How to track this error down? When not using cabal adding OldNew.hs to the include path everything is fine --- command run ------ $ rm -fr dist; cWC; cat MW/OldNew.hs; ghc --show-iface dist/build/MW/OldNew.hi; cd testapp/; cat test.hs; ghc -package mwutil --make test.hs' &> /tmp/f cWC does ./setup configure, build, install notice the cd testapp to ensure OldNew.hs is not in include path. -- output annotated in haskell style -------------- {- configuring and instaling succesfully: -} setup: No command given (try --help) setup: Warning: No license-file field. Configuring mwutil-0.1... configure: Dependency base-any: using base-2.0 configure: Dependency haskell98-any: using haskell98-1.0 configure: Using install prefix: /usr/local configure: Binaries installed in: /usr/local/bin configure: Libraries installed in: /usr/local/lib/mwutil-0.1/ghc-6.6 configure: Private binaries installed in: /usr/local/libexec configure: Data files installed in: /usr/local/share/mwutil-0.1 configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.6 configure: Using package tool: /usr/bin/ghc-pkg configure: Using ar found on system at: /usr/bin/ar configure: Using haddock found on system at: /usr/local/bin/haddock configure: No pfesetup found configure: Using ranlib found on system at: /usr/bin/ranlib configure: Using runghc found on system at: /usr/bin/runghc configure: Using runhugs found on system at: /usr/bin/runhugs configure: Using happy: /usr/bin/happy configure: Using alex: /usr/bin/alex configure: Using hsc2hs: /usr/bin/hsc2hs configure: Using c2hs: /usr/bin/c2hs configure: No cpphs found configure: Using greencard: /usr/local/bin/greencard Preprocessing library mwutil-0.1... Building mwutil-0.1... [1 of 1] Compiling MW.OldNew ( MW/OldNew.hs, dist/build/MW/OldNew.o ) /usr/bin/ar: creating dist/build/libHSmwutil-0.1.a Installing: /usr/local/lib/mwutil-0.1/ghc-6.6 & /usr/local/bin mwutil-0.1... Registering mwutil-0.1... Reading package info from ".installed-pkg-config" ... done. Saving old package config file... done. Writing new package config file... done. {- the file causing the error -} module MW.OldNew where data Old a = Old a data New a = New a data OldNew a = OldNew a a; unOld :: Old a -> a unOld (Old o) = o unNew :: New n -> n unNew (New n) = n a=a+1 {- output of --show-iface -} interface mwutil:MW.OldNew 1 6060 where export mwutil:MW.OldNew OldNew|{OldNew OldNew} a Old|{Old Old} New|{New New} unOld unNew module dependencies: package dependencies: base orphans: base:GHC.Base base:GHC.ForeignPtr data OldNew a Variances [(True, False)] RecFlag NonRecursive Generics: no = OldNew a a Stricts: _ _ data New a Variances [(True, False)] RecFlag NonRecursive Generics: no = New a Stricts: _ data Old a Variances [(True, False)] RecFlag NonRecursive Generics: no = Old a Stricts: _ unOld :: Old a -> a unNew :: New n -> n a :: base:GHC.Num.Integer {- the test app -} module Main where import MW.OldNew main = do print "test" {- compiling test app with -package mwutil -} [1 of 1] Compiling Main ( test.hs, test.o ) test.hs:2:0: Bad interface file: /usr/local/lib/mwutil-0.1/ghc-6.6/MW/OldNew.hi Something is amiss; requested module mwutil-0.1:MW.OldNew differs from name found in the interface file mwutil:MW.OldNew Marc