Install this version:
emerge -a =dev-haskell/mtl-compat-0.2.2-r1
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =dev-haskell/mtl-compat-0.2.2-r1
Or alternatively:
emerge --autounmask-write -a =dev-haskell/mtl-compat-0.2.2-r1
# Copyright 1999-2026 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
# ebuild generated by hackport 0.9.1.0.9999
CABAL_FEATURES="lib profile haddock hoogle hscolour"
inherit haskell-cabal
DESCRIPTION="Backported Control.Monad.Except module from mtl"
HOMEPAGE="https://github.com/haskell-compat/mtl-compat"
LICENSE="BSD"
SLOT="0/${PV}"
KEYWORDS="~amd64 ~arm64 ~ppc64 ~riscv ~x86"
RDEPEND=">=dev-lang/ghc-9.0.2:=
"
DEPEND="${RDEPEND}
>=dev-haskell/cabal-3.4.1.0
"
>=dev-lang/ghc-9.0.2:= >=dev-haskell/cabal-3.4.1.0
>=dev-lang/ghc-9.0.2:=