[Haskell-cafe] Pointer equality for nullary constructors

[David Feuer]

Yes, by the way, insert would be a poor use case, since the "same value
pointer" case will be fairly rare. The actual use-case I have in mind is
alterF, where that case is much more likely.

On Feb 22, 2018 3:12 PM, "David Feuer" <david.feuer at gmail.com> wrote:

> Double-barreled continuations don't seem to work well when you want to
> abort construction of a recursive structure. Think about
> Data.HashMap.Strict.insert. We don't really want to have to walk all the
> way back up to the top if we discover that the value pointer we're
> inserting is the same as the I've already in the map.
>
> On Feb 22, 2018 2:55 PM, "Carter Schonwald" <carter.schonwald at gmail.com>
> wrote:
>
> 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/compilingwithcont
> inuationscontinued.pdf  aka the so called "double barrelled cps"
> transform?
>
> On Wed, Feb 21, 2018 at 11:21 PM, David Feuer <david.feuer at 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 at 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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>
>
>
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