So far the only functions in the table are:

    • integer: the value is a number and an integer
    • anything: the value is not nil
    • file: the value is an open file
      • filein: the value is an open, readable file
      • fileout: the value is an open, writable file
  • If none of the above, and the value is table, argcheck will try to match the table's metatable to the constraint.

    • if the metatable has a __type, __typename or _type field that matches the constraint
    • if the metatable has a __typeinfo field such that mt.__typeinfo[constraint] is true
    • if the metatable has a __type field that's a function and mt.__type(value) == constraint
    • if there's a _G[constraint] or _ENV[constraint] that is the same as the metatable
    • if there's an entry in the registry such that debug.getregistry[constraint] == mt
    • if there's an upvalue whose name matches the constraint and which is the same as the metatable (this only works if the correct metatable is accessed in the function being checked)

    If there is a match, then the correct metatable for the constraint is stored in a cache for quicker subsequent access.

3. Requirements

Lua (5.1 or 5.2) or LuaJIT.

4. Issues


5. Wishlist

  • More comprehensive condition checking
  • Pluggable check functions
  • Pluggable object type checkers
  • Compile the checks to a function

6. Alternatives


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