Upgrading packages/Updating to GHC 6.10

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 6.8

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.

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.

ghc --make

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

   ghc --make --package base-3.0.3.0

runhaskell

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

   --constraint="base<4"

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

   --constraint="base<4"

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.

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.

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.