Hi Simon, Thanks for the reply. The work on accelerated is indeed relevant: in accelerate-cuda, CUDA kernels (which are composed of C code) get compiled to a binary file invoking an external compiler, and then (this is the relevant part) the compiled binary is executed used the CUDA library. The problem is that while CUDA kernels are executable through a libraries, compiled functions using the C calling convention aren't. So I can't really call compiled C functions like accelerate-cuda invokes CUDA kernels. Relatedly, if I have some function pointer known at runtime that addresses a C function that takes some arguments, I have no way to invoke it from Haskell, since all FFI imports must be declared at compile-time, and I don't know the address of the symbol I want to execute at compile time. I am exploring the possibility of adding such capabilities to GHC, since as I mentioned I don't see any particular roadblock. Francesco On 11 February 2015 at 11:40, Simon Peyton Jones wrote:

This may be relevant. http://www.cse.unsw.edu.au/~chak/papers/CMCK14.html Manuel gave a talk about something like this at the Haskell Symposium.

Simon

| -----Original Message----- | From: Haskell-Cafe [mailto:haskell-cafe-bounces@haskell.org] On Behalf | Of Francesco Mazzoli | Sent: 11 February 2015 10:26 | To: haskell; ghc-devs@haskell.org | Subject: [Haskell-cafe] Anonymous FFI calls | | Hi, | | I am in a situation where it would be very useful to call C functions | without an explicit FFI import. For example, I'd like to be able to | do | | (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 | | instead of declaring the foreign import explicitely at the top level. | | Is there a way to do this or to achieve similar results in some other | way? | | If not, I imagine it would be easy to implement such a facility in | GHC, given that the code implementing calling to C functions must | already be present to implement "proper" FFI imports. I think such an | addition would be useful in many cases. | | Thanks, | Francesco | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe

Quoth Sylvain Henry ,

2015-02-11 16:18 GMT+01:00 Francesco Mazzoli :

...

Relatedly, if I have some function pointer known at runtime that addresses a C function that takes some arguments, I have no way to invoke it from Haskell, since all FFI imports must be declared at compile-time, and I don't know the address of the symbol I want to execute at compile time.

You can use FunPtr and wrapper imports to convert a FunPtr into a Haskell function.

For example, foreign import ccall "get_type1_fn" get_type1_fn :: IO (FunPtr (CInt -> IO CInt)) foreign import ccall "dynamic" type1FunPtrToFun :: FunPtr (CInt -> IO CInt) -> (CInt -> IO CInt) main = do fp <- get_type1_fn let f = type1FunPtrToFun fp f 5 >>= print I guess the key point in this demo is that a C function is able to return a FunPtr. Donn

On 11 February 2015 at 18:26, Sylvain Henry wrote:

You can use FunPtr and wrapper imports to convert a FunPtr into a Haskell function. https://hackage.haskell.org/package/base-4.7.0.2/docs/Foreign-Ptr.html#g:2

The problem with that scheme is that you have to define a FFI import per type. TH solutions are not viable (and cumbersome) for our use case -- the biggest problem being related to the TH staging restriction.

Code using http://hackage.haskell.org/package/libffi looks pretty similar, though the specification needs terms (such as argCInt) instead of a mandatory type signature. GHC itself includes libffi so maybe the foreign import "dynamic" calls end up doing exactly the same thing as the hackage libffi does.

Thanks! That seems like a good workaround for the time being -- we had actually considered for a split second re-implementing the C calling convention, I should have suspected that such a library existed already. However, while that library works for pointers, it does not (and can't) have the facilities to refer to symbols. In any case, there still is the need for such a facility built-in GHC, since such solutions are always going to incur in some overhead, compared to calling C functions directly. For example in the linked bindings to `libffi' the arguments are passed as a list, which is certainly a big difference from storing them into registers. And again, it seems like 95% of the work is already done, since `foreign import's are already desugared to Haskell functions with a primitive call to the C function. Francesco

I'd love for the implementation to be as powerful as the documentation suggests it is :D However, yes, in the meantime, fixing the documentation would be great! Also, I meant to say that addTopDecls is only exported by "Language.Haskell.TH.Syntax". While this is a digression, there are a few other handy functions that are oddly left out of "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly more. -Michael On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones wrote:

I would LOVE someone to improve the documentation for addTopDecls. Manuel Chakravarty and Geoff Mainland were responsible for the implementation.

