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Upgrading packages/Updating to GHC 6.10

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A list of things that need updating when porting packages to newer library/cabal versions.

If you maintain a Haskell package this is for you. For older versions of this document:

Updating to GHC 6.10 and Cabal 1.6

When upgrading to GHC 6.10, any of your packages that worked against the base-3 library will continue to work. GHC 6.10 provides both the old base-3 library and the new base-4.

To ensure your old code continues to work, you can have the code compile and link against base-3, and then, over time, migrate code to the base-4 series.


1 Adding base-3 constraints

How to do this depends on how you build your Haskell code. We'll start with the most simplistic build mechanisms. cabal-install, the most sophisticated tool, will sort this all out for you anyway, so things should change.

1.1 ghc --make

Force use of package base-3 when using --make,

   ghc --make --package base-

1.2 runhaskell

If you build your packages with the 'runhaskell Setup.hs configure' method, then you can force the use of base-3,


1.3 cabal-install

It is worth upgrading cabal-install immediately (maybe before installing GHC). This way you can use the smart dependency solver to work out what to install for you.

* install HTTP from hackage
* install zlib from hackage

Then build cabal-install.

You can also override the dependencies when using the 'cabal' binary, with


2 Typical breakages with GHC 6.10

54 new packages break with the current GHC 6.10 release candidate.

The primary reasons are:

  • Changes to Arrow class definition
  • Changes to Map monadic types
  • Cabal changes
  • Changes to ghc-api
  • Changes to when 'forall' is parsed
  • GHC.Prim was moved,
  • Changes to -fvia-C and headers?
  • GADT changes,
  • pragma warnings tightened
  • Integer constructors have moved
  • New warnings and used -Werror
  • Exception handling: catch, finally, throwIO don't work anymore

Each of these has a standard way to solve the problem. Techniques should be attached here.

2.1 Changes to Arrow class definition

The relevant change is essentially that Arrow became a superclass of Category. To be exact:

  • (>>>) was removed from Arrow, instead we have (.) in Category
  • id is a new function, in Category

What this means for code is that you need to declare an instance of Category as well as Arrow. A brute-force fix which should work:

  • Add the following imports:
   import Control.Category
   import Prelude hiding (id,(.)) -- conflicts with Category otherwise
  • Add instance Category [your type] where for any Arrows
  • Move your (>>>) definition into Category, and change f >>> g = ... into g . f = ...
  • Define id in Category. (Actually, I'm not 100% sure if this is strictly necessary for code that used to work in 6.8, but it's safest.)

And you're done.

3 Backwards compatibility

The new, suggested Cabal version range syntax,

  build-depends: base-3.*

is not backwards compatible with older versions of Cabal. Users will need to upgrade to the newer Cabal to build packages that start using this syntax.