load function fails with nil args

[technomancy]

This code, as you might expect, prints "2":

print(load("return (1 + 1)")())

However, this code fails, even though it should work exactly the same:

print(load("return (1 + 1)", nil)())
{"init terminating in do_boot",{{lua_error,{badarg,load,[<<"return (1 + 1)">>,nil]},{luerl,{array,13,100,undefined,{{{table,{array,0,10,nil,10},{{{{empty,<<"_G">>,{tref,0},empty,<<"_VERSION">>,<<"Lua 5.2">>,empty},<<"assert">>,{function,#Fun<luerl_lib_basic.1.62990734>},{empty,<<"bit32">>,{tref,5},empty},<<"collectgarbage">>,{function,#Fun<luerl_lib_basic.2.62990734>},{empty,<<"debug">>,{tref,12},empty}},<<"dofile">>,{function,#Fun<luerl_lib_basic.3.62990734>},{{empty,<<"eprint">>,{function,#Fun<luerl_lib_basic.4.62990734>},empty},<<"error">>,{function,#Fun<luerl_lib_basic.0.62990734>},{empty,<<"getmetatable">>,{function,#Fun<luerl_lib_basic.5.62990734>},empty,<<"io">>,{tref,6},empty}}},<<"ipairs">>,{function,#Fun<luerl_lib_basic.6.62990734>},{{{empty,<<"load">>,{function,#Fun<luerl_lib_basic.7.62990734>},empty},<<"loadfile">>,{function,#Fun<luerl_lib_basic.8.62990734>},{empty,<<"loadstring">>,{function,#Fun<luerl_lib_basic.9.62990734>},empty,<<"math">>,{tref,7},empty}},<<"next">>,{function,#Fun<luerl_lib_basic.10.62990734>},{{empty,<<"os">>,{tref,8},empty},<<"package">>,{tref,4},{empty,<<"pairs">>,{function,#Fun<luerl_lib_basic.11.62990734>},empty},<<"pcall">>,{function,#Fun<luerl_lib_basic.12.62990734>},{empty,<<"print">>,{function,#Fun<luerl_lib_basic.13.62990734>},empty}}},<<"rawequal">>,{function,#Fun<luerl_lib_basic.14.62990734>},{{{empty,<<"rawget">>,{function,#Fun<luerl_lib_basic.15.62990734>},empty},<<"rawlen">>,{function,#Fun<luerl_lib_basic.16.62990734>},{empty,<<"rawset">>,{function,#Fun<luerl_lib_basic.17.62990734>},empty,<<"require">>,{function,#Fun<luerl_lib_package.0.46355753>},empty}},<<"select">>,{function,#Fun<luerl_lib_basic.18.62990734>},{{empty,<<"setmetatable">>,{function,#Fun<luerl_lib_basic.19.62990734>},empty},<<"string">>,{tref,9},{empty,<<"table">>,{tref,11},empty}},<<"tonumber">>,{function,#Fun<luerl_lib_basic.20.62990734>},{{empty,<<"tostring">>,{function,#Fun<luerl_lib_basic.21.62990734>},empty},<<"type">>,{function,#Fun<luerl_lib_basic.22.62990734>},{empty,<<"unpack">>,{function,#Fun<luerl_lib_basic.23.62990734>},empty}}}},nil},{table,{array,3,10,nil,{nil,{function,#Fun<luerl_lib_package.2.46355753>},{function,#Fun<luerl_lib_package.3.46355753>},nil,nil,nil,nil,nil,nil,nil}},empty,nil},{table,{array,0,10,nil,10},{{{empty,<<"_G">>,{tref,0},empty},<<"bit32">>,{tref,5},{empty,<<"debug">>,{tref,12},empty},<<"io">>,{tref,6},{empty,<<"math">>,{tref,7},empty}},<<"os">>,{tref,8},{{empty,<<"package">>,{tref,4},empty},<<"string">>,{tref,9},{empty,<<"table">>,{tref,11},empty}}},nil},{table,{array,0,10,nil,10},empty,nil},{table,{array,0,10,nil,10},{{empty,<<"config">>,<<47,10,59,10,63,10,33,10,45,10>>,empty},<<"loaded">>,{tref,2},{empty,<<"path">>,<<"./?.lua;./?/init.lua">>,empty},<<"preload">>,{tref,3},{empty,<<"searchers">>,{tref,1},empty,<<"searchpath">>,{function,#Fun<luerl_lib_package.1.46355753>},empty}},nil},{table,{array,0,10,nil,10},{{{empty,<<"arshift">>,{function,#Fun<luerl_lib_bit32.7.29927074>},empty,<<"band">>,{function,#Fun<luerl_lib_bit32.0.29927074>},empty},<<"bnot">>,{function,#Fun<luerl_lib_bit32.1.29927074>},{empty,<<"bor">>,{function,#Fun<luerl_lib_bit32.2.29927074>},empty}},<<"btest">>,{function,#Fun<luerl_lib_bit32.3.29927074>},{{empty,<<"bxor">>,{function,#Fun<luerl_lib_bit32.4.29927074>},empty},<<"extract">>,{function,#Fun<luerl_lib_bit32.10.29927074>},{empty,<<"lrotate">>,{function,#Fun<luerl_lib_bit32.8.29927074>},empty}},<<"lshift">>,{function,#Fun<luerl_lib_bit32.5.29927074>},{{empty,<<"replace">>,{function,#Fun<luerl_lib_bit32.11.29927074>},empty},<<"rrotate">>,{function,#Fun<luerl_lib_bit32.9.29927074>},{empty,<<"rshift">>,{function,#Fun<luerl_lib_bit32.6.29927074>},empty}}},nil},{table,{array,0,10,nil,10},{empty,<<"flush">>,{function,#Fun<luerl_lib_io.0.101991831>},empty,<<"write">>,{function,#Fun<luerl_lib_io.1.101991831>},empty},nil},{table,{array,0,10,nil,10},{{{{empty,<<"abs">>,{function,#Fun<luerl_lib_math.2.97665772>},empty},<<"acos">>,{function,#Fun<luerl_lib_math.3.97665772>},{empty,<<"asin">>,{function,#Fun<luerl_lib_math.4.97665772>},empty}},<<"atan">>,{function,#Fun<luerl_lib_math.5.97665772>},{{empty,<<"atan2">>,{function