Question

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

Using C, at runtime, I can:

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

Is a similar thing possible with Template Haskell?

Answer 1

Yes, it’s 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 sophisticated, shows one general technique that even provides a modicum of type safety. Template Haskell expressions are build using the Meta type, which can then be compiled and loaded into a usable 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 an ExpQ, and first runs it in IO to create a plain Exp. The Exp is then pretty-printed into source code, which is compiled and loaded at run-time. In practice, I’ve found that one of the more difficult obstacles is specifying the correct imports in the generated TH code.