Resolving dependencies... Error: cabal: Could not resolve dependencies: [__0] trying: paynow-zw-0.1.0.0 (user goal) [__1] next goal: bytestring (dependency of paynow-zw) [__1] rejecting: bytestring-0.11.3.1/installed-0.11.3.1 (conflict: paynow-zw => bytestring>=0.10.12 && <0.11) [__1] skipping: bytestring-0.11.3.1, bytestring-0.11.3.0, bytestring-0.11.2.0, bytestring-0.11.1.0, bytestring-0.11.0.0 (has the same characteristics that caused the previous version to fail: excluded by constraint '>=0.10.12 && <0.11' from 'paynow-zw') [__1] trying: bytestring-0.10.12.1 [__2] next goal: base (dependency of paynow-zw) [__2] rejecting: base-4.16.3.0/installed-4.16.3.0 (conflict: bytestring => base>=4.2 && <4.16) [__2] skipping: base-4.17.0.0, base-4.16.4.0, base-4.16.3.0, base-4.16.2.0, base-4.16.1.0, base-4.16.0.0 (has the same characteristics that caused the previous version to fail: excluded by constraint '>=4.2 && <4.16' from 'bytestring') [__2] rejecting: base-4.15.1.0, base-4.15.0.0, base-4.14.3.0, base-4.14.2.0, base-4.14.1.0, base-4.14.0.0, base-4.13.0.0, 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) [__2] fail (backjumping, conflict set: base, bytestring, paynow-zw) After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: bytestring, base, paynow-zw