On Fri, Aug 23, 2013 at 12:21 PM, Graydon Hoare <gray...@mozilla.com> wrote:
> On 13-08-23 12:16 PM, Simon Sapin wrote: > > Le 23/08/2013 19:47, Matthieu Monrocq a écrit : > >> The most trivial mistake is to have `+=` and `+` defined so that `a += > >> 5` has a different result than `a = a + 5` > > > > Would it work to make `a += b` always expand to `a = a + b`, and have > > that not be overridable? Or am I missing something else? > > Generally these operators want to be able to implement optimized > versions that do not produce and destroy temporaries. > I sympathize with this point, however I think it should be achieved in by means of something like the GHC RULES pragma[1] where programmers are allowed to tell the compiler about simple optimizations (rewrite rules), in their modules. So if you're doing a math module you could detect a*b+c and change it to a fmadd using a rule, and if there's an earlier pass that expands a+=b to a=a+b, you could then have a rules pragma that says a=a+b rewrites to an in-place add. In practice, rewrite rules have been extraordinarily powerful and pretty much essential in certain cases. E.g. efficient stream processing (stream fusion) relies on it. [1] http://www.haskell.org/ghc/docs/latest/html/users_guide/rewrite-rules.html -- Sebastian Sylvan
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