[Haskell-cafe] Re: Where do I put the seq?

Peter Verswyvelen bugfact at gmail.com
Fri Aug 21 05:04:19 EDT 2009

IO also seems to use unboxed (hence strict?) tuples

newtype IO a = IO (State# RealWorld -> (# State# RealWorld, a #))

Not sure if this is just for performance, but if the strictness is required,
here we have the horrible hack again then (would behave different without
it?). I guess it works because when applying primitive function likes
putChar#, these could be considered as fully strict, since putChar# c really
does force evaluation of c strictly and puts in the screen. This is
different from the lazy IO situation, where a string is concatenated lazily,
and put on the screen by the consumer as soon as it's available. Ah I'm
having troubles to explain myself formally, never mind :)

Actually RealWorld is not defined in that file, it is defined here, but


But I don't understand the comment

data *RealWorld*Source<file:///C:/app/ghp/doc/libraries/ghc-prim/src/GHC-Prim.html#RealWorld>
RealWorld is deeply magical. It is *primitive*, but it is not *unlifted*
 (hence ptrArg). We never manipulate values of type RealWorld; it's only
used in the type system, to parameterise State#.
Maybe I should reread the papers, but it seems lots of magic is needed to
get IO right (such as the existential types to make sure different state
threads are kept separate)

On Fri, Aug 21, 2009 at 10:52 AM, Bayley, Alistair <
Alistair.Bayley at invesco.com> wrote:

> > From: haskell-cafe-bounces at haskell.org
> > [mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Bulat Ziganshin
> > To: Peter Verswyvelen
> >
> > > But how does GHC implement the RealWorld internally? I guess
> >
> > look the "base" library sources for "RealWorld"
> http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/GHC-IOBas
> e.html#IO
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