Hello,
Removing the MR seems reasonable. I am a little less certain about
the MPB rule though. I suspect that, as the wiki page points out,
many uses of pattern bindings are monomorphic but still, there seem to
be a number of examples on the wiki where people have run into this
problem. So I am not sure that the conclusion that nobody has noticed
the change is valid. I do believe that you can probably work around
the problem in many situations but the question in my mind is "why
should we have to work around stuff when we have a system that already
works?" In other words, what problem do MBPs solve?
I should also point out that if we were to adopt the MBP rule, we
would have to adjust the definition of what pattern bindings mean.
For example, I think that this is how things are desugared at the
moment:
(x,y) = e
becomes
new_var = e
x = case new_var of (v,_) -> v
y = case new_var of (_,v) -> v
It seems that under MBP the second program is not equivalent to the
first because it is more polymorphic.
-Iavor
On Wed, Apr 23, 2008 at 10:32 AM, Simon Marlow
Folks,
The current proposal on the table for what to do about the monomorphism restriction (henceforth MR) is
* remove the MR entirely * adopt Monomorphic Pattern Bindings (MPB)
Right now, the committee is almost uniformly in favour of dropping the MR, and most of us are coming round to the idea of MPB. Since this area has historically been difficult to achieve a concensus on, I'm excited that we appear to be close to making a decision, and a good one at that!
The arguments for removing the MR are pretty well summarised on the wiki:
http://hackage.haskell.org/trac/haskell-prime/wiki/MonomorphismRestriction
You can read about MPB here:
http://hackage.haskell.org/trac/haskell-prime/wiki/MonomorphicPatternBinding...
GHC has implemented MPB by default (i.e. we deviate slightly from Haskell 98) since 6.8.1.
The nice thing about the combination of removing MR and adopting MPB is that we retain a way to explicitly declare monomorphic bindings. These are monomorphic bindings:
~x = e x@_ = e
or if you don't mind a strict binding: !x = e. The wiki points out that
(x) = e
would also be monomorphic, but arguably this is in poor taste since we expect (x) to mean the same as x everywhere.
Cheers, Simon _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime