"eval" in ghc(i)?

Donald Bruce Stewart dons at cse.unsw.edu.au
Wed May 3 19:33:50 EDT 2006

> 	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.

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

So if your program critically relies on this its possible to do.

-- Don

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