Simon Marlow <[email protected]>:
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?
Cheers,
Manuel
Simon Marlow <[email protected]>:
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: https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi
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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