[Haskell-beginners] Tips for a FFI-wrapped C library?

Patrick LeBoutillier patrick.leboutillier at gmail.com
Mon Nov 30 16:34:17 EST 2009

Hi all,

I have wrapped, using FFI,  a C-library I am writing that calls into
different programming languages. I put
everything in the IO monad. So far it works as expected but I find the
resulting usage code somewhat
ugly (especially the bits musing 'sequence'). I tried using finalizers
to delete the resources but it couldn't
respect my dependencies.

Can anyone offer any advice on making it feel more natural in Haskell?

Note: The FFI code is available here:

  import qualified MLP
  import qualified MLP.Error
  import qualified MLP.Context
  import qualified MLP.Language
  import qualified MLP.Runtime
  import qualified MLP.Value
  import qualified MLP.Object

  main = do
    ctx <- MLP.Context.new                            -- create a context
    MLP.Context.with ctx $ do
      lang <- MLP.Language.init ctx "perl"            -- init a
language, i.e. perl
      rt <- MLP.Runtime.new ctx lang                   -- create a
runtime for the language
      MLP.Runtime.loadLibrary ctx rt "calc"           -- load a
library, i.e. calc.pm
      val <- MLP.Object.new ctx rt "calc" [] ""       -- create a calc
object, i.e. my $obj = new calc() ;
      obj <- MLP.Value.takeObject val                 -- extract the
object from the value
      MLP.Value.delete ctx val

      args_add <- sequence [MLP.Value.newLong 2, MLP.Value.newLong 3]
      args_mul <- sequence [MLP.Value.newDouble 3.5, MLP.Value.newDouble 4.6]
      val_add <- MLP.Object.callMethod ctx rt obj "add" args_add ""
      sequence $ map (MLP.Value.delete ctx) args_add
      val_mul <- MLP.Object.callMethod ctx rt obj "mul" args_mul ""
      sequence $ map (MLP.Value.delete ctx) args_mul
      MLP.Object.delete ctx obj

      MLP.Value.getLong val_add >>= putStrLn . show >>
MLP.Value.delete ctx val_add
      MLP.Value.getDouble val_mul >>= putStrLn . show >>
MLP.Value.delete ctx val_mul

      MLP.Runtime.delete ctx rt
      MLP.Language.delete ctx lang



Patrick LeBoutillier
Rosemère, Québec, Canada

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