What is the relationship between ghc-pkg and cabal?

Question

With respect to how packages are created, installed and used in Haskell, what is the relationship between ghc-pkg and cabal?

What are their roles - when would you use on

Answer 1

Graphically, the dependencies are:

Packages GHC can use
    | 
Are registered with "ghc-pkg register"
    |
And (almost always) built with Cabal
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With build dependencies resolved by cabal-install
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From Hackage.