,#Fun<luerl_lib_math.6.97665772>},empty},<<"ceil">>,{function,#Fun<luerl_lib_math.7.97665772>},{empty,<<"cos">>,{function,#Fun<luerl_lib_math.8.97665772>},empty}}},<<"cosh">>,{function,#Fun<luerl_lib_math.9.97665772>},{{{empty,<<"deg">>,{function,#Fun<luerl_lib_math.10.97665772>},empty},<<"exp">>,{function,#Fun<luerl_lib_math.11.97665772>},{empty,<<"floor">>,{function,#Fun<luerl_lib_math.12.97665772>},empty}},<<"fmod">>,{function,#Fun<luerl_lib_math.13.97665772>},{{empty,<<"frexp">>,{function,#Fun<luerl_lib_math.14.97665772>},empty},<<"huge">>,1.797693e+308,{empty,<<"ldexp">>,{function,#Fun<luerl_lib_math.15.97665772>},empty}}},<<"log">>,{function,#Fun<luerl_lib_math.16.97665772>},{{{empty,<<"log10">>,{function,#Fun<luerl_lib_math.17.97665772>},empty},<<"max">>,{function,#Fun<luerl_lib_math.0.97665772>},{empty,<<"min">>,{function,#Fun<luerl_lib_math.1.97665772>},empty}},<<"modf">>,{function,#Fun<luerl_lib_math.18.97665772>},{{empty,<<"pi">>,3.141593e+00,empty},<<"pow">>,{function,#Fun<luerl_lib_math.19.97665772>},{empty,<<"rad">>,{function,#Fun<luerl_lib_math.20.97665772>},empty}},<<"random">>,{function,#Fun<luerl_lib_math.21.97665772>},{{empty,<<"randomseed">>,{function,#Fun<luerl_lib_math.22.97665772>},empty},<<"sin">>,{function,#Fun<luerl_lib_math.23.97665772>},{empty,<<"sinh">>,{function,#Fun<luerl_lib_math.24.97665772>},empty},<<"sqrt">>,{function,#Fun<luerl_lib_math.25.97665772>},{empty,<<"tan">>,{function,#Fun<luerl_lib_math.26.97665772>},empty,<<"tanh">>,{function,#Fun<luerl_lib_math.27.97665772>},empty}}}},nil},{table,{array,0,10,nil,10},{{empty,<<"clock">>,{function,#Fun<luerl_lib_os.0.64788999>},empty},<<"date">>,{function,#Fun<luerl_lib_os.1.64788999>},{empty,<<"difftime">>,{function,#Fun<luerl_lib_os.2.64788999>},empty},<<"getenv">>,{function,#Fun<luerl_lib_os.3.64788999>},{empty,<<"time">>,{function,#Fun<luerl_lib_os.4.64788999>},empty}},nil},{table,{array,0,10,nil,10},{{{empty,<<"byte">>,{function,#Fun<luerl_lib_string.0.23292798>},empty},<<"char">>,{function,#Fun<luerl_lib_string.1.23292798>},{empty,<<"dump">>,{function,#Fun<luerl_lib_string.2.23292798>},empty}},<<"find">>,{function,#Fun<luerl_lib_string.3.23292798>},{{empty,<<"format">>,{function,#Fun<luerl_lib_string.4.23292798>},empty},<<"gmatch">>,{function,#Fun<luerl_lib_string.5.23292798>},{empty,<<"gsub">>,{function,#Fun<luerl_lib_string.6.23292798>},empty}},<<"len">>,{function,#Fun<luerl_lib_string.7.23292798>},{{empty,<<"lower">>,{function,#Fun<luerl_lib_string.8.23292798>},empty},<<"match">>,{function,#Fun<luerl_lib_string.9.23292798>},{empty,<<"rep">>,{function,#Fun<luerl_lib_string.10.23292798>},empty},<<"reverse">>,{function,#Fun<luerl_lib_string.11.23292798>},{empty,<<"sub">>,{function,#Fun<luerl_lib_string.12.23292798>},empty,<<"upper">>,{function,#Fun<luerl_lib_string.13.23292798>},empty}}},nil}},{{table,{array,0,10,nil,10},{empty,<<"__index">>,{tref,9},empty},nil},{table,{array,0,10,nil,10},{{empty,<<"concat">>,{function,#Fun<luerl_lib_table.0.83559307>},empty},<<"insert">>,{function,#Fun<luerl_lib_table.1.83559307>},{empty,<<"pack">>,{function,#Fun<luerl_lib_table.2.83559307>},empty},<<"remove">>,{function,#Fun<luerl_lib_table.3.83559307>},{empty,<<"sort">>,{function,#Fun<luerl_lib_table.4.83559307>},empty,<<"unpack">>,{function,#Fun<luerl_lib_table.5.83559307>},empty}},nil},{table,{array,0,10,nil,10},{{empty,<<"getmetatable">>,{function,#Fun<luerl_lib_debug.0.33065617>},empty},<<"getuservalue">>,{function,#Fun<luerl_lib_debug.1.33065617>},{empty,<<"setmetatable">>,{function,#Fun<luerl_lib_debug.2.33065617>},empty,<<"setuservalue">>,{function,#Fun<luerl_lib_debug.3.33065617>},empty}},nil},undefined,undefined,undefined,undefined,undefined,undefined,undefined},10,10,10,10,10,10,10,10,10}},[],13,{array,0,10,undefined,10},[],0,{tref,0},[{call_frame,[{[]}],[]},{function,#Fun<luerl_lib_basic.13.62990734>}],{meta,nil,nil,nil,{tref,10}},#Ref<0.0.2.79>}},[{luerl_lib,lua_error,2,[{file,"src/luerl_lib.erl"},{line,43}]},{luerl_emul,functioncall,4,[{file,"src/luerl_emul.erl"},{line,838}]},{luerl_emul,functioncall,7,[{file,"src/luerl_emul.erl"},{line,741}]},{luerl_emul,functioncall,5,[{file,"src/luerl_emul.erl"},{line,851}]},{luerl_emul,functioncall,4,[{file,"src/luerl_emul.erl"},{line,826}]},{luerl_emul,functioncall,3,[{file,"src/luerl_emul.erl"},{line,731}]},{luerl_emul,call,3,[{file,"src/luerl_emul.erl"},{line,392}]},{init,start_em,1,[]}]}}

