#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: Type: bug | Status: new Priority: low | Milestone: Component: libraries/base | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Incorrect result | Difficulty: Easy (less than 1 at runtime | hour) Test Case: acosh(-1) :: | Blocked By: Complex Double | Related Tickets: Blocking: | -------------------------------------+------------------------------------- Comment (by leftaroundabout): Replying to [comment:1 carter]:

does this behave as expected in the a small radius around -1? (worth at least plotting it out)

For real arguments, it sure enough works. {{{ GHCi, version 7.4.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :m +Data.Complex Prelude Data.Complex> :m +Graphics.Rendering.Chart.Simple Prelude Data.Complex Graphics.Rendering.Chart.Simple> :set prompt "> "

let re f = realPart . f . realToFrac let im f = imagPart . f . realToFrac plotPNG "acosh-on-reals.png" [-2, -1.99 .. 2] (re acosh) (im acosh) }}}

If you put in complex numbers, there's a problem anyway: the imaginary part jumps from {{{+π}}} to {{{-π}}} as {{{Im z}}} becomes negative. Still, ([http://www.wolframalpha.com/input/?i=acosh]) it's apparently convention that for {{{Im z = 0}}}, we always have {{{Im (acosh(z)) ≥ 0)}}}, including {{{z = -1}}}. At any rate, the real part is 0 there; but in the current implementation that is also {{{NaN}}}. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: Type: bug | Status: new Priority: low | Milestone: Component: libraries/base | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: Incorrect result | Difficulty: Easy (less than 1 at runtime | hour) Test Case: acosh(-1) :: | Blocked By: Complex Double | Related Tickets: Blocking: | -------------------------------------+------------------------------------- Comment (by ekmett): We do pick the standard branch cut location. That said, it is possible to give a more general form with a specified branch cut. I do so for all of these functions and more for quaternions in http://hackage.haskell.org/package/linear-1.10.1.1/docs/Linear- Quaternion.html You pick the location of the branch cut by supplying a unit quaternion. Adapting this logic to a choice of point on the unit circle could be done, but I confess I'd rather hash that out in libraries before asserting it is the right approach for something like base. Almost nobody knows how to use the more exotic cuts, but importantly they can be formed from the standard cut and identities by somone with sufficient background to be aware of the issue. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: alekzcb Type: bug | Status: new Priority: low | Milestone: Component: Core Libraries | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect result | Test Case: acosh(-1) :: at runtime | Complex Double Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): Do let me know if you encounter trouble, alekzcb. Thanks for picking this up! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: alekzcb Type: bug | Status: new Priority: low | Milestone: Component: Core Libraries | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect result | Test Case: acosh(-1) :: at runtime | Complex Double Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): Oh no! What are the symptoms? Feel free to drop by `#ghc` on `irc.freenode.net` if you want help. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: alekzcb Type: bug | Status: new Priority: low | Milestone: Component: Core Libraries | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Incorrect result | Test Case: acosh(-1) :: at runtime | Complex Double Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): For the record, I suspect the issue was that alekzcb didn't have an editor in his `PATH`. Unfortunately it seems that arcanist doesn't deal with this terribly gracefully. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#8532: Hyperbolic arc cosine fails on (-1) :: Complex r. -------------------------------------+------------------------------------- Reporter: leftaroundabout | Owner: alekzcb Type: bug | Status: new Priority: low | Milestone: 8.4.1 Component: Core Libraries | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect result | Unknown/Multiple at runtime | Test Case: numrun016 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3916 Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari ): In [changeset:"6458b8dcbd33b3c32f3c23f7e5b08fdc6e73ed46/ghc" 6458b8dc/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="6458b8dcbd33b3c32f3c23f7e5b08fdc6e73ed46" base: Update acosh to handle -1::Complex Summary: Fixes #8532 Reviewers: austin, hvr, bgamari Reviewed By: bgamari Subscribers: rwbarton, thomie GHC Trac Issues: #8532 Differential Revision: https://phabricator.haskell.org/D3916 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8532#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler