On Thu, Aug 20, 2009 at 4:41 PM, Peter Verswyvelen
But how does GHC implement the RealWorld internally? I guess this can't be done using standard Haskell stuff? It feels to me that if I would implement it, I would need seq again, or a strict field, or some incrementing "time" value that is a strict argument of each of the IO primitives. In any case, I would need strictness to control the dependencies no? I might be wrong (again) but this is all very interesting ;-)
The RealWorld is just a token that GHC uses to force IO computations
to have the correct data dependencies. If you look at code like
"putChar 'x' >> getChar", there's no obvious data dependency that
would prevent executing getChar before putChar, so internally the IO
monad passes around the RealWorld token to guarantee the ordering.
I don't know the exact details of GHC's IO internals, but I'd expect
putChar 'x' >> getChar to translate into something like this,
\rw0 -> let ((), rw1) = putChar# 'x' rw0 in getChar# rw1
The important things to note are (1) getChar# depends on the token
returned by putChar#, thus guaranteeing that putChar# gets executed
first, and (2) putChar# and getChar# are impure and cannot normally be
defined in Haskell.
--
Dave Menendez