Cabal project seems to be importing multiple versions of Cabal Library breaking Setup.hs - haskell

Question

Short:
I have a cabal project that depends on a library built using cabal sandbox add-source. This library exports a preprocessor function BuildInfo -> LocalBuildInfo -> PreProcessor. When I try to use that preprocessor in the Setup.hs for my main cabal project, I get the error:
Couldn't match type ‘Cabal-1.18.1.3:Distribution.PackageDescription.BuildInfo’
with ‘BuildInfo’
NB: ‘Cabal-1.18.1.3:Distribution.PackageDescription.BuildInfo’
is defined in ‘Distribution.PackageDescription’
in package ‘Cabal-1.18.1.3’
‘BuildInfo’
is defined in ‘Distribution.PackageDescription’
in package ‘Cabal-1.20.0.1’
Less Short:
I currently have the following directory structure in my cabal sandbox:
├── main.cabal
├── Setup.hs
├── Main.hs
├── lib
│   ├── myPP.cabal
│   ├── MyPP.hs
│   └── myPP
│   ├── MyPP.hs
│   ├── myPP.cabal
My main package has myPP has a build depends. myPP is a library with MyPP as the exposed module and Cabal and ghc as dependencies. the MyPP module exports a function
myPP :: BuildInfo -> LocalBuildInfo -> PreProcessor
I then run
cabal sandbox add-source ./lib/*
cabal install myPP
In my Setup.hs I have:
module Main (main) where
import Distribution.Simple
import Distribution.Simple.PreProcess
import Distribution.Simple.Utils
import Distribution.PackageDescription
import Distribution.Simple.LocalBuildInfo
import MyPP(myPP)
main :: IO ()
main = let hooks = simpleUserHooks
pp = ("pp",myPP)
in defaultMainWithHooks hooks {hookedPreProcessors = pp:knownSuffixHandlers}
I then attempt to actually cabal build only to get the error:
Couldn't match type ‘Cabal-1.18.1.3:Distribution.PackageDescription.BuildInfo’
with ‘BuildInfo’
NB: ‘Cabal-1.18.1.3:Distribution.PackageDescription.BuildInfo’
is defined in ‘Distribution.PackageDescription’
in package ‘Cabal-1.18.1.3’
‘BuildInfo’
is defined in ‘Distribution.PackageDescription’
in package ‘Cabal-1.20.0.1’
Attempt to diagnose the error:
My understanding is that when you have a package that depends on both ghc and Cabal, then you get the version of Cabal that is linked to when ghc was built. As a result, the myPP package is built with that old version of cabal. When the custom Setup.hs is run by cabal, it imports its version of cabal (which is the one that I have installed). This then causes the error. However, I am hoping that there is a better solution than to just downgrade my version of Cabal....

Answer 1

You won't like the other solution: build ghc from the current HEAD in order to get a newer Cabal (now at 1.21.1.0). Unfortunately, there's just no way to make two different versions of a package coexist in a single build. Downgrading Cabal is probably the least-painful option.