Proposal: NF newtype

[David Feuer]

+1, generally. Two thoughts:

1. There are, as far as I know, two reasonable senses of NF that one
might care about. The weak sense is a guarantee that the structure has
no accessible bottoms within it that would be reached if the structure
were evaluated to normal form, along with a guarantee that it is
impossible to use it to force lazy IO. A stronger sense offers some
kind of performance guarantee, which seems much trickier to specify.
Whether something is *actually* in normal form is not observable
without using something like ticky-ticky profiling, so I don't think
we're allowed to care about that. Since I support the idea of
unsafeMakeNF, and perhaps other special NF-creation functions, I think
it's important to be clear about just what it's supposed to mean.

2. I think it makes a lot of sense to put unsafeMakeNF into a separate
Unchecked module, presumably declared unsafe for Safe Haskell.

On Tue, Feb 10, 2015 at 6:45 AM, Michael Snoyman <michael at snoyman.com> wrote:
> Forgot to mention: I'm +1 on the proposal in general, though it would be
> nice if these types/functions were available from a compatibility library as
> well, since deepseq is now bundled with GHC and therefore difficult to
> upgrade on its own.
>
> On Tue Feb 10 2015 at 1:43:52 PM Michael Snoyman <michael at snoyman.com>
> wrote:
>>
>> On Tue Feb 10 2015 at 1:41:48 PM Herbert Valerio Riedel <hvr at gnu.org>
>> wrote:
>>>
>>> On 2015-02-10 at 09:46:37 +0100, Edward Z. Yang wrote:
>>> > I propose the following (abstract) data type, functions, and instance
>>> > be
>>> > added to deepseq (naming amenable to bikeshedding):
>>> >
>>> >     newtype NF a = NF a -- abstract
>>> >     makeNF :: NFData a => a -> NF a
>>> >     getNF :: NF a -> a
>>>
>>> >     instance NFData (NF a) where
>>> >         rnf x = x `seq` ()
>>>
>>> I generally like it, so count me as +1
>>>
>>> bikeshed-wondering though, why did you make it abstract w/ separate
>>> construct/deconstructors?
>>>
>>>
>>
>> If the real constructor was available, then someone would be free to write
>> something like:
>>
>>     let foo = NF $ 1 : undefined
>>
>> which would break the guarantees that this type is trying to provide.
>>
>>>
>>> as a related concept (and hoping not to derail the discussion too much),
>>> I had attempted some time ago to implement TH predicates  that could
>>> infer if WHNF=NF holds:
>>>
>>>
>>> http://hackage.haskell.org/package/deepseq-th-0.1.0.4/docs/Control-DeepSeq-TH.html#v:typeWhnfIsNf
>>>
>>> and back then I was wondering, if GHC couldn't help us out here (maybe
>>> by providing an implicit NF=WHNF class-instance -- and btw, I recently
>>> found out, Haskell 1.4 had an implicit 'Eval'-class declaring
>>> `seq`-ability)
>>>
>>> I'm not sure though if WHNF=NF can always be decided...
>>>
>>>
>>> Cheers,
>>>   hvr
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://www.haskell.org/mailman/listinfo/libraries
>
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>