Attempted to follow the "Getting Started" instructions, couldn't successfully import MCPrelude.

[derifatives]

I followed the "Getting Started" instructions at https://mightybyte.github.io/monad-challenges/. The cabal install ran for a long time (maybe 20 minutes? I wandered off) but seemed to complete successfully. However, trying to ghci a simple toy file failed to import the prelude:

rif@rifo1:~/tmp/mc/monad-challenges/monad-challenges-code$ cat Set1.hs
{-# LANGUAGE MonadComprehensions #-}
{-# LANGUAGE RebindableSyntax  #-}

module Set1 where

import MCPrelude

fiveRands:: [Integer]
fiveRands = [3, 4, 5]

rif@rifo1:~/tmp/mc/monad-challenges/monad-challenges-code$ ghci Set1.hs
GHCi, version 8.8.4: https://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Set1             ( Set1.hs, interpreted )

Set1.hs:6:1: error:
    Could not find module ‘MCPrelude’
    Perhaps you meant Prelude (from base-4.13.0.0)
    Use -v (or `:set -v` in ghci) to see a list of the files searched for.
  |
6 | import MCPrelude
  | ^^^^^^^^^^^^^^^^
Failed, no modules loaded.
Prelude> 

Is there maybe some missing piece of newbie instructions here? Do I have to somehow add MCPrelude to a path or something?

[derifatives]

It's also possible I've done something previously that messed up my system. I don't really understand Haskell install or what is / isn't hermetic.

[georgefst]

I was just about to open an issue about this. The problem is that the guide uses the old-fashioned way of installing libraries. Cabal has changed a lot in the last few years.

I think the simplest solution (assuming that you're using Cabal 3+, check with cabal -V) would be to create a folder for the Haskell source files you're going to write, then run cabal install --package-env /PATH_TO_YOUR_FOLDER --lib instead of just cabal install. This will create a file called something like .ghc.environment.x86_64-darwin-8.10.3, which will tell calls to ghc and ghci from that folder, where to find MCPrelude.

@mightybyte Should I open a PR modifying/expanding the instructions, or do you have a preferred solution?

[derifatives]

This does seem to work, thanks!

[georgefst]

That's a bizarre error @Aster89.

Is there any chance you've tampered with your ghci? Do you have another ghci on your PATH, or have you set an alias? What happens if you run which ghci?

[Aster89]

@georgefst , I had an alias :/