Remote GHCi

Manuel M T Chakravarty chak at
Sat Nov 21 03:38:49 UTC 2015

> Simon Marlow <marlowsd at>:
> 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 Value 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?)

The GHC API basically assumes that the ”result” of statement execution is the *side-effect* of printing the result to stdout. This is not sufficient for an interactive graphical environment as

(1) I want to have the result (even if it is a string) separate from anything else interpreted code execution writes to stdout. (In Haskell for Mac, these things are displayed in different places.)

(2) I want results that are not just strings. For example, a result (of running Haskell code) may be a ForeignPtr to a C-land data structure representing an image (e.g., an in-memory representation of a PNG image rendered by Diagrams).

For the latter, I’m actually using `compileExpr`, then `unsafeCoerce` the `hValue` into `IO (ForeignPtr ())` and `try` that (to also catch any exceptions). When this code runs, in some cases, it calls back and forth between interpreted Haskell code and the host application using the FFI.

> 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 difficult in my case, because the resulting value is used in the GUI code written in Swift. Code running in a different process cannot call the Cocoa framework methods for the GUI of the main process.

>> 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.

On a fundamental level: The game engine runs on OpenGL. If it is in a different process, it cannot access the OpenGL context of the main process (which it needs to do to render into a specific view of a specific window of the main process).

In practice, it is not just an OpenGL problem as I’m using a framework called SpriteKit with its own event and rendering loop that in turn uses OpenGL for the actual rendering. It does a lot of things behind the scenes (which makes it convenient to use), which requires you to be careful which threads you use to execute some operations. Running in an entire different process is surely going to break things.

>   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 understand that, but I also think that it is an artefact of Haskell mostly being used in a command line program set up. I don’t think, it is just by chance that the IHaskell people do some quite similar things to at least some of what I’m doing. Once you want a more interactive experience, call patterns get more complicated.

>> 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

Yes, I understand that and, as I wrote, I do like the idea of running in a separate process. However, it would also be a shame to prevent richer and more interactive experiences than CLI applications.

I have thought a bit more about what the fundamental obstacle is. I think, it is two things:

(1) I have interpreted Haskell code that (via a compiled Haskell library) uses FFI calls to call Cocoa system framework methods to create Cocoa objects. In Haskell, these Cocoa objects are referenced via a ForeignPtr and I need the interpreter to be able to return these foreign pointers. The ForeignPtr’s need to refer to memory of the main host process; hence, the FFI calls need to run the Cocoa framework code in the host process.

(2) The Cocoa objects from (1) include both StablePtrs as well as C function pointers created via foreign dynamic wrapper. At least some of the StablePtrs refer to Haskell heap structures that need to be accessed by interpreted Haskell code. And calling the dynamic wrapper code from Swift in the main process needs to execute Haskell code that may refer to closures created by interpreted code.

So, the issue really is that I would need FFI calls in the interpreter process that call Cocoa code in the main process and dynamic wrapper entry code in the main process that needs to call Haskell code in the interpreter process. (Crossing the FFI language chasm corresponds to cross-process calls.)

I cannot move the Cocoa code from the main process to the interpreter process, as Cocoa requires that it runs on the *main* thread of the main process (to interact with the GUI and also to render via OpenGL).

Does that make sense?


>>> 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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