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.

For more information, see: https://wiki.haskell.org/Cabal/NewBuild

Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] trying: butterflies-0.3.0.2 (user goal)
[__1] trying: repa-devil-0.3.2.6 (dependency of butterflies)
[__2] next goal: transformers (dependency of repa-devil)
[__2] rejecting: transformers-0.5.5.0/installed-0.5..., transformers-0.5.5.0,
transformers-0.5.4.0, transformers-0.5.2.0, transformers-0.5.1.0,
transformers-0.5.0.1, transformers-0.5.0.0 (conflict: repa-devil =>
transformers>=0.2 && <0.5)
[__2] trying: transformers-0.4.3.0
[__3] next goal: base (dependency of butterflies)
[__3] rejecting: base-4.12.0.0/installed-4.1... (conflict: transformers =>
base>=2 && <4.9)
[__3] 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)
[__3] fail (backjumping, conflict set: base, butterflies, transformers)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: transformers, repa-devil, base,
butterflies