In ghc 7.8.3 on the Mac, for some packages cabal reinstalls the package instead of telling me I've already installed it. ghc-pkg check shows I have no issues, e.g.: bash-3.2$ cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 Updating documentation index /Users/gcolpitts/Library/Haskell/share/doc/index.html bash-3.2$ cabal install cabal-install Resolving dependencies... Configuring cabal-install-1.20.0.3... Building cabal-install-1.20.0.3... Installed cabal-install-1.20.0.3 Updating documentation index /Users/gcolpitts/Library/Haskell/share/doc/index.html bash-3.2$ ghc-pkg check bash-3.2$ Other packages that have this problem include alex, and an example of a package that doesn't have this problem is vector. Should I file a bug? bash-3.2$ ghc --info [("Project name","The Glorious Glasgow Haskell Compilation System") ,("GCC extra via C opts"," -fwrapv") ,("C compiler command","/usr/bin/gcc") ,("C compiler flags"," -m64 -fno-stack-protector") ,("C compiler link flags"," -m64") ,("Haskell CPP command","/usr/bin/gcc") ,("Haskell CPP flags","-E -undef -traditional -Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs") ,("ld command","/usr/bin/ld") ,("ld flags"," -arch x86_64") ,("ld supports compact unwind","YES") ,("ld supports build-id","NO") ,("ld supports filelist","YES") ,("ld is GNU ld","NO") ,("ar command","/usr/bin/ar") ,("ar flags","clqs") ,("ar supports at file","NO") ,("touch command","touch") ,("dllwrap command","/bin/false") ,("windres command","/bin/false") ,("libtool command","libtool") ,("perl command","/usr/bin/perl") ,("target os","OSDarwin") ,("target arch","ArchX86_64") ,("target word size","8") ,("target has GNU nonexec stack","False") ,("target has .ident directive","True") ,("target has subsections via symbols","True") ,("Unregisterised","NO") ,("LLVM llc command","llc") ,("LLVM opt command","opt") ,("Project version","7.8.3") ,("Booter version","7.6.3") ,("Stage","2") ,("Build platform","x86_64-apple-darwin") ,("Host platform","x86_64-apple-darwin") ,("Target platform","x86_64-apple-darwin") ,("Have interpreter","YES") ,("Object splitting supported","YES") ,("Have native code generator","YES") ,("Support SMP","YES") ,("Tables next to code","YES") ,("RTS ways","l debug thr thr_debug thr_l thr_p dyn debug_dyn thr_dyn thr_debug_dyn l_dyn thr_l_dyn") ,("Support dynamic-too","YES") ,("Support parallel --make","YES") ,("Dynamic by default","NO") ,("GHC Dynamic","YES") ,("Leading underscore","YES") ,("Debug on","False") ,("LibDir","/Library/Frameworks/GHC.framework/Versions/7.8.3-x86_64/usr/lib/ghc-7.8.3") ,("Global Package DB","/Library/Frameworks/GHC.framework/Versions/7.8.3-x86_64/usr/lib/ghc-7.8.3/package.conf.d") ] bash-3.2$ ghc-pkg list /Library/Frameworks/GHC.framework/Versions/7.8.3-x86_64/usr/lib/ghc-7.8.3/package.conf.d: Cabal-1.18.1.4 GLURaw-1.4.0.1 GLUT-2.5.1.1 HTTP-4000.2.10 HUnit-1.2.5.2 OpenGL-2.9.2.0 OpenGLRaw-1.5.0.0 QuickCheck-2.6 array-0.5.0.0 async-2.0.1.5 attoparsec-0.10.4.0 base-4.7.0.1 bin-package-db-0.0.0.0 binary-0.7.1.0 rts-1.0 bytestring-0.10.4.0 case-insensitive-1.1.0.3 containers-0.5.5.1 deepseq-1.3.0.2 directory-1.2.1.0 fgl-5.5.0.1 filepath-1.3.0.2 (ghc-7.8.3) ghc-prim-0.3.1.0 hashable-1.2.2.0 haskeline-0.7.1.2 haskell-src-1.0.1.6 (haskell2010-1.1.2.0) (haskell98-2.0.0.3) hoopl-3.10.0.1 hpc-0.6.0.1 hscolour-1.20.3 html-1.0.1.2 integer-gmp-0.5.1.0 mtl-2.1.3.1 network-2.4.2.3 old-locale-1.0.0.6 old-time-1.1.0.2 parallel-3.2.0.4 parsec-3.1.5 pretty-1.1.1.1 primitive-0.5.2.1 process-1.2.0.0 random-1.0.1.1 regex-base-0.93.2 regex-compat-0.95.1 regex-posix-0.95.2 split-0.2.2 stm-2.4.2 syb-0.4.1 template-haskell-2.9.0.0 terminfo-0.4.0.0 text-1.1.0.0 time-1.4.2 transformers-0.3.0.0 unix-2.7.0.1 unordered-containers-0.2.4.0 vector-0.10.9.1 xhtml-3000.2.1 zlib-0.5.4.1 /Users/gcolpitts/.ghc/x86_64-darwin-7.8.3/package.conf.d: Cabal-1.20.0.2 MonadRandom-0.3 aeson-0.6.2.1 auto-update-0.1.1.2 bifunctors-4.1.1.1 blaze-builder-0.3.3.3 cmdargs-0.10.9 comonad-4.2.2 conduit-1.2.0.2 contravariant-1.2 cpphs-1.18.5 data-ordlist-0.4.6.1 distributive-0.4.4 djinn-ghc-0.0.2.2 djinn-lib-0.0.1.2 dlist-0.7.1 either-4.3.1 exceptions-0.6.1 free-4.9 ghc-mod-5.0.1.2 ghc-paths-0.1.0.9 ghc-syb-utils-0.2.2 haskell-src-exts-1.15.0.1 hasktags-0.69.0 hlint-1.9.4 hoogle-4.2.34 http-date-0.0.4 http-types-0.8.5 io-choice-0.0.5 json-0.7 lifted-base-0.2.3.0 mmorph-1.0.4 monad-control-0.3.3.0 monad-journal-0.2.3.2 mwc-random-0.13.2.0 nats-0.2 polyparse-1.9 prelude-extras-0.4 profunctors-4.2.0.1 resourcet-1.1.2.3 safe-0.3.8 scientific-0.3.3.1 semigroupoids-4.2 semigroups-0.15.3 shake-0.13.2 simple-sendfile-0.2.17 streaming-commons-0.1.4.2 strict-0.3.2 stylish-haskell-0.5.10.2 tagged-0.7.2 tagsoup-0.13.2 text-1.2.0.0 transformers-base-0.4.3 transformers-compat-0.3.3.4 uniplate-1.6.12 unix-compat-0.4.1.3 unordered-containers-0.2.5.0 utf8-string-0.3.8 vault-0.3.0.3 vector-0.10.11.0 vector-algorithms-0.6.0.3 void-0.6.1 wai-3.0.1.1 warp-3.0.1.1 yaml-0.8.9.1 bash-3.2$