Simon

| -----Original Message----- | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Michael | Sloan | Sent: 11 February 2015 23:19 | To: Francesco Mazzoli | Cc: ghc-devs@haskell.org; haskell | Subject: Re: [Haskell-cafe] Anonymous FFI calls | | It seems like addTopDecls[1] will able to help here. Unfortunately, | the function is not well documented and not very discoverable because | it's only exported by Language.Haskell.TH. | | The documentation doesn't mention that it can only be used to create | new top level functions and FFI imports[2]. I think that adding FFI | imports was the main motivation for implementing it. In the past | I've wanted to generate instances via this function, but unfortunately | it's not implemented.. | | Hope that helps! | -Michael | | [1] http://hackage.haskell.org/package/template-haskell- | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls | | [2] | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33f78a/ | compiler/typecheck/TcSplice.hs#L818 | | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli wrote: | > Hi, | > | > I am in a situation where it would be very useful to call C functions | > without an explicit FFI import. For example, I'd like to be able to do | > | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 | > | > instead of declaring the foreign import explicitely at the top level. | > | > Is there a way to do this or to achieve similar results in some other | > way? | > | > If not, I imagine it would be easy to implement such a facility in GHC, | > given that the code implementing calling to C functions must already be | > present to implement "proper" FFI imports. I think such an addition | > would be useful in many cases. | > | > Thanks, | > Francesco | > _______________________________________________ | > Haskell-Cafe mailing list | > Haskell-Cafe@haskell.org | > http://www.haskell.org/mailman/listinfo/haskell-cafe | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://www.haskell.org/mailman/listinfo/ghc-devs

Thanks to everyone who replied! It seems like that through a combination of facilities like `libffi' and `addTopDecls' I can do everything that I wanted to do. I still want to take a shot at implementing anonymous FFI calls, since IMHO I think they are a very small but useful addition to the language. Francesco On 12 February 2015 at 09:29, Simon Peyton Jones wrote:

| Also, I meant to say that addTopDecls is only exported by | "Language.Haskell.TH.Syntax". While this is a digression, there are a | few other handy functions that are oddly left out of | "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly | more.

That does seem wrong. Do make a patch!

SIMon

| | -Michael | | On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones | wrote: | > I would LOVE someone to improve the documentation for addTopDecls. | Manuel Chakravarty and Geoff Mainland were responsible for the | implementation. | > | > Simon | > | > | -----Original Message----- | > | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of | > | Michael Sloan | > | Sent: 11 February 2015 23:19 | > | To: Francesco Mazzoli | > | Cc: ghc-devs@haskell.org; haskell | > | Subject: Re: [Haskell-cafe] Anonymous FFI calls | > | | > | It seems like addTopDecls[1] will able to help here. | Unfortunately, | > | the function is not well documented and not very discoverable | > | because it's only exported by Language.Haskell.TH. | > | | > | The documentation doesn't mention that it can only be used to | create | > | new top level functions and FFI imports[2]. I think that adding | FFI | > | imports was the main motivation for implementing it. In the past | > | I've wanted to generate instances via this function, but | > | unfortunately it's not implemented.. | > | | > | Hope that helps! | > | -Michael | > | | > | [1] http://hackage.haskell.org/package/template-haskell- | > | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls | > | | > | [2] | > | | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33 | > | f78a/ | > | compiler/typecheck/TcSplice.hs#L818 | > | | > | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli | wrote: | > | > Hi, | > | > | > | > I am in a situation where it would be very useful to call C | > | > functions without an explicit FFI import. For example, I'd like | > | > to be able to do | > | > | > | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 | > | > | > | > instead of declaring the foreign import explicitely at the top | level. | > | > | > | > Is there a way to do this or to achieve similar results in some | > | > other way? | > | > | > | > If not, I imagine it would be easy to implement such a facility | in | > | > GHC, given that the code implementing calling to C functions | must | > | > already be present to implement "proper" FFI imports. I think | > | > such an addition would be useful in many cases. | > | > | > | > Thanks, | > | > Francesco | > | > _______________________________________________ | > | > Haskell-Cafe mailing list | > | > Haskell-Cafe@haskell.org | > | > http://www.haskell.org/mailman/listinfo/haskell-cafe | > | _______________________________________________ | > | ghc-devs mailing list | > | ghc-devs@haskell.org | > | http://www.haskell.org/mailman/listinfo/ghc-devs