ImplicitParams and MonoLocalBinds

[Iavor Diatchki]

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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>> Glasgow-haskell-users at haskell.org
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>>
>
>
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