If the Control.DeepSeq module exports an error-yielding instance, then one would need to newtype all their ForeignPtrs in order to define their own correct NFData instance. Naturally I’m biased, but that seems like quite a hoop to jump through to get correct but subtle behavior. I’m not sure the core libraries go to such lengths to hide correct-but-tricky behavior anywhere else; I’d be interested in seeing existing examples of this. On Wed, Jan 15, 2020 at 1:13 AM Henning Thielemann < lemming@henning-thielemann.de> wrote:

On Wed, 15 Jan 2020, Travis Whitaker wrote:

...

If such an error-yielding instance were added, how would users who need the correct-but-potentially-confusing behaving NFData instance cope with this?

In the criterion/record example they would manually implement the NFData instance for the record, as you already suggested.