Iavor Diatchki wrote:
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
The report doesn't actually mention this translation although it is
widely used to implement pattern bindings, and in some compilers (not
GHC) the translation is done before type checking.
What's interesting to me is that perhaps this gives us a way to
understand what the static semantics of pattern bindings should be,
absent MPB. e.g.
(x,y) = (negate,show)
(Simon's example) translates to
z = (negate,show)
x = fst z
y = snd z
and we can see why both x and y end up generalised over both
constraints, because
z :: (Num a, Show b) => (a -> a, b -> String)
and this also explains why the pattern-bound variables don't have to be
generalised over all the type variables. e.g. in
z = (id,id)
x = fst z
y = snd z
we'd get
z :: forall a b . (a->a, b->b)
x :: forall a . a -> a
not
x :: forall a b . a -> a
because the generalisation step for x only generalises over the type
variables in the type arising from its right-hand side.
Cheers,
Simon
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 <[EMAIL PROTECTED]> wrote:
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/MonomorphicPatternBindings
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
[EMAIL PROTECTED] = 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
_______________________________________________
Haskell-prime mailing list
Haskell-prime@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-prime
