I just tried creating my first package with Cabal, and chose a package version, "0", that obeys the rules documented by Cabal: "a sequence of one or more integers separated by dots"[1]. But at the end of the build, I got "ghc-pkg: ambiguous package identifier: foo-0". (I am using ghc 6.4.2.) Simon Marlow already commented on this silly problem[2], but it appears it was never resolved. Can someone just decide on the systax of a package identifier? Cabal's documented rules seem sane to me: A package is identified by a globally-unique package name, which consists of one or more alphanumeric words separated by hyphens. To avoid ambiguity, each of these words should contain at least one letter.... A particular version of the package is distinguished by a version number, consisting of a sequence of one or more integers separated by dots. These can be combined to form a single text string called the package ID, using a hyphen to separate the name from the version, e.g. “HUnit-1.1”. Andrew [1] http://www.haskell.org/ghc/docs/latest/html/Cabal/ [2] http://www.haskell.org/pipermail/glasgow-haskell-users/2005-February/007801....