[Haskell-cafe] Pointer equality for nullary constructors

[David Feuer]

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/compilingwithcontinuationscont
inued.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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