On 5/5/06, Geoffrey Alan Washburn <[EMAIL PROTECTED]> wrote:
Lemmih wrote: > On 5/4/06, Donald Bruce Stewart <[EMAIL PROTECTED]> 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 _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
