Question

The HsLua examples on the Haskell Wiki are broken (dostring and dofile are not defined). Looks like the API has changed since the examples were written.

However I have been trying to modify the examples to match the current API and not getting much success. Here’s a program that should really work!

main = do
    l <- Lua.newstate
    Lua.openlibs l
    Lua.loadfile l "configfile.lua"
    [name,pwd] <- Lua.callfunc l "getuserpwd" "mail.google.com"
    putStrLn name
    Lua.close l

However, this doesn’t even compile, giving me this strange error message –

No instance for (Lua.StackValue [String])
  arising from a use of `Lua.callfunc'
Possible fix:
  add an instance declaration for (Lua.StackValue [String])
In a stmt of a 'do' expression:
    [name, pwd] <- Lua.callfunc l "getuserpwd" "mail.google.com"
In the expression:
  do { l <- Lua.newstate;
       Lua.openlibs l;
       Lua.loadfile l "configfile.lua";
       [name, pwd] <- Lua.callfunc l "getuserpwd" "mail.google.com";
       .... }
In an equation for `main':
    main
      = do { l <- Lua.newstate;
             Lua.openlibs l;
             Lua.loadfile l "configfile.lua";
             .... }

While the content of the file configfile.lua probably doesn’t matter (because the haskell code doesn’t even compile), it is as below (same as on the wiki page) –

function getuserpwd (site)
  local cookies = { ["www.ibm.com"] = {"joe", "secret"}
                  , ["www.sun.com"] = {"hoe", "another secret"}
                  }
  if cookies[site] then
    return cookies[site]
  elseif site:match("[.]google[.]com$") then
    return {"boss", "boss"}
  else
    return { os.getenv("USER") or "God"
           , os.getenv("PWD")  or "dontdisturb" }
  end
end

Could someone please provide me with a working example of Haskell->Lua and Lua->Haskell calls?

Edit

Okay I changed the type of the return value to a String (from the earlier array of String), and that program does compile! However it now fails at runtime. Here’s the modified program –

main = do
    l <- Lua.newstate
    Lua.openlibs l
    Lua.loadfile l "configfile.lua"
    Lua.callfunc l "getuserpwd" "mail.google.com" >>= putStrLn
    Lua.close l

And here’s configfile.lua –

function getuserpwd (site)
  return "boss"
end

And the runtime error message is as follows –

**** Exception: user error (attempt to call a nil value)

Answer 1

You have to execute loaded Lua chunk prior to calling any functions:

main = do
    l <- Lua.newstate
    Lua.openlibs l
    Lua.loadfile l "configfile.lua"
    Lua.call l 0 0
    Lua.callfunc l "getuserpwd" "mail.google.com" >>= putStrLn
    Lua.close l

Method dofile was a wrapper for loadfile and call, I don’t know reasons for removing it.

Edit.
This code calls function that returns table and iterates over it. It is based on this traversal example. I am not sure how to do it via callfunc.

import qualified Scripting.Lua as Lua
import Control.Monad.Loops
import Data.Maybe

print_table l = do
    Lua.pushnil l
    whileM_ (Lua.next l 1) (Lua.tostring l (-1) >>= putStrLn >> Lua.pop l 1)

main = do
  l <- Lua.newstate
  Lua.openlibs l
  Lua.loadfile l "configfile.lua"
  Lua.call l 0 0
  Lua.getglobal l "getuserpwd"
  Lua.pushstring l "mail.google.com"
  Lua.call l 1 (-1) -- calls function without Haskell extensions
  print_table l
  Lua.close l

It turns out that HsLua implementation is a very simple wrapper and there are no suitable Haskell bindings for Lua tables.