"eval" in ghc(i)?

[Lemmih]

On 5/5/06, Geoffrey Alan Washburn <geoffw at cis.upenn.edu> wrote:
> Lemmih wrote:
> > 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.
>
>         Thanks!  I think this is a bit closer to what I'm looking for than the
> hs-plugins eval.  It is possible to get runStmt to output the result of
> the session to a string rather than stdout?

Yeah:

Prelude GHC GHC.Exts> Just n <- compileExpr session "show (add1 2)"
Prelude GHC GHC.Exts> let n' = unsafeCoerce# n :: String
Prelude GHC GHC.Exts> n'
"3"

--
Friendly,
  Lemmih