I want to get it done within the next few weeks. Currently GHCi is
mostly working, and the main missing pieces are TH and the debugger. I
plan to backport it to the 7.10 branch so that I can have it in our
local GHC builds at Facebook.
My WIP branch is here: https://github.com/simonmar/ghc/commits/prof-ghci
Cheers
Simon
On 24/11/2015 11:04, Alan & Kim Zimmerman wrote:
What kind of timescale can we expect on this, and will it be possible to
backport it (via a ghci-ng or similar)?
We are currently wrestling with ghci stdio issues in haskell-ide-engine.
If this will be out soon we can wait for it.
Alan
On Tue, Nov 24, 2015 at 2:25 AM, Manuel M T Chakravarty
<[email protected] <mailto:[email protected]>> wrote:
Hi Simon,
Sounds great!
This may very well what you have got in mind anyway, but I could
imagine to run the interpreter on a different thread in the
-fno-external-interpreter case and arrange communication through the
same messaging API that you outlined for the seperate-process
interpreter. Then, the essential difference between the two modes
would be whether memory is shared or not (i.e., multithreading vs
multi-process).
Cheers,
Manuel
> Simon Marlow <[email protected] <mailto:[email protected]>>:
>
> Hi Manuel,
>
> Thanks for the detailed reply, I have a much better understanding
of your requirements now.
>
> I'm going to support both models of running interpreted code.
The current plan is to have a flag, -fexternal-interpreter, which
GHC will use by default when running Template Haskell during
compilation, and perhaps for GHCi, but for compatibility with
applications like yours I'll probably leave it off for GHC API users.
>
> There's really no downside to doing this, it's not much more
complicated than implementing the separate-process model.
>
> Cheers,
> Simon
>
> On 21/11/2015 03:38, Manuel M T Chakravarty wrote:
>>> Simon Marlow <[email protected] <mailto:[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] <mailto:[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
>>>>> _______________________________________________
>>>>> ghc-devs mailing list
>>>>> [email protected] <mailto:[email protected]>
>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>>>
>>
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