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.diatc...@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 <r...@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 >> >> _______________________________________________ >> Glasgow-haskell-users mailing list >> Glasgow-haskell-users@haskell.org >> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users >> > >
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