How does dependency hell happen in Cabal-install?
I read the following at Cabal/Survival - HaskellWiki:
1. What is the difficulty caused by Cabal-install?
The main difficulty with Cabal is otherwise known as 'dependency hell', in which the cabal-install does not manage to install a desired package for a reason or another, leading to large amount of manual work. As an example of this difficulty, consider a case where the user wishes to install packages A and B. Both of these work with package C, but not with the same version of C.
I understand why this would make sense generally, but not with Cabal-install/ghc-pkg, because you can have multiple versions of the same package installed.
It's as if every version is an entirely different package, with how that in many respects the version becomes a part of the package name (e.g. mustaches-0.1.0.0) -- and maybe it really is for ghc-pkg (I'm not entirely familiar with it, but it would make sense).
You have scenarios like this:
Where both B and C depend on A. However, if they were installed at different times, they may depend on different versions of A. For example, A version 1 export type T = Int, but in version 2 it exports type T = Bool.
Only when you try to build D do you expose the problem that B and C were build against different versions of A, and you can't compare T version 1 against T version 2.
The problem is that you cannot link your program against those different versions of package C. A and B have to find a common version of C to use the same implementation of a certain function. One solution to this problem is OSGi but requires stuff like Classloaders which can be used to load different versions of package C in the same process w/o conflicts.
One way dependency hell happens is when several different projects you are working on interfere. While a single project has a solvable constraint set, two different project might not and since they're sharing a single package DB, there will be trouble. This variant of dependency hell is solved by cabal sandboxes.
来源：https://stackoverflow.com/questions/25869041/whats-the-reason-behind-cabal-dependency-hell