Runtime importing of modules in GHC

[Andre Pang]

On Mon, May 27, 2002 at 10:57:18AM +0100, Simon Marlow wrote:

> > So, is there some wizardry out there which allows using the RTS
> > Linker to load up .o modules which are not created with the FFI?
> > I've been experimenting with it, and I just get segfaults if
> > I try to load the adder_closure, adder_entry, adder_fast1 or
> > adder_info symbols.
> 
> You *should* be able to just get the address of adder_closure and coerce
> it to the right type.  The difficulty with this is that lookupSymbol
> returns a Ptr, which is an unboxed type, whereas the function type is a
> boxed type.  Coercing unboxed types to boxed types is highly unsafe,
> because they have different representations on the stack.
> 
> However, GHCi does exactly this, and coerces from one to the other using
> addrToHValue# (see ByteCodeLink.lookupCE).  It's dangerous, but we'd
> know pretty quickly if it didn't work.

Welp, after fumbling in the dark for about 30 minutes, I think it
works:

    Adder2.hs:
    
	module Adder2 where

	adder :: Int -> Int
	adder n = n + 3

    Main.hs:

	loadFunction :: String -> IO (Int -> Int)
	loadFunction sym_to_find = do 
	  m <- lookupSymbol sym_to_find
	  case m of
	    Just (Ptr addr) -> case addrToHValue# addr of
			       (# hval #) -> return hval
	    Nothing -> error ("Couldn't find symbol name " ++ sym_to_find ++ ", please try again later")

        main = do
	         ...
	         adderFunction <- loadFunction (moduleName ++ "_adder_closure")
	         let result = adderFunction 6
	         putStr (show result ++ "\n")
		 ...

    22:34(0) .../project/runtime_import-2% ./a.out Adder2 
    Loaded Adder2
    Resolved Object Symbols: True
    9
    Yay!

Yay indeed :).  Thanks so much for your help, Simon.  Hopefully
it'll work with arbitrary data types next; that'll be my next
test.

(What's with all the Simons in the Haskell world, anyway?  I bet
it's some evil Haskell conspiracy I don't know about.)


-- 
#ozone/algorithm <ozone@algorithm.com.au>          - trust.in.love.to.save