deepseq: Add more instances. #50

[Travis Whitaker]

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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