[Haskell-cafe] Best approach to avoid dependency hells

[Albert Y. C. Lai]

On 12-12-08 07:39 AM, Ivan Perez wrote:
> When you install A, you may not know that you'll need to depend on a
> lower version of bytestring later on. Cabal will pick the highest
> version available (0.10 if present). If a program you install later on
> depends on A (needs  bytestring-0.10) and ghc (needs bytestring-0.9),
> you'll have a conflict.

You are saying, before "cabal install A", you already have both 
bytestring 0.9.2 and 0.10 installed. This is new data not revealed last 
time. And it is important, this is exactly the cause, you have too many 
bytestring's installed.

(You cannot be saying, you start without bytestring 0.10, and "cabal 
install A" brings in bytestring 0.10. You start with GHC, and since it 
depends on bytestring 0.9.2, you start with bytestring 0.9.2 too. And 
you don't already have another bytestring. Then "cabal install A" will 
stick with bytestring 0.9.2.)