-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 10/2/10 18:07 , Christian Höner zu Siederdissen wrote:
cd syb-0.2.1 * remove base<4.3 constraint from syb.cabal cabal install * syb-0.2.1 is now installed and works!
$ cabal install parsec-3.1.0 cabal: cannot configure syb-0.2.1. It requires base >=4.0 && <4.3
This is rather annoying: the constraint is solved (eg. syb is installed) but we /still/ assume that we can not continue. So obviously, I would like to be able to have cabal assume constraints fullfilled if the package is installed.
This is unsafe because the package database (which isn't maintained by cabal, but by ghc; see the ghc-pkg command) doesn't store the constraints. So cabal can't tell if that *particular* build was against the version of base it needs for everything else. (ghc can have multiple versions of base around; see the 6.10 series. I *think* ghc is constrained to only supporting it for different major versions, but it can't tell Cabal that.) Now, it is arguable that if there is no different release of base in the package database then it should assume the visible release is the one used; but you again run into the limitations of ghc-pkg. If I "ghc-pkg hide" a given package release, dependents of that package don't also get hidden because (again) ghc-pkg doesn't store that information. So now you end up with the same problem except it will show up when you try to use one of the dependents, and (worse) because Cabal can't tell this happened you will at best get an unexpected type mismatch at compile time and at worst the program will compile but dump core (if the mismatch is in base. It's hard to validate that runtimes are compatible, and base is very tightly bound to the runtime). - -- brandon s. allbery [linux,solaris,freebsd,perl] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkynvHgACgkQIn7hlCsL25XttACfQ+9G4bIAOnQVwJddDBizaPba uU4AoJjFXPkeqcloqp8w2COQ4I0bh+nk =sZhw -----END PGP SIGNATURE-----