Sending again, this time including ghc-devs. Edward Kmett writes:

Note: From skimming your readme it is worth noting that log1p _is_ in base now (alongside expm1, log1pexp, and log1mexp). We added them all a couple of years back as a result of the very thread linked in your README.

You need to `import Numeric` to see them, though.

Switching to more accurate functions for doubles and floats for asinh, atanh, etc. to exploit this sort of functionality at least seems to make a lot of sense.

That can be done locally without any user API impact as the current definitions aren't supplied as defaults, merely as pointwise implementations instance by instance. Things will just become more accurate.

In that same spirit, we can probably crib a better version for complex numbers from somewhere as well, as it follows the same general simplistic formula right now, even if it can't be plugged directly into the equations you've given. For that matter, the log1p definition we're using for complex numbers was the best I could come up with, but there may well be a more accurate version you can find down in the mines of libm or another math library written by real analysts.

[snip]

So, here's a +1 from the libraries committee side towards improving the situation.

From there, it's a small matter of implementation.

Here's where I'd usually get Ben involved. Hi Ben!

Hi Edward and Matt! Indeed the sinh sinh situation should already be improved in 8.6. See #14927 and GHC commit 3ea33411d7cbf32c20940cc72ca07df6830eeed7. I suspect this should fix Matt's issue. Regarding log1p, I'd happily accept patches improving on the status quo. Cheers, - Ben

Hi George, Not a stupid question. I don't have a single source at hand, but I think I read in a few places on the wiki that calling out to the system math library is not an option due to the variety of system math libraries on the platforms GHC supports. It'd be great if I got the wrong impression and this could just be a call to C. Can anyone set me straight on this point? Matt On Thu, Aug 2, 2018 at 5:13 AM, George Colpitts wrote:

Hello Mat

Just curious, why the preferred solution isn't to call the system math library? As it says in the README you reference below,

One good solution would be to always call the system math library for these functions.

Hope this is isn't a stupid question.

Thanks

George

On Sat, Jun 2, 2018 at 2:23 AM Matt Peddie wrote:

...

Hi devs,

I tried to use asinh :: Double -> Double and discovered that it's inaccurate compared to my system library (GNU libm), even returning -Infinity in place of finite values in the neighborhood of -22 for large negative arguments. `atanh` is also inaccurate compared to the system library. I wrote up a more detailed description of the problem including plots in the README file at https://github.com/peddie/ghc-inverse-hyperbolic -- this repository is package that can help you examine the error for yourself or generate the plots, and it also contains accurate pure-Haskell translations of the system library's implementation for these functions. What's the next step to fixing this in GHC?

Cheers

Matt Peddie _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

Thanks, Ben, for chiming in. I think calling out to C for these functions is the way to go if it's now feasible. (Calling out to libm is the workaround I'm using in the application that led me to discover the inaccuracy.) Matt On Thu, Aug 2, 2018 at 11:33 AM, Ben Gamari wrote:

Matt Peddie writes:

...

Hi George,

Not a stupid question. I don't have a single source at hand, but I think I read in a few places on the wiki that calling out to the system math library is not an option due to the variety of system math libraries on the platforms GHC supports. It'd be great if I got the wrong impression and this could just be a call to C. Can anyone set me straight on this point?

Indeed it's not a stupid question at all. Indeed this is precisely what we do for the simpler transcendentals (e.g. sin, asin, log). We very well could move in this direction in the case of asinh/atanh as well. I believe the reason we don't currently is that atanh was only standardized in C99, which we only started requiring a few releases ago. Perhaps this is ultimately the right direction.

Cheers,

- Ben

Here is the patch: https://phabricator.haskell.org/D5034 -- Best, Artem On Thu, 2 Aug 2018 at 06:26 Artem Pelenitsyn wrote:

I'd be willing to do this.

-- Best wishes, Artem

On Thu, 2 Aug 2018, 04:38 Matt Peddie, wrote:

...

Thanks, Ben, for chiming in. I think calling out to C for these functions is the way to go if it's now feasible. (Calling out to libm is the workaround I'm using in the application that led me to discover the inaccuracy.)

Matt

On Thu, Aug 2, 2018 at 11:33 AM, Ben Gamari wrote:

...

Matt Peddie writes:

...

Hi George,

Not a stupid question. I don't have a single source at hand, but I think I read in a few places on the wiki that calling out to the system math library is not an option due to the variety of system math libraries on the platforms GHC supports. It'd be great if I got the wrong impression and this could just be a call to C. Can anyone set me straight on this point?

Indeed it's not a stupid question at all. Indeed this is precisely what we do for the simpler transcendentals (e.g. sin, asin, log). We very well could move in this direction in the case of asinh/atanh as well. I believe the reason we don't currently is that atanh was only standardized in C99, which we only started requiring a few releases ago. Perhaps this is ultimately the right direction.

Cheers,

- Ben

---

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Thanks David! Indeed, here is the commit and ticket: https://github.com/ghc/ghc/commit/3ea33411d7cbf32c20940cc72ca07df6830eeed7 https://ghc.haskell.org/trac/ghc/ticket/14927 This concerns only `asinh` though. The implementation is closer to what Matt proposes in his package but simpler. Nevertheless, the original issue about `Infinity` on large negative numbers seems to be fixed with this. So, I guess, feel free to kill the patch. -- Best, Artem On Thu, 2 Aug 2018 at 23:19 David Feuer wrote:

Wasn't there a very recent commit to improve these functions, by leftaroundabout?

...

Here is the patch: https://phabricator.haskell.org/D5034

-- Best, Artem

On Thu, 2 Aug 2018 at 06:26 Artem Pelenitsyn wrote:

...

I'd be willing to do this.

-- Best wishes, Artem

On Thu, 2 Aug 2018, 04:38 Matt Peddie, wrote:

...

Thanks, Ben, for chiming in. I think calling out to C for these functions is the way to go if it's now feasible. (Calling out to libm is the workaround I'm using in the application that led me to discover the inaccuracy.)

Matt

On Thu, Aug 2, 2018 at 11:33 AM, Ben Gamari wrote:

...

Matt Peddie writes:

...

Hi George,

Not a stupid question. I don't have a single source at hand, but I think I read in a few places on the wiki that calling out to the system math library is not an option due to the variety of system math libraries on the platforms GHC supports. It'd be great if I got

On Thursday, August 2, 2018 8:16:10 AM EDT Artem Pelenitsyn wrote: the

...

...

...

...

...

wrong impression and this could just be a call to C. Can anyone set me straight on this point?

Indeed it's not a stupid question at all. Indeed this is precisely what we do for the simpler transcendentals (e.g. sin, asin, log). We very well could move in this direction in the case of asinh/atanh as well. I believe the reason we don't currently is that atanh was only standardized in C99, which we only started requiring a few releases ago. Perhaps this is ultimately the right direction.

Cheers,

- Ben

---

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-- David Feuer Well-Typed