Hiding versions of random from GHCi

问题

I am working with the newsynth package and am running into a bug that I am starting to suspect has to do with the update random-1.2.0 over the summer (newsynth was last updated in late 2019, and random-1.2.0 came out in June. I have run cabal update since then, which is why both seem to be installed.) Here is the code that I ran in GHCi:

 λ> import System.Random
 λ> import Quantum.Synthesis.Ring
 λ> import Quantum.Synthesis.Diophantine
 λ> g <- getStdGen
 λ> diophantine g (RootTwo 5 0)

<interactive>:5:1: error:
    • No instance for (random-1.1:System.Random.RandomGen StdGen)
        arising from a use of ‘diophantine’
      There are instances for similar types:
        instance random-1.1:System.Random.RandomGen
                   random-1.1:System.Random.StdGen
          -- Defined in ‘random-1.1:System.Random’
    • In the expression: diophantine g (RootTwo 5 0)
      In an equation for ‘it’: it = diophantine g (RootTwo 5 0)

As was suggested to me in another SO post, I tried hiding the other copy of random from GHCi. I ran

$ ghc-pkg --global --package-db ~/.cabal/store/ghc-8.8.3/package.db list random
~/.ghcup/ghc/8.8.3/lib/ghc-8.8.3/package.conf.d
    (no packages)
~/.cabal/store/ghc-8.8.3/package.db
    random-1.1
    random-1.2.0

so I tried $ ghc-pkg --global --package-db ~/.cabal/store/ghc-8.8.3/package.db hide random-1.2.0. However, when the bug persisted, I also tried $ ghc-pkg --global --package-db ~/.cabal/store/ghc-8.8.3/package.db hide random-1.1 (just to make sure hide did what I thought it would), yet inexplicably found that in GHCi (even after restarting Terminal) I was able to run λ> import System.Random despite all known instances of random being hidden. Am I hiding packages in the wrong way?

Thank you in advance.

(I asked a somewhat related question the other day but since I think this issue is kinda different I thought it made sense be separate. If this is not the case I can update the other post.)

回答1:

Use cabal or stack when you want to manage packages. But first, reset to clean state:

rm -rf ~/.ghc ~/.cabal
cabal update
cabal v2-repl -b newsynth

This will get you into a GHCi session with the desired package available.

回答2:

I you want to work directly in ghci without going through cabal repl or stack ghci, one way is to create a local GHC package environment in a folder, using cabal-install:

cabal install --lib --constraint="random == 1.1" --package-env .  random

Besides "random" itself, you'll need to explicitly list other packages that you want to be present in the environment. Likewise, you might add multiple --constraint arguments.

This will create a file named ".ghc.environment.xxx" in the folder, which should be picked up by standalone ghci or ghc invocations made there.

来源：https://stackoverflow.com/questions/64362142/hiding-versions-of-random-from-ghci

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