[rvirding]

How were you calling this?

[technomancy]

Great question! I'm actually using luerl with Fennel, which is a small compiler which targets Lua: https://github.com/bakpakin/Fennel/blob/master/fennel.lua#L64

I have been investigating some of the less mainstream Lua implementations to see if I could uncover any bugs with Fennel. But I believe Erlang semantics might be leaking here--Lua semantics would dictate that there is basically no way for a function to tell the difference between being called like f(1) vs f(1, nil, nil, nil); unless f calls select on ... these two calls should be equivalent.

That appears to not be the case here, at least with load but possibly with other functions as well?

[rvirding]

So load can handle multiple arguments but it is strict on the types of the first 3 arguments. If they are given they must be strings otherwise you get an argument error. The 4 can be anything. Currently only the first argument is used, the others are ignored. This needs some more work. The question is how to interpret nil as an argument here. Is calling load("return 1+1", nil, "bt") legal? The manual doesn't say.

This should only be a problem with functions in the standard libraries and not in functions defined in Lua.

[technomancy]

If they are given they must be strings otherwise you get an argument error

I think this is where the problem lies with this statement. In Erlang it's impossible to have the 3rd argument be provided if the second isn't, but in Lua this is a perfectly normal occurrence. Lua is different from most lisps in that nil actually consistently means the absence of a value, so "when this value is not given" means when it's nil. I think the lower-arity versions just need to be wrappers that call the max-arity versions filling out the args with nils, and the max-arity just needs to expect that any optional argument can be nil.

Is calling load("return 1+1", nil, "bt") legal? The manual doesn't say.

Depending on your interpretation of the phrasing in the manual, I could see how this could be considered technically undefined, but of the 4 other Lua implementations I've tried they have all supported it.

[rvirding]

How would it be interpreted? One way would be that all the nil arguments get their default value. Would this be a reasonable interpretation? Would it apply everywhere?

[technomancy]

One way would be that all the nil arguments get their default value.

Oh, yes that's exactly what I meant; sorry, I realize I wasn't clear about that. It should be a universal rule for Lua functions with optional arguments; I can't think of any exceptions.

[rvirding]

I will have a go and try to fix it. There is one limitation with load which is a bit more difficult to fix and that is that it currently only accepts a file name as first and not a function. This is more difficult to fix as calling lua functions from within erlang is a bit more tricky. We shall see if I can fix that as well.

[rvirding]

I will gradually upgrade the internal functions. If you come across where an explicit nil as an argument does not mean use the default then let me know.

[technomancy]

That sounds good; thanks!