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