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
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