Different imports according to version of dependencies

I have a module that uses Control.Ex开发者_C百科ception in Base < 4 which is Control.OldException in Base >= 4. How can I, using cabal or any other tool, get rid of the version dependency (just depend on Base and not Base < 4) and import Control.OldException when using Base >= 4 and Control.Exception when using Base < 4?

cabal automatically sets certain CPP definitions based on the version of packages used.

So for your case I would:

{-# LANGUAGE CPP #-}
module Blah where
#if MIN_VERSION_base(0,4,0)
import Control.OldException
#else
import Control.Exception
#endif

This method builds fine with cabal.

(actually, I would use new exceptions and wouldn't bother supporting base < 4, but that's just me)

With Cabal this is done with "flags" and its constraint solving algorithm. An example (from control-monad-exception on Hackage):

Flag extensibleExceptions
  description: Use extensible-exception package
  default: False

(...)

  if flag(extensibleExceptions)
    build-depends:
      extensible-exceptions >= 0.1 && <0.2,
      base >= 3.0 && <4
  else
    build-depends:
      base >= 4 && < 5

On a machine with an older version of base, Cabal will try to solve the dependency with extensibleExceptions False, fail, then retry with it True and use the different build-depends, which will succeed. (You can also turn the flag on from the command line.)

http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.html#configurations documents this mechanism, and the rest of the page describes other mechanisms including direct conditionals such as if impl(ghc >= 6.10.0).

This is a language agnostic answer, so it may not apply for you.

There's a couple options

1. Wrap both exceptions in a SuperException that has both implementations. Give it a parameter that tells it what implementation to use based on Base.
2. Refactor Exception to be a child of OldException with overriden calls. (best option)