check-pvp: Check whether module and package imports conform to the PVP

Check whether the version ranges used in the Build-Depends field matches the style of module imports according to the Package Versioning Policy (PVP). See http://www.haskell.org/haskellwiki/Package_versioning_policy. The tool essentially looks for any dependency like containers >=0.5 && <0.7 that allows the addition of identifiers to modules within the version range. Then it checks whether all module imports from containers are protected against name clashes that could be caused by addition of identifiers.

You must run the tool in a directory containing a Cabal package.

$ check-pvp

This requires that the package is configured, since only then the association of packages to modules is known. If you want to run the tool on a non-configured package you may just check all imports for addition-proof style.

$ check-pvp --include-all

It follows a detailed description of the procedure and the rationale behind it.

First the program classifies all dependencies in the Cabal file of the package. You can show all classifications with the --classify-dependencies option, otherwise only problematic dependencies are shown.

A dependency like containers >=0.5.0.3 && <0.5.1 does not allow changes of the API of containers and thus the program does not check its imports. Clashing import abbreviations are an exception.

The dependency containers >=0.5.1 && <0.7 requires more care when importing modules from containers and this is what the program is going to check next. This is the main purpose of the program! I warmly recommend this kind of dependency range since it greatly reduces the work to keep your package going together with its imported packages.

Dependencies like containers >=0.5 or containers >=0.5 && <1 are always problematic, since within the specified version ranges identifier can disappear. There is no import style that protects against removed identifiers.

An inclusive upper bound as in containers >=0.5 && <=0.6 will also cause a warning, because it is unnecessarily strict. If you know that containers-0.6 works for you, then containers-0.6.0.1 or containers-0.6.1 will also work, depending on your import style. A special case of inclusive upper bounds are specific versions like in containers ==0.6. The argument for the warning remains the same.

Please note that the check of ranges is performed entirely on the package description. The program will not inspect the imported module contents. E.g. if you depend on containers >=0.5 && <0.7 but import in a way that risks name clashes, then you may just extend the dependency to containers >=0.5 && <0.7.1 in order to let the checker fall silent. If you use the dependency containers >=0.5 && <0.7.1 then the checker expects that you have verified that your package works with all versions of kind 0.5.x and the version 0.6.0. Other versions would then work, too, due to the constraints imposed by package versioning policy.

Let us now look at imports that must be protected against identifier additions.

The program may complain about a lax import. This means you have imported like

import Data.Map as Map

Additions to Data.Map may clash with other identifiers, thus you must import either

import qualified Data.Map as Map

or

import Data.Map (Map)

The program emits an error on clashing module abbreviations like

import qualified Data.Map.Lazy as Map
import qualified Data.Map.Strict as Map

This error is raised whenever multiple modules are imported with the same abbreviation, where at least one module is open for additions. Our test is overly strict in the sense that it also blames

import qualified Data.Map as Map
import qualified Data.Map as Map

but I think it is good idea to avoid redundant imports anyway.

Additionally there are warnings on imports that are consistent with large version ranges, but complicate API changing updates of your dependencies. You can disable these warnings with --disable-warnings.

The program warns about an open list of constructors as in

import Data.Sequence (ViewL(..))

Additions of constructors to ViewL may also conflict with other identifiers, but additions of constructors are considered API changes since they may turn a complete case analysis into an incomplete one. Similarly additionally class methods can turn a complete class instance into a partial one. Thus addition of constructors and class methods require a version bump from x.y.z to x.y+1. Nonetheless it is a good idea to import either

import Data.Sequence (ViewL(EmptyL, (:<)))

or

import qualified Data.Sequence as Seq

because you document the origin of identifiers this way. This is especially important when the imported identifiers are moved or removed in the future. If you use constructors only for constructions and not for pattern matches or if you only call class methods but do not define instances, then with explicit imports or qualified imports your modules survive such additions in your dependent packages without modifications.

More warnings are issued for hiding imports. The import

import Data.Map hiding (insert)

is not bad in the sense of the PVP, but this way you depend on the existence of the identifier insert although you do not need it. If it is removed in a later version of containers, then your import breaks although you did not use the identifier.

Finally you can control what items are checked. First of all you can select the imports that are checked. Normally the imports are checked that belong to lax dependencies like containers >=0.5 && <0.7. However this requires the package to be configured in order to know which import belongs to which dependency. E.g. Data.Map belongs to containers. You can just check all imports for being addition-proof using the --include-all option. Following you can write the options --include-import, --exclude-import, --include-dependency, --exclude-dependency that allow to additionally check or ignore imports from certain modules or packages. These modifiers are applied from left to right. E.g. --exclude-import=Prelude will accept any import style for Prelude and --exclude-dependency=foobar will ignore the package foobar, say, because it does not conform to the PVP.

Secondly, you may ignore certain modules or components of the package using the options --exclude-module, --exclude-library, --exclude-executables, --exclude-testsuites, --exclude-benchmarks. E.g. --exclude-module=Paths_PKG will exclude the Paths module that is generated by Cabal. I assume that it will always be free of name clashes.

Known problems:

• The program cannot automatically filter out the Paths module.

• The program cannot find and check preprocessed modules.

• The program may yield wrong results in the presence of Cabal conditions.

If this program proves to be useful it might eventually be integrated in the check command of cabal-install. See https://github.com/haskell/cabal/issues/1703.

Alternative: If you want to allow exclusively large version ranges, i.e. >=x.y && <x.y+1, then you may also add the option -fwarn-missing-import-lists to the GHC-Options fields of your Cabal file. See https://ghc.haskell.org/trac/ghc/ticket/4977. Unfortunately there is no GHC warning on clashing module abbreviations. See https://ghc.haskell.org/trac/ghc/ticket/4980.

Related: There are programs that check PVP compliance of exports:

• cabal-bounds: http://hackage.haskell.org/package/cabal-bounds simplifies extending the version bounds in Build-Depends fields.

• precis: http://hackage.haskell.org/package/precis

• apidiff: http://code.haskell.org/gtk2hs/tools/apidiff/