Re: ImplicitParams and MonoLocalBinds

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