[Haskell-cafe] using ghc as a library

[Daniel Gorín]

On Oct 25, 2008, at 8:39 PM, Anatoly Yakovenko wrote:

> so I am trying to figure out how to use ghc as a library.  following
> this example, http://www.haskell.org/haskellwiki/GHC/As_a_library, i
> can load a module and examine its symbols:
> [...]
>
> given Test.hs:
>
> module Test where
>
> hello = "hello"
> world = "world"
> one = 1
> two = 2
>
> i get this output:
>
> $ ./Main ./Test.hs
> "[]"
> "[Test.hello, Test.one, Test.two, Test.world]"
>
> which is what i expect.  My question is, how do manipulate the symbols
> exported by Test?  Is there a way to test the types?  lets say i
> wanted to sum all the numbers and concatenate all the strings in
> Test.hs, how would i do that?

Hi, Anatoly

Sorry for don't answering your question in the first place, but for  
this kind of tasks I believe you might be better off using some  
lightweight wrapper of the GHC Api. For instance, using   http://hackage.haskell.org/cgi-bin/hackage-scripts/package/hint 
  you write:

import Language.Haskell.Interpreter.GHC
import Control.Monad.Trans ( liftIO  )
import Control.Monad       ( filterM )

test_module = "Test"

main :: IO ()
main = do s <- newSession
           withSession s $ do
               loadModules [test_module]        -- loads Test.hs...
               setTopLevelModules [test_module] -- ...and puts it in  
scope
               setImports ["Prelude"]           -- put the Prelude in  
scope too
               --
               exports <- getModuleExports "Test" -- get Test's symbols
               let ids = [f | Fun f <- exports]
               --
               strings <- filterM (hasType "[Char]") ids
               conc <- concat `fmap` mapM (\e -> interpret e infer)  
strings
               liftIO $ putStrLn conc
               --
               ns <- filterM (hasType "Integer") ids
               sum_ns <- sum `fmap` mapM (\e -> interpret e (as ::  
Integer)) ns
               liftIO $ putStrLn (show sum_ns)


hasType :: String -> Id -> Interpreter Bool
hasType t e = do type_of_e <- typeOf e
                  return (type_of_e == t)

$ ./Main
helloworld
3

The version in hackage of hint works only with GHC 6.6.x and 6.8.x,  
mind you, but a new version is coming soon....

Good luck,

Daniel