Proposal: NF newtype

[Michael Snoyman]

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
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>
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