"eval" in ghc(i)?

[Lemmih]

On 5/4/06, Donald Bruce Stewart <dons at cse.unsw.edu.au> wrote:
> geoffw:
> >
> >       I have an application written in OCaml that I'm interested in
> >       porting over to Haskell, and I was wondering what the best way to replace
> > the following OCaml function would be:
> >
> > Toploop.initialize_toplevel_env();;
> >
> > let eval txt = let lb = (Lexing.from_string txt) in
> >  let phr = !Toploop.parse_toplevel_phrase lb in
> >  Toploop.execute_phrase true Format.std_formatter phr;;
> >
> > eval "let add1 x = x +1;;";;
> > eval "add1 2;;";;
> >
> > Where I would like to be able to "eval" Haskell-code instead.  It looks
> > like I might be able to achieve something like this using hs-plugins,
> > but it looks a bit more complex.  Is hs-plugins the best choice for this
> > kind of "meta"-programming?  I'm pretty sure Template Haskell will not
> > work for me, at least as I understand it I can only manipulate program
> > fragments that will be compiled later and as such that it will not be
> > possible to execute them until the next "phase".
>
> You can do some forms of runtime metaprogrammign with hs-plugins, yes.
> E.g.
>
> Prelude System.Eval.Haskell> v <- eval "1 + 2 :: Int" [] :: IO (Maybe Int)
> Prelude System.Eval.Haskell> v
> Just 3
>
> Prelude System.Eval.Haskell> mf <- eval "\\x -> x + 1 :: Int" [] :: IO (Maybe (Int -> Int))
> Prelude System.Eval.Haskell> let f = fromJust mf
> Prelude System.Eval.Haskell> :t f
> f :: Int -> Int
> Prelude System.Eval.Haskell> f 7
> 8
>
> So if your program critically relies on this its possible to do.

You can also use the GHC library:
Prelude> :m GHC
Prelude GHC> GHC.init (Just "/home/david/coding/haskell/ghc/usr/lib/ghc-6.5")
Prelude GHC> session <- newSession Interactive
Prelude GHC> setSessionDynFlags session =<< initPackages =<<
getSessionDynFlags session
Prelude GHC> setContext session [] [mkModule "Prelude"]
Prelude GHC> runStmt session "let add1 x = x + 1"
Prelude GHC> runStmt session "add1 2"
3
Prelude GHC> :q
Leaving GHCi.


--
Friendly,
  Lemmih