[Haskell-cafe] Anonymous FFI calls

[Michael Sloan]

Hi!

I don't mean to be accusatory with the whole documentation thing - I
totally understand that most folks have a lot going on!  In this
particular case, I got very excited when I initially saw addTopDecls,
because it says it allows you to "Add additional top-level
declarations".  After implementing something to use it, I got rather
disappointed as it couldn't add instances.

So, when I say "not implemented", it's just that the docs say it
allows you to add additional top-level decls, when you can really only
add functions, variable bindings, and foreign import decls.

I didn't realize that not re-exporting these functions is due to their
experimental status.  That makes sense!  I'm glad it was a conscious
decision.  I just tried writing a demo of addTopDecls, but
unfortunately using it to add a function and referencing this function
causes a GHC internal error:
https://gist.github.com/mgsloan/53d7fa50338c696e5c80 .  I haven't
tried it with a foreign import yet.

-Michael

On Wed, Feb 11, 2015 at 3:51 PM, Geoffrey Mainland
<mainland at cs.drexel.edu> wrote:
> Typed Template Haskell happened at the end of my tenure at MSR, and
> there was a mad rush to get it in to the compiler in time for 7.8.3 at
> the same time I was starting a new job. I'm afraid the documentation is
> indeed wanting, but the alternative was no typed Template Haskell.
>
> One side-effect was that the functions you note as present only in
> Language.Haskell.TH.Syntax were not thoroughly vetted, so we didn't
> re-export them from Language.Haskell.TH.
>
> I am willing to help with documentation if people want to use this
> functionality. Has anyone attempted to use addTopDecls? Michael, when
> you say "it's not implemented," what do you mean? What, exactly, is not
> implemented?
>
> Cheers,
> Geoff
>
> On 2/11/15 6:32 PM, Michael Sloan wrote:
>> 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
>> <simonpj at microsoft.com> 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 at haskell.org] On Behalf Of Michael
>>> | Sloan
>>> | Sent: 11 February 2015 23:19
>>> | To: Francesco Mazzoli
>>> | Cc: ghc-devs at 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 <f at mazzo.li> 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
>>> | > _______________________________________________
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>