Re: lua-l@...

[Daurnimator <quae@...>]

On 7 May 2016 at 17:58, Satoru Kawahara <s1311350@coins.tsukuba.ac.jp> wrote:
> Hello,
>
> Below code can be compiled but says an error "attempt to call a nil value"
>
> ```
> print"hello"
>
> (function() print"world" end)()
> ```
>
> luac creates this instructions:
>
> ```
> main <foo.lua:0,0> (7 instructions at 0x6a5a50)
> 0+ params, 2 slots, 1 upvalue, 0 locals, 2 constants, 1 function
>         1       [1]     GETTABUP        0 0 -1  ; _ENV "print"
>         2       [1]     LOADK           1 -2    ; "hello"
>         3       [1]     CALL            0 2 2
>         4       [3]     CLOSURE         1 0     ; 0x6a5d30
>         5       [1]     CALL            0 2 2
>         6       [1]     CALL            0 1 1
>         7       [3]     RETURN          0 1
> constants (2) for 0x6a5a50:
>         1       "print"
>         2       "hello"
> locals (0) for 0x6a5a50:
> upvalues (1) for 0x6a5a50:
>         0       _ENV    1       0
>
> function <foo.lua:3,3> (4 instructions at 0x6a5d30)
> 0 params, 2 slots, 1 upvalue, 0 locals, 2 constants, 0 functions
>         1       [3]     GETTABUP        0 0 -1  ; _ENV "print"
>         2       [3]     LOADK           1 -2    ; "world"
>         3       [3]     CALL            0 2 1
>         4       [3]     RETURN          0 1
> constants (2) for 0x6a5d30:
>         1       "print"
>         2       "world"
> locals (0) for 0x6a5d30:
> upvalues (1) for 0x6a5d30:
>         0       _ENV    0       0
>
> ```
>
> Look at the 4th and 5th instructions of main chunk.
> At 4th instruction, it registers the annonymous function to register 1, and
> 5th instruction calls a function from register 0, which is empty.
> That is why the code says "attempt to call a nil value".
>
> Generally speaking, the code makes a invalid instruction like this:
>
> ```
> funcall()
> (exp)
> ```
>
> This code can't be compiled with lua5.1, but CAN be compiled with 5.2, 5.3
>
> It is not a problem that `exp`  is function definition or variables (, even
> if the type is not function), but it can write number, string, and so on.
> I think it should not be compiled.
>
> And invalid instruction insertion IS a bug.
>
> --
> Kawahara Satoru
> College of Information Science
> University of Tsukuba

I do not see any issue here.
newlines are insignificant in lua. your example is the same as:

print("hello")(function() print"world" end)()

i.e.
1. call print with "hello",
2. call the result of that with an argument of a function
3. call the result of that with no arguments.

As the result of print is nothing; at step 2 you attempt to call
`nil`, and hence get the (correct) error message.