https://buildd.debian.org/status/package.php?p=haskell-operational&suite=sid

...
[1 of 1] Compiling Main             ( doc/examples/TicTacToe.hs, dist-ghc/build/operational-TicTacToe/operational-TicTacToe-tmp/Main.o )

src/Control/Monad/Operational.hs:26:1: warning: [-Wunused-imports]
    The import of ‘Control.Applicative’ is redundant
      except perhaps to import instances from ‘Control.Applicative’
    To import instances alone, use: import Control.Applicative()
   |
26 | import Control.Applicative
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Control/Monad/Operational.hs:26:1: warning: [-Wunused-imports]
    The import of ‘Control.Applicative’ is redundant
      except perhaps to import instances from ‘Control.Applicative’
    To import instances alone, use: import Control.Applicative()
   |
26 | import Control.Applicative
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^

doc/examples/TicTacToe.hs:46:13: error:
    Ambiguous occurrence ‘singleton’
    It could refer to
       either ‘Control.Monad.Operational.singleton’,
              imported from ‘Control.Monad.Operational’ at doc/examples/TicTacToe.hs:19:1-32
           or ‘Data.List.singleton’,
              imported from ‘Data.List’ at doc/examples/TicTacToe.hs:23:1-16
              (and originally defined in ‘base-4.15.1.0:Data.OldList’)
   |
46 | readBoard = singleton ReadBoard
   |             ^^^^^^^^^

doc/examples/TicTacToe.hs:47:13: error:
    Ambiguous occurrence ‘singleton’
    It could refer to
       either ‘Control.Monad.Operational.singleton’,
              imported from ‘Control.Monad.Operational’ at doc/examples/TicTacToe.hs:19:1-32
           or ‘Data.List.singleton’,
              imported from ‘Data.List’ at doc/examples/TicTacToe.hs:23:1-16
              (and originally defined in ‘base-4.15.1.0:Data.OldList’)
   |
47 | playMove  = singleton . PlayMove
   |             ^^^^^^^^^
 at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 107.
	Debian::Debhelper::Buildsystem::Haskell::Recipes::run_quiet("debian/hlibrary.setup", "build", "--builddir=dist-ghc") called at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 131
	Debian::Debhelper::Buildsystem::Haskell::Recipes::run("debian/hlibrary.setup", "build", "--builddir=dist-ghc") called at /usr/share/perl5/Debian/Debhelper/Buildsystem/Haskell/Recipes.pm line 640
	Debian::Debhelper::Buildsystem::Haskell::Recipes::build_recipe() called at -e line 1
make: *** [/usr/share/cdbs/1/class/hlibrary.mk:153: build-ghc-stamp] Error 25

We believe that the bug you reported is fixed in the latest version of
haskell-operational, which is due to be installed in the Debian FTP archive.

A summary of the changes between this version and the previous one is
attached.

Thank you for reporting the bug, which will now be closed.  If you
have further comments please address them to 1013787@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.

Debian distribution maintenance software
pp.
Ilias Tsitsimpis <iliastsi@debian.org> (supplier of updated haskell-operational package)

(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
Format: 1.8
Date: Sun, 03 Jul 2022 13:22:05 +0300
Source: haskell-operational
Architecture: source
Version: 0.2.4.1-1
Distribution: unstable
Urgency: medium
Maintainer: Debian Haskell Group <pkg-haskell-maintainers@lists.alioth.debian.org>
Changed-By: Ilias Tsitsimpis <iliastsi@debian.org>
Closes: 1013787
Changes:
 haskell-operational (0.2.4.1-1) unstable; urgency=medium
 .
   * New upstream release (Closes: #1013787)
   * Declare compliance with Debian policy 4.6.1
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