Could not find module ‘Control.Monad.State’ even though mtl is installed

Question

When I tried loading a module containing import Control.Monad.State I got

Could not find module ‘Control.Monad.State’
Perhaps you meant
  Control.Monad.ST (from base-4.8.2.0)
  Control.Monad.ST.Safe (from base-4.8.2.0)
  Control.Monad.Fix (from base-4.8.2.0)
Use -v to see a list of the files searched for.
Failed, modules loaded: none.

I installed the Haskell Platform, which has cabal and mtl installed.

When I run cabal update and then cabal install mtl:

Resolving dependencies...
All the requested packages are already installed:
mtl-2.2.1
Use --reinstall if you want to reinstall anyway.

I tried the reinstall flag but it just keeps giving me warnings that I will break other packages so I didn't continue.

My ghc --version is version 7.10.3. cabal --version is 1.22.5.0

This is what I got when I ran ghc-pkg list mtl:

WARNING: cache is out of date: Library/Frameworks/GHC.framework/Versions/7.10.3-x86_64/usr/lib/ghc-7.10.3/package.conf.d/package.cache
ghc will see an old view of this package db. Use 'ghc-pkg recache' to fix.
/Library/Frameworks/GHC.framework/Versions/7.10.3-x86_64/usr/lib/ghc-7.10.3/package.conf.d
   mtl-2.2.1

And when I ran ghc-pkg check :

WARNING: cache is out of date: /Library/Frameworks/GHC.framework/Versions/7.10.3-x86_64/usr/lib/ghc-7.10.3/package.conf.d/package.cache
ghc will see an old view of this package db. Use 'ghc-pkg recache' to fix.

And when I ran ghc-pkg recache

ghc-pkg: /Library/Frameworks/GHC.framework/Versions/7.10.3-x86_64/usr/lib/ghc-7.10.3/package.conf.d/package.cache: you don't have permission to modify this file

How did you run the compiler in order to get the first message? I mean the exact command. — Bartek Banachewicz, Jan 07 '16 at 11:51
I did `:l Editor.hs` to load the Editor module. I got the same message when I did `import Control.Monad.State`. — mehnj, Jan 07 '16 at 11:59
I did `$ ghci`. I also tried `$ cabal repl` but didn't work. — mehnj, Jan 07 '16 at 12:20
Possible explanation: `mtl` is installed, but hidden. Alternate possible explanation: `cabal` is using a different version of `ghc` (at your hidden, forgotten request). The output of `ghc-pkg list mtl` would help for evaluating both hypotheses. — Daniel Wagner, Jan 07 '16 at 18:05
try `$ ghci -package mtl`. If you have a `.cabal` file include `mtl` in the required packages field. — vivian, Jan 07 '16 at 22:29
Did you try `sudo ghc-pkg recache`? You might need temporarily elevated privileges to write the cache file, since the path to it is in a Mac OS system directory (`/Library/...`). — scooter me fecit, Jan 08 '16 at 01:28
I just tried `sudo ghc-pkg recache` and it worked! Still no idea what happened though — mehnj, Jan 08 '16 at 01:30
Open a haskell-platform bug, if you can reproduce it. Seems like the same issue as https://github.com/haskell/haskell-platform/issues/189, but then for Mac. — thomie, Jan 08 '16 at 01:41

Could not find module ‘Control.Monad.State’ even though mtl is installed

0 Answers0

Linked