How lose can we be with strictness

[Joachim Breitner]

Hi,

yes, of course. What GHC is doing now is correct, safe and what an
(exprienced) programmer expects. Especially if he is using `x==x` to
deeply force x…

But then I presume that there are many uses of code like this where the
effect of `==` forcing its arguments is a necessary side effect of
deciding the equality, and if the compiler finds a different way of
deciding the equality, e.g. statically, the programmer would be happy
to have `n` as dead code, with all the advantages this provides.

And I am wondering if there is a way for GHC to discriminate between
these two options somehow…

Joachim


Am Samstag, den 20.05.2017, 12:07 -0500 schrieb Christopher Allen:
> Correct me if I'm wrong here, but it's preserving the effect of (==)
> forcing its arguments. This makes the optimization less surprising,
> in
> my opinion.
> 
> Is there a benefit to not doing this?
> 
> On Sat, May 20, 2017 at 11:56 AM, Joachim Breitner
> <mail at joachim-breitner.de> wrote:
> > Hi,
> > 
> > I just observed that GHC optimizes
> > 
> > fun (n::Int) = n == 0 + n
> > 
> > to essentially
> > 
> > fun (n::Int) = n `seq` True
> > 
> > 
> > I am wondering under what circumstances we would be happy to
> > transform
> > this further to
> > 
> > fun _ = True
> > 
> > 
> > Clearly, we do not want to drop `seq`s in general. But is there
> > some
> > commonly accepted rule about which strictness we generally allow
> > the
> > compiler to get rid of, if it turns out that the compiler can do
> > without? Or are all such transformations out of bounds for GHC?
> > 
> > Greetings,
> > Joachim
> > 
> > --
> > Joachim Breitner
> >   mail at joachim-breitner.de
> >   http://www.joachim-breitner.de/
> > 
> > _______________________________________________
> > ghc-devs mailing list
> > ghc-devs at haskell.org
> > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> > 
> 
> 
> 
-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/
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