Bindings, functions, and utilities enabling the integration of a lua interpreter into a haskell project.

We are trying to keep error handling on the haskell side very simple and intuitive. However, when combined with error handling on the Lua side, it get's tricky: We can call Haskell from Lua which calls Lua again etc. At each language boundary we should check for errors and propagate them properly to the next level in stack. Hslua does this for you when returning from Lua to Haskell, but care must be taken when passing errors back into Lua.

Let's say we have this call stack: (stack grows upwards)

Haskell function
Lua function
Haskell program

and we want to report an error in the top-most Haskell function. We can't use lua_error from the Lua C API, because it uses longjmp, which means it skips layers of abstractions, including the Haskell RTS. There's no way to prevent this longjmp. lua_pcall sets the jump target, but even with lua_pcall it's not safe. Consider this call stack:

Haskell function which calls lua_error
Lua function, uses pcall
Haskell program

This program jumps to Lua function, skipping Haskell RTS code that would run before Haskell function returns. For this reason we can use lua_pcall (pcall) only for catching errors from Lua, and even in that case we need to make sure there are no Haskell calls between error-throwing Lua call and our pcall call.

To be able to catch errors from Haskell functions in Lua, we need to find a convention. Currently hslua does this: lerror has same type as Lua's lua_error, but instead of calling real lua_error, it's returning two values: A special value _HASKELLERR and error message as a string.

Using this, we can write a function to catch errors from Haskell like this:

function catch_haskell(ret, err_msg)
    if ret == _HASKELLERR then
      print("Error caught from Haskell land: " .. err_msg)
      return
    end
    return ret
end

(_HASKELLERR is created by newstate)

(Type errors in Haskell functions are also handled using this convention. E.g., if you pass a Lua value with the wrong type to a Haskell function, the error will be reported in this way)

At this point our call stack is like this:

Lua function (Haskell function returned with error, which we caught)
Haskell program

If we further want to propagate the error message to Haskell program, we we can just use standard error function and use pcall in Haskell side. Note that if we use error on the Lua side and forget to use pcall in the calling Haskell function, we would be starting to skip layers of abstractions and would get a segfault in the best case. That's why hslua wraps all API functions that can potentially fail in custom C functions. Those functions behave idential to the functions they wrap, but catch all errors and return error codes instead. Using error within Lua should hence be safe.

However, the raw C API bindings in RawBindings don't provide these guarantees. Even an apparently harmless operations like accessing a field via lua_getfield can call a meta method and trigger a longjmp, causing the host program to crash.

The pcall function is not wrapped in additional C code but still safe. The reason it's safe is because the lua_pcall C function is calling the Lua function using Lua C API, and when the called Lua function calls error it longjmps to lua_pcall C function, without skipping any layers of abstraction. lua_pcall then returns to Haskell.

NOTE: If you're loading a hslua program compiled to a dynamic library from a Lua program, you need to define _HASKELLERR = {} manually, after creating the Lua state.