Remote GHCi

Simon Marlow marlowsd at
Wed Nov 18 09:45:26 UTC 2015

On 18/11/2015 01:41, Manuel M T Chakravarty wrote:
> Hi Simon,
> While this is an interesting proposal, Haskell for Mac strongly
> relies on running interpreted code in the same process. I’m using
> ’dynCompileExpr’ as well as ’hscStmtWithLocation’ and some other
> stuff.

Let me say first of all that I'm not going to remove anything, so 
there's no need to worry.  But I'd like to explore exactly what you 
need, so that we can see whether there's a way to accommodate it with a 
separate-process implementation.

hscStmtWithLocation is part of the core GHCi functionality, it is 
definitely supported.  It has a slightly different signature:

hscStmtWithLocation :: HscEnv
                     -> String -- ^ The statement
                     -> String -- ^ The source
                     -> Int    -- ^ Starting line
                     -> IO ( Maybe ([Id]
                           , RemoteHValue {- IO [HValue] -}
                           , FixityEnv))

RemoteHValue is a reference to a value in the interpreter's context. 
These have to be evaluated via an explicit API, rather than just 
unsafeCoercing HValue as we do now.  (this is not strictly speaking part 
of the GHC API, so a separate but interesting question is: why did you 
need to use this directly, and what should we add to the GHC API?)

I believe that many uses of dynCompileExpr can be changed so that the 
code using the resulting value is moved into the interpreter's context, 
and then there's no problem.

> This is quite crucial for some of the interactive
> functionality. Imagine a game where the game engine is in Swift
> linked into the main application and the game logic is in
> *interpreted* Haskell code. The engine calls into the Haskell code
> multiple times per frame of the animation and for all
> keyboard/mouse/etc input (using StablePtr and ForeignPtr to construct
> the scene graph across the Swift and Haskell heap).

So my question is, why wouldn't you run the whole game engine in the 
interpreter's context?  That's what would happen if you were to load the 
program into GHCi and run it.  Directly calling back and forth between 
the client of the GHC API and the program being interpreted is arguably 
a strange thing to do, and it's kind of accidental that we allow it.

> I actually also might have a use for the architecture that you are
> proposing. However, I really would like to keep the ability to, at
> least, optionally run interpreted code in the same process (without
> profiling etc). Do you think we could have both?

We can certainly have both, it's straightforward to implement, but I 
don't get to throw away some of the hacks we have to support 
same-process execution, which would be a shame.  We just add more code 
rather than

> Cheers,
> Manuel
>> Simon Marlow <marlowsd at>:
>> Hi folks - I've been thinking about changing the way we run interpreted code so that it would be run in a separate process.  It turns out this has quite a few benefits, and would let us kill some of the really awkward hacks we have in GHC to work around problems that arise because we're running interpreted code and the compiler on the same runtime.
>> I summarised the idea here:
>> I'd be interested to hear if anyone has any thoughts around this, particularly if doing this would make your life difficult in some way. Are people relying on dynCompileExpr for anything?
>> Cheers,
>> Simon
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