[Haskell-cafe] ANN: Elerea, another FRP library

[Peter Verswyvelen]

Well, the documentation says:
Use {-# NOINLINE foo #-} as a pragma on any function foo that calls
unsafePerformIO<file:///C:/app/ghc-6.10.1/doc/libraries/base/System-IO-Unsafe.html#v%3AunsafePerformIO>.
If the call is inlined, the I/O may be performed more than once.

So you claim this does not prevent GHC to inline it anyway? That feels like
a bug then, both in the documentation and NOINLINE

On Thu, Apr 16, 2009 at 10:18 AM, Lennart Augustsson <lennart at augustsson.net
> wrote:

> There's no guarantee about unsafePerformIO not being inlined, that's
> just how ghc treats it.
>
> 2009/4/16 Patai Gergely <patai_gergely at fastmail.fm>:
> >> On the other hand, breaking referential transparency in the
> >> external interface is a very bad idea, in my opinion. Actually,
> >> this means that the library user would have to turn certain
> >> compiler optimizations off to get the intended behavior.
> > However, in practice you can compile Elerea with -O2 without ill
> > effects. In fact, that's what happens if you install it with cabal.
> >
> >> Just have a look at the Haddock docs of unsafePerformIO.
> > Yes, I did that too, and came up with the following checklist:
> >
> > - the order of side effects doesn't matter much, since the resulting
> > networks are equivalent if we don't rely on the automatic delay feature
> > (applicative optimisations can be different, but still with the same net
> > effect)
> > - unsafePerformIO is apparently never inlined, i.e. each instance is
> > executed once, so sharing works as desired
> > - let-floating is no problem, because all instances of unsafePerformIO
> > rely on surrounding function arguments
> > - CSE is no problem either, it even helps if it's performed (and it is
> > with optimisations turned on), since it results in smaller equivalent
> > networks
> >
> > I think we can expect it to be fairly well-behaving, because the 'side
> > effect' of Elerea primitives is basically the same as that of pure
> > values in general: upon evaluation a value is created in the memory and
> > we get a reference to it. We only have an extra constraint for the
> > compiler: never duplicate these values. Merging identical ones is okay,
> > and in fact desirable. The following code demonstrates this if you
> > compile it with and without optimisations:
> >
> > import Control.Applicative
> > import Control.Monad
> > import FRP.Elerea
> > import System.IO.Unsafe
> >
> > cint a b = unsafePerformIO (putStrLn "!") `seq`
> >           transfer 0 (\dt x x0 -> x0+x*dt) b
> >
> > mysig = (latcher 0 (b >@ 0.3) (const (cint a b) <$> cint a b)) +
> >        (cint a b) + (cint a b) + a
> >    where a = pure 4
> >          b = stateful 0 (+)
> >
> > main = replicateM 10 (superstep mysig 0.1) >>= print
> >
> > I'd like to see an example where optimisation does make a difference,
> > because I'm still unsure about the consequences of 'unsafeness'.
> >
> > Gergely
> >
> > --
> > http://www.fastmail.fm - Or how I learned to stop worrying and
> >                          love email again
> >
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