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July 2009

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41 discussions

base library and GHC 6.12
by Ian Lynagh 01 Jul '09

01 Jul '09

Hi all, There has been some discussion recently about the base library. In particular, base is supposed to follow the PvP, which means that certain sorts of changes require increasing the version number. When the version number is increased, this makes work for lots of people, as any packages correctly using an upper bound on base's version number need to be updated. For modules like Data.List this makes sense, but base also contains a number of modules in the GHC.* hierarchy which, while exposed, are really internal, and much less stable than the "public" API. There is also another issue: Currently, it is not possible for a package to specify, in its dependencies, whether or not it uses these GHC.* modules. We therefore can't easily tell whether a package is sensitive to changes in those modules, or whether it can be used with hugs, nhc98, etc. We've come up with 3 possible ways forward. Comments, suggestions and criticisms welcomed! Option 1 -------- In order to solve the version number issue, we could simply state that "base follows the PvP, but only for shared module hierarchies". However, it would be impossible for packages which /do/ need GHC.* modules to give accurately versioned dependencies, and it wouldn't solve the other issue at all. Option 2 -------- Another possible solution would be to rename the base package to base-internals, and to make base only re-export the "public" modules. This would require either renaming each module M to Internal.M, or for every implementation to support something like GHC's PackageImports extension. Option 3 -------- The other alternative is to try to split base into two parts: The shared "public" modules, and the internal GHC.* modules. Then GHC would have ghc-base, hugs would have hugs-base, etc, and there would be a common base package built on top of the appropriate impl-base. To do this with minimal loss of code sharing is a large task, and hard to just do in a branch, as merging changes made in the base HEAD is a pain when the file the patch applies to has moved to another repository. Thus in the short term we would expect to give up a reasonable amount of code sharing, but we hope that once we have separate impl-base packages it will be easier to untangle their contents (as impl-base will be significantly smaller than base currently is, and because we can rearrange imports and code inside impl-base without having to worry about breaking other implementations), and then regain as much sharing as possible. I've had a look at what can be done, and the first cut looks like this: We start off with 143 modules in base, 89 of which are public and 54 of which are GHC.*. Afterwards, base has 53 modules, 2 of which are GHC.*: * GHC.Exts, which could be moved into ghc-base, but would take Data.String with it, or it could go into a ghc-exts package. As this is "more public" than the other GHC.* modules, this seems like a reasonable thing to do anyway * GHC.Desugar, which could be put into its own package if nothing else Here's the module graph, which looks quite sane: http://community.haskell.org/~igloo/base-small.png That leaves 90 modules in ghc-base, 52 of which are GHC.*, and 38 of which are in the portable namespace. So almost all GHC.* modules have moved, and more than half of the portable modules are in base. These 38 are the interesting ones: Control.Exception.Base Control.Monad Data.Bits Data.Char Data.Dynamic Data.Either Data.HashTable Data.Int Data.List Data.Maybe Data.Tuple Data.Typeable Data.Word Foreign Foreign.C Foreign.C.Error Foreign.C.String Foreign.C.Types Foreign.ForeignPtr Foreign.Marshal Foreign.Marshal.Alloc Foreign.Marshal.Array Foreign.Marshal.Error Foreign.Marshal.Pool Foreign.Marshal.Utils Foreign.Ptr Foreign.StablePtr Foreign.Storable Numeric System.IO.Error System.IO.Unsafe System.Posix.Internals System.Posix.Types Text.ParserCombinators.ReadP Text.ParserCombinators.ReadPrec Text.Read.Lex Text.Show Unsafe.Coerce Some of them are easy to move back into base, e.g. Foreign contains no code, and Numeric is only needed so that GHC.Ptr can use showHex for its Show instance. Some are more integral to the implementation of the rest of GHC.*. I've only tried this for amd64/Linux, so it's possible that dependencies on other platforms could cause additional problems. Here's the module graph, which is somewhat messier than base's: http://community.haskell.org/~igloo/ghc-base-small.png As with option 2, for each implementation, either impl-base needs to rename the public modules M to Impl.M, or it needs to implement something like GHC's PackageImports extension. Thanks Ian

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