Quick Q: do all FFI (non-primop) calls involve State# and RealWorld?

[Csaba Hruska]

Hi,

I've also observed that in the final lowered STG the State# is always
passed to effectful primops and FFI calls, but the returning new State# is
removed from the result type. The State# has VoidRep representation in in
cmm, so no register gets allocated for it and eventually the State#
function argument is compiled to nothing in the machine code. i.e. the
compilation steps for the code above is:

foreign import ccall "math.h sin" sin :: CDouble -> CDouble


   1.  Initial STG type is: CDouble -> State# RealWorld -> (# State#
   RealWorld, CDouble #)
   2.  Lowered STG type is: CDouble -> State# RealWorld -> (# CDouble #)
   3.  FFI C function should be: double sin(double x);

Regards,
Csaba

On Mon, Oct 28, 2019 at 10:59 AM Christopher Done <chrisdone at gmail.com>
wrote:

> Hi all,
>
> I tried compiling this file:
>
> {-# LANGUAGE NoImplicitPrelude #-}-- | Demonstrate various use of the FFI.module Foreign whereimport Foreign.Cforeign import ccall "math.h sin" sin :: CDouble -> CDoubleit :: CDoubleit = sin 1
>
> And I’ve noticed that the annotated type given for this foreign op in
> Core, is (# State# RealWorld, CDouble #), whereas I would have expected
> e.g. CDouble.
>
> Meanwhile, the foreign op call is passed a RealWorld argument.
>
> Additionally, code that consumes the result of this foreign call expects a (#
> CDouble #) as a return value.
>
> So there are some assumptions I put in my interpreter to test this FFI
> call out:
>
>    1. Despite claiming to return the real world in a tuple, it actually
>    should just return an unboxed tuple of the value.
>    2. It should ignore the RealWorld argument entirely.
>
> I assume, if I were to lift this function into returning IO, that I
> should indeed return the RealWorld argument given. So the lesson is:
>
> All FFI functions accept a RealWorld, and may return a 2-tuple of State#
> RealWorld *if* it’s impure, else it’ll return a 1-tuple of the value.
> Correct?
>
> Can someone confirm that my observations are right? Also, if so, is there
> somewhere I can read more about this?
>
> Cheers
>
> Chris
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