"eval" in ghc(i)?

[Simon Marlow]

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.

Nice example, I've added it to the wiki page:

   http://haskell.org/haskellwiki/GHC/As_a_library

However, there's a caveat with doing this:  the RTS linker only has a 
single symbol table, so the running GHCi will be sharing this symbol 
table with the new GHC session.  Loading object code in both sessions 
will probably lead to problems.

Cheers,
	Simon