Re: C API const correctness

[Roberto Ierusalimschy <roberto@...>]

> It would be nice if someday the C API was made const correct.  I'm referring
> to the lua_State argument of all the API functions.  If a function does not
> affect the state, the state pointer argument should be qualified as const.

The problem is that there is a difference between a "function that does 
not change the struct lua_State" and a "function that does not affect the 
state seen through the API". The `struct lua_State' being const in the eyes 
of the C compiler is too implementation dependent, and so it is not a good 
idea to export such details to the API. 

For instance, in the current implementation, the `lua_insert' function 
(that changes elements in the stack) could be declared const, while 
lua_getn (that only returns the size of an array) could not. (The first one 
changes only the stack, but not its size, so lua_State is constant. The 
second may need to create a string, an therefore can potentially resize 
some internal structures, changing lua_State). 

-- Roberto