Hi all,

I tried compiling this file:

{-# LANGUAGE NoImplicitPrelude #-}
-- | Demonstrate various use of the FFI.
module Foreign where
import Foreign.C
foreign import ccall "math.h sin" sin :: CDouble -> CDouble
it :: CDouble
it = 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