Resolving dependencies...
cabal: Could not resolve dependencies:
next goal: Pugs (user goal)
rejecting: Pugs-6.2.13.20150815, 6.2.13.20130611, 6.2.13.20120717,
6.2.13.20120203, 6.2.13.20111008, 6.2.13.20110925 (global constraint requires
==6.2.13.20110519)
trying: Pugs-6.2.13.20110519
trying: pugs-DrIFT-2.2.3.20150815 (dependency of Pugs-6.2.13.20110519)
next goal: mtl (dependency of Pugs-6.2.13.20110519)
rejecting: mtl-2.2.1, 2.2.0.1, 2.2, 2.1.3.1, 2.1.2, 2.1.1, 2.1, 2.0.1.1,
2.0.1.0, 2.0.0.0 (conflict: Pugs => mtl<2.0.0.0)
rejecting: mtl-1.1.1.1, 1.1.1.0, 1.1.0.2, 1.1.0.1, 1.1.0.0, 1.0 (conflict:
pugs-DrIFT => mtl>=2.0.0.0)
Dependency tree exhaustively searched.