And of course the biggest reason for this change, is we want GHC to continue to become "smarter". Remember, Haskell is a high level language. The original promise, is that the code should be algebraically optimizable by the compiler itself. Yes, of course many Haskell coders have learned to deal with the places where Haskell is all to close to the metal. But we hope that this will not always be the case. The interesting part, may not be that you can add the pragma yourself, but that in the future, you won't have to change anything at all.

Tim

---------- Původní zpráva ----------
Od: Joachim Breitner <breitner@kit.edu>
Datum: 29. 8. 2012
Předmět: Re: [Haskell-cafe] A first glimps on the {-# NOUPDATE #-} pragma

Hi Facundo,

Am Mittwoch, den 29.08.2012, 10:26 -0300 schrieb Facundo Domínguez:
> > upd_noupd n =
> > let l = myenum' 0 n
> > in last l + length l
>
> This could be rewritten as
>
> > upd_noupd n =
> > let l n = myenum' 0 n
> > in last (l n) + length (l n)
>
> Or a special form of let could be introduced to define locally-scoped macros:
>
> > upd_noupd n =
> > let# l = myenum' 0 n
> > in last l + length l
>
> What's the strength of the {-# NOUPDATE #-} approach?

it does not require refactoring of the code. There is not always a
parameter handy that you can use to prevent sharing, and using () for
that sometimes fails due to the full-lazyness-transformation.

And a locally-scoped macros would not help in this case:

test g n = g (myenum' 0 n)

Now you still might want to prevent the long list to be stored, but here
it cannot be done just by inlining or macro expansion.

Greetings,
Joachim

--
Dipl.-Math. Dipl.-Inform. Joachim Breitner
Wissenschaftlicher Mitarbeiter
http://pp.info.uni-karlsruhe.de/~breitner