Proposal: NF newtype

[Ivan Lazar Miljenovic]

On 2 May 2015 at 11:24, Edward Z. Yang <ezyang at mit.edu> wrote:
> I just realized that the thread of this conversation was not fully
> finished.
>
> For Show/Read, there are two flex points:
>
> (1) Should `show (mkNF 2)` output "mkNF 2" or "UnsafeNF 2"?
>     I have a preference for the former but then we need to handwrite
>     the instance.  We could also change the definition of NF to just
>     name its constructor NF and export a helper function unsafeMkNF

I think it should output "mkNF 2", unless you're going to be exporting
the constructor for people to use.

>
> (2) Should `read "mkNF 2"` execute rnf on the result of the inner
>     Read instance?  If the answer is no, we can use the default; if
>     the answer is yes, we want an instance like:
>
>         instance (NFData a, Read a) => Read (NF a) where
>             readsPrec = parens . prec 10 $ do
>                             Ident "makeNF" <- lexP
>                             m <- step readPrec
>                             return (m `deepseq` UnsafeNF m)

I think this is the right approach, though maybe you should just use
"makeNF m" internally rather than copying it's definition in the last
line.

>
> I lean towards a safe by default API, with an unsafeReadNF, but
> maybe if default Read instances are not lazy, we should be OK
> (e.g.  (read $ show (repeat 1)) :: [Int] hangs; it isn't identity).
>
> Edward
>
> Excerpts from Roman Cheplyaka's message of 2015-04-22 13:28:10 -0700:
>> On 22/04/15 21:54, Edward Z. Yang wrote:
>> > But it is an interesting question whether or not 'UnsafeNF' should be
>> > used, since the value read in is known to be in normal form.
>>
>> Is it?
>>
>> newtype X = X Int
>>   deriving Show
>> instance Read X where
>>   readsPrec n = map (first $ X . trace "eval") . readsPrec n
>>
>> > length (read "[1,2,3]" :: [X])
>> 3
>> > read "[1,2,3]" :: [X]
>> [X eval
>> 1,X eval
>> 2,X eval
>> 3]
>>
>> Or did you mean something else?
>>
>> > Excerpts from Henning Thielemann's message of 2015-04-22 19:49:56 +0100:
>> >>
>> >> On Wed, 22 Apr 2015, Dan Burton wrote:
>> >>
>> >>> A hand-written read makes more sense to me in this case:
>> >>> read = makeNF . read
>> >>> show = show . getNF
>> >>
>> >> Show and Read instances should process Strings representing Haskell code,
>> >> and I guess, Haskell code with the same type as the represented value.
>> >> Thus the NF should be part of the formatted value.
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>> >
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-- 
Ivan Lazar Miljenovic
Ivan.Miljenovic at gmail.com
http://IvanMiljenovic.wordpress.com