Install this version:
emerge -a =dev-haskell/dual-tree-0.2.3.1
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =dev-haskell/dual-tree-0.2.3.1
Or alternatively:
emerge --autounmask-write -a =dev-haskell/dual-tree-0.2.3.1
# Copyright 1999-2026 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
# ebuild generated by hackport 0.8.5.1.9999
CABAL_HACKAGE_REVISION=4
CABAL_FEATURES="lib profile haddock hoogle hscolour test-suite"
inherit haskell-cabal
DESCRIPTION="Rose trees with cached and accumulating monoidal annotations"
HOMEPAGE="https://hackage.haskell.org/package/dual-tree"
LICENSE="BSD"
SLOT="0/${PV}"
KEYWORDS="~amd64 ~arm64 ~ppc64 ~riscv ~x86"
RDEPEND=">=dev-haskell/monoid-extras-0.6:=[profile?] <dev-haskell/monoid-extras-0.7:=[profile?]
>=dev-haskell/semigroups-0.8:=[profile?] <dev-haskell/semigroups-0.21:=[profile?]
>=dev-lang/ghc-9.0.2:=
"
DEPEND="${RDEPEND}
>=dev-haskell/cabal-3.4.1.0
test? ( dev-haskell/quickcheck
dev-haskell/testing-feat )
"
>=dev-haskell/monoid-extras-0.6:=[profile?] <dev-haskell/monoid-extras-0.7:=[profile?] >=dev-haskell/semigroups-0.8:=[profile?] <dev-haskell/semigroups-0.21:=[profile?] >=dev-lang/ghc-9.0.2:= >=dev-haskell/cabal-3.4.1.0 test? ( dev-haskell/quickcheck dev-haskell/testing-feat )
>=dev-haskell/monoid-extras-0.6:=[profile?] <dev-haskell/monoid-extras-0.7:=[profile?] >=dev-haskell/semigroups-0.8:=[profile?] <dev-haskell/semigroups-0.21:=[profile?] >=dev-lang/ghc-9.0.2:=