Resolving dependencies...
cabal: Could not resolve dependencies:
[__0] next goal: butter (user goal)
[__0] rejecting: butter-0.1.0.1 (constraint from user target requires
==0.1.0.0)
[__0] trying: butter-0.1.0.0
[__1] trying: forkable-monad-0.1.1 (dependency of butter)
[__2] next goal: base (dependency of butter)
[__2] rejecting: base-4.10.1.0/installed-4.1... (conflict: forkable-monad =>
base>3.0 && <4.4)
[__2] rejecting: 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)
After searching the rest of the dependency tree exhaustively, these were the
goals I've had most trouble fulfilling: base, butter, forkable-monad