Is it possible to generate and run the generated TemplateHaskell code at run time?

Question

Is it possible to generate and run the generated TemplateHaskell code at run time?

Using C, at runtime I can:

• create the source code of the function,
• call gcc to compile it in .so (linux) (or use llvm, etc.)
• download .so and
• function call.

Is this possible with the Haskell pattern?

Answer 1

Yes it is possible. The GHC API will compile Template Haskell. A proof of concept is available at https://github.com/JohnLato/meta-th , which, although not very complicated, shows one general method that even provides a security type modification. Haskell pattern expressions are constructed using the Meta type, which can then be compiled and loaded into a useful function.

 {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE TemplateHaskell #-} {-# OPTIONS_GHC -Wall #-} module Data.Meta.Meta ( -- * Meta type Meta (..) -- * Functions , metaCompile ) where import Language.Haskell.TH import Data.Typeable as Typ import Control.Exception (bracket) import System.Plugins -- from plugins import System.IO import System.Directory newtype Meta a = Meta { unMeta :: ExpQ } -- | Super-dodgy for the moment, the Meta type should register the -- imports it needs. metaCompile :: forall a. Typeable a => Meta a -> IO (Either String a) metaCompile (Meta expr) = do expr' <- runQ expr -- pretty-print the TH expression as source code to be compiled at -- run-time let interpStr = pprint expr' typeTypeRep = Typ.typeOf (undefined :: a) let opener = do (tfile, h) <- openTempFile "." "fooTmpFile.hs" hPutStr h (unlines [ "module TempMod where" , "import Prelude" , "import Language.Haskell.TH" , "import GHC.Num" , "import GHC.Base" , "" , "myFunc :: " ++ show typeTypeRep , "myFunc = " ++ interpStr] ) hFlush h hClose h return tfile bracket opener removeFile $ \tfile -> do res <- make tfile ["-O2", "-ddump-simpl"] let ofile = case res of MakeSuccess _ fp -> fp MakeFailure errs -> error $ show errs print $ "loading from: " ++ show ofile r2 <- load (ofile) [] [] "myFunc" print "loaded" case r2 of LoadFailure er -> return (Left (show er)) LoadSuccess _ (fn :: a) -> return $ Right fn 

This function takes ExpQ and first runs it in IO to create a simple Exp . Exp then pretty printed into the source code, which is compiled and loaded at runtime. In practice, I have found that one of the most difficult hurdles is specifying proper import into the generated TH code.

Answer 2

From what I understand, you want to create and run code at runtime, which I think you can use with the GHC API , but I'm not very sure about the amount of what you can achieve. If you want something like a hot-code exchange, you can look at the hotswap package.