deepseq: Add more instances. #50

David Feuer david.feuer at
Wed Jan 15 16:04:30 UTC 2020

NFData predates compact regions by many years, and they've never been very
precisely matched.

On Wed, Jan 15, 2020, 10:42 AM Elliot Cameron <eacameron at> wrote:

> And similar to Show, many of us wish there was a *different *class that
> was mostly for debugging, not for serialization. If there's a perception
> that "NFData" means "able to be put in a compact region" then we definitely
> don't want these instances. But perhaps what we want is two classes with
> different guarantees.
> On Wed, Jan 15, 2020 at 10:40 AM Elliot Cameron <eacameron at>
> wrote:
>> As long as "Show" is the "other side of Read", a Show instance for
>> reference types is completely silly. NFData doesn't have *that* complication
>> at least.
>> On Wed, Jan 15, 2020 at 6:43 AM Andrew Martin <andrew.thaddeus at>
>> wrote:
>>> 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>
>>> wrote:
>>>> Greetings Haskellers,
>>>> I am writing to draw your attention to
>>>> 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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