[Haskell-cafe] How to execute a function loaded via GHCI.ObjLink?

[Moritz Kiefer]

Hi,
I have a blogpost which demonstrates this
https://purelyfunctional.org/posts/2016-05-20-dynamic-loading-haskell-module.html
Basically you need to unwrap the address and then use addrToAny.
Obviously this is not typesafe so use it carefully.

Cheers,
Moritz

On 07/27/2017 04:20 PM, Saurabh Nanda wrote:
> Hi,
> 
> I'm playing around
> with https://www.stackage.org/haddock/lts-9.0/ghci-8.0.2/GHCi-ObjLink.html
> to see if I can come up with some sort of hot-loading/plugin system in
> Haskell. 
> 
> I've managed to load a shared object (compiled from a Haskell source via
> -rdynamic) with the following code:
> 
>     {-# LANGUAGE OverloadedStrings #-}
>     {-# LANGUAGE MagicHash, UnboxedTuples #-}
>     
>     module Main where
>     
>     import GHC.Exts         ( addrToAny# )
>     import GHC.Ptr          ( Ptr(..) )
>     import System.Info      ( os, arch )
>     import Encoding
>     import GHCi.ObjLink
>     import Debug.Trace
>     
>     main :: IO ()
>     main = do
>       traceM "before initObjLinker"
>       initObjLinker
>       traceM "before loadObj"
>       loadObj
> "/Users/saurabhnanda/projects/test-plugins/test-plugins/app/PluginMarkup.o"
>       traceM "after loadObj"
>     
>       -- NOTE: I've hardcoded the symbol name that I obtained from
> running `symbols PluginMarkup.o`
>       sym <- lookupSymbol "PluginMarkup_foliage_info"
>       traceM "after lookupsymbol"
>       traceM (show sym)
> 
> I'm getting the following output, which mean that I'm probably getting
> the Ptr to the function.
> 
>     before initObjLinker
>     before loadObj
>     after loadObj
>     after lookupsymbol
>     Just 0x0000000104be49a8
> 
> Question is, how do I run the function in the same Haskell
> environment/runtime? The underlying function is actually `foliage ::
> UtcTime -> Html`
> 
> The code is available
> at https://github.com/vacationlabs/hint-test/blob/dll/app/Main.hs and
> the DLL is available
> at https://github.com/vacationlabs/hint-test/blob/dll/app/PluginMarkup.o
> 
> -- Saurabh.
> 
> 
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