David: 

i'm inclined to agree with Doug here.

Phrased differently: what is the example change in overheads in micro or milliseconds? 

what is an example tiny program where those overheads are a significant part of  program overhead?

why woulnd't they use something like https://www.microsoft.com/en-us/research/wp-content/uploads/2007/10/compilingwithcontinuationscontinued.pdf  aka the so called "double barrelled cps" transform?

On Wed, Feb 21, 2018 at 11:21 PM, David Feuer <david.feuer@gmail.com> wrote:

Because sometimes the sanctioned way is inefficient. throwIO always
wraps its exception argument in a SomeException constructor before
calling raiseIO# on the result. That extra baggage is likely enough to
make the implementation I'm considering too slow to bother with, so I
care right now in 2018. I'd very much prefer to get an
officially-approved way to do what I want, but barring that I'll take
one that works.

On Wed, Feb 21, 2018 at 9:33 AM, Doug McIlroy <doug@cs.dartmouth.edu> wrote:
>
>> > Can I use reallyUnsafePtrEquality# reliably to identify whether a value is
>> a nullary constructor of a particular type?
>
> Can this "optimization" possibly save enough time to justify
> nonstandard trickery?
> This kind of obscure brittle coding may have been OK 50 years
> ago. But why do it now?
>
> Doug
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