ImplicitParams and MonoLocalBinds

[Roman Cheplyaka]

Right! I knew about this, but somehow it didn't pop up in my mind in
this case. Thanks a lot!

Roman

* Iavor Diatchki <iavor.diatchki at gmail.com> [2013-03-28 17:07:18-0700]
> Hi,
> 
> Aha! This page explains what is going on:
> http://hackage.haskell.org/trac/ghc/blog/LetGeneralisationInGhc7
> 
> The summary is that the definition of what is "local" is not what one might
> expect:  only things that depend
> on variables in scope are considered to be locals, other bindings, that
> could be lifted out (e.g., like `p` in both examples)
> are not considered local and are generalized.  Of course, with implicit
> parameters this is not what one might hope for...
> 
> A while back there was a discussion about adding a construct for
> monomorphic bindings to the language (I think the proposed notation was
> something like "x := 2").
> Perhaps we should revisit it, it seems much simpler than the rather
> surprising behavior of `MonoLocalBinds`.
> 
> -Iavor
> 
> 
> 
> 
> 
> 
> On Thu, Mar 28, 2013 at 4:39 PM, Iavor Diatchki <iavor.diatchki at gmail.com>wrote:
> 
> > Hi,
> > This does not appear to be related to ImplicitParameters, rather
> > `MonoLocalBinds` is not working as expected.
> >
> > Here is an example without implicit parameters that compiles just fine,
> > but would be rejected if `p` was monomorphic:
> >
> > {-# LANGUAGE NoMonomorphismRestriction, MonoLocalBinds #-}
> >
> > class C a where
> >   f :: a -> ()
> >
> > instance C Bool where f = const ()
> > instance C Char where f = const ()
> >
> > g = let p = f
> >     in (p 'a', p True)
> >
> > -Iavor
> >
> >
> >
> >
> > On Fri, Mar 22, 2013 at 1:39 AM, Roman Cheplyaka <roma at ro-che.info> wrote:
> >
> >> The value of the following expression
> >>
> >>   let ?y = 2  in
> >>   let  p = ?y in
> >>   let ?y = 1  in
> >>   p
> >>
> >> depends on whether the second binding is generalised.
> >>
> >> MonomorphismRestriction makes it not generalise, hence the value is 2.
> >>
> >> What surprises me is that MonoLocalBinds doesn't have this effect.
> >>
> >>   Prelude> :set -XImplicitParams -XNoMonomorphismRestriction
> >> -XMonoLocalBinds
> >>   Prelude> let ?y = 2 in let p = ?y in let ?y = 1 in p
> >>   1
> >>
> >> What's going on here?
> >>
> >> Roman
> >>
> >> _______________________________________________
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> >> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> >>
> >
> >