deepseq: Add more instances. #50

[Andrew Martin]

My understanding of the argument for and against is:

* For: Typeclass instances should be provided for as many types as
possible, and IORef, TVar, etc. only admit a single way to define NFData
* Against: Intuitively, NFData means that data is plain old sums and
products all the way down, all of which can be forced. This would mean that
anything with an NFData instance should be able to go into a compact region
(except that pinned byte arrays, and consequently bytestrings, cannot)

I'm inclined to agree with most of the opinions expressed earlier on the
thread. That is, given how minor the convenience is, the confusion is
probably not worth the convenience. I'll think about this more today.

On Tue, Jan 14, 2020 at 7:55 PM Travis Whitaker <pi.boy.travis at gmail.com>
wrote:

> Greetings Haskellers,
>
> I am writing to draw your attention to
> https://github.com/haskell/deepseq/pull/50.
>
> This PR adds NFData instances for UArray, ForeignPtr, and TVar. The
> instances for ForeignPtr and TVar have proven to be somewhat controversial.
> I'll refer to these as "reference types" from here on out, since similar
> concerns have been raised with respect to e.g. IORef. I think the arguments
> presented here apply equally well to IORef.
>
> In summary: the argument against these instances is that rnf forces the
> evaluation of the reference value itself, not the value referred to by the
> reference. This is potentially confusing to NFData users. For example, a
> user might expect that calling force on a TVar value will leave the value
> to which the TVar refers in normal form. If this assumption doesn't hold,
> the user's program will leak thunks.
>
> The argument for these instances is as follows: whether or not a reference
> value is in normal form has nothing to do with whether or not the
> referred-to value is in normal form. For example, consider ForeignPtr.
> ForeignPtr's type constructor argument is just a phantom. Each ForeignPtr
> value is just an Addr# with some finalizers. Whether or not these values
> are in normal form has nothing to do with whether or not the value the
> enclosed address may (or may not!) point to is in normal form. Indeed, an
> NFData instance for ForeignPtr, TVar, or IORef that attempted to force the
> referred-to value would require unsafe IO.
>
> I'm curious to hear other's thoughts about these arguments. I'm hopeful
> that the proposed instances may be added, since I've encountered these
> instances as orphans on numerous occasions.
>
> Thanks for your time,
>
> Travis Whitaker
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-- 
-Andrew Thaddeus Martin
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