Warning: The install command is a part of the legacy v1 style of cabal usage.

Please switch to using either the new project style and the new-install
command or the legacy v1-install alias as new-style projects will become the
default in the next version of cabal-install. Please file a bug if you cannot
replicate a working v1- use case with the new-style commands.

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Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: hakyll-sass-0.2.4 (user goal)
[__1] trying: hakyll-4.12.4.0 (dependency of hakyll-sass)
[__2] trying: hakyll:+usepandoc
[__3] next goal: pandoc (dependency of hakyll +usepandoc)
[__3] rejecting: pandoc-2.5, pandoc-2.4, pandoc-2.3.1, pandoc-2.3 (conflict:
hakyll +usepandoc => pandoc>=2.0.5 && <2.3)
[__3] trying: pandoc-2.2.3.2
[__4] next goal: haddock-library (dependency of pandoc)
[__4] rejecting: haddock-library-1.7.0 (conflict: pandoc =>
haddock-library>=1.6 && <1.7)
[__4] trying: haddock-library-1.6.0
[__5] next goal: base (dependency of hakyll-sass)
[__5] rejecting: base-4.12.0.0/installed-4.1... (conflict: haddock-library =>
base>=4.5 && <4.12)
[__5] rejecting: base-4.12.0.0, base-4.11.1.0, base-4.11.0.0, base-4.10.1.0,
base-4.10.0.0, base-4.9.1.0, base-4.9.0.0, base-4.8.2.0, base-4.8.1.0,
base-4.8.0.0, base-4.7.0.2, base-4.7.0.1, base-4.7.0.0, base-4.6.0.1,
base-4.6.0.0, base-4.5.1.0, base-4.5.0.0, base-4.4.1.0, base-4.4.0.0,
base-4.3.1.0, base-4.3.0.0, base-4.2.0.2, base-4.2.0.1, base-4.2.0.0,
base-4.1.0.0, base-4.0.0.0, base-3.0.3.2, base-3.0.3.1 (constraint from
non-upgradeable package requires installed instance)
[__5] fail (backjumping, conflict set: base, haddock-library, hakyll-sass)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: pandoc, haddock-library, hakyll, base,
hakyll:usepandoc, hakyll-sass