Re: "eval" in ghc(i)?

On 5/5/06, Geoffrey Alan Washburn wrote:

Lemmih wrote:

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On 5/4/06, Donald Bruce Stewart wrote:

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geoffw:

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I have an application written in OCaml that I'm interested in porting over to Haskell, and I was wondering what the best way

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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

to replace this

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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