deepseq: Add more instances. #50

[John Villarreal]

 I too agree that NFData instances for reference types are more trouble than worth the modest convenience they buy us. We don't have Show instances for reference-like types either and I don't think anybody would be arguing for those either.

John

    On Wednesday, 15 January 2020, 07:55:40 UTC, Henning Thielemann <lemming at henning-thielemann.de> wrote: 
> David put my thoughts pretty clearly: is it more useful or more 
> confusing? I am also leaning toward more confusing. Hopefully more can 
> weigh in on this.

I also think NFData instances for reference types are more dangerous than 
helpful. Since laziness and strictness account for subtle programming 
mistakes, it is better if GHC tells you that 'rnf tvar' does possibly not 
perform what you expect. I'd even consider to add an instance with a type 
error message to show the user that this instance has been omitted by 
intention.
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