Installation in Arch Linux

[Xavier Góngora]

Problem

I manage to successfully install Euterpea following the instructions on the web page but I wasn´t able to import it into ghci.

Context

Kernel: 6.0.6-zen1-1-zen
DE: KDE Plasma
OS: Garuda Linux

To reproduce

1. Make sure to not have any previous configuration for the Haskell toolchain. In a terminal run rm -rf .cabal .ghc
2. Install ghcup from upstream script
3. In a terminal run ghcup tui and install ghc 8.10.7. During ghcup installation you should be prompted for cabal installation, which at this time installs 9.2.4 by default (which is ok, you can have more than one version of ghc without conflict)
4. From within ghcup tui set ghc 8.10.7
5. Clone this repo, and from inside its location run cabal v1-install --allow-newer (without the last flag it fails in my system).
6. Run ghci and then import Euterpea

This results in

$ ghci
Loaded package environment from /home/user/.ghc/x86_64-linux-9.2.4/environments/default
GHCi, version 9.2.4: https://www.haskell.org/ghc/  :? for help
ghci> import Euterpea

<no location info>: error:
Could not find module ‘Euterpea’
It is not a module in the current program, or in any known package.

Using the cabal v1 repl also fails: cabal v1-repl and then import Euterpea results in the same error.

Of notice:

• I'm have the Haskell toolchain installed vía ghcup and use statically linked packages installed from cabal only (with no Arch Linux official repository haskell-* packages).
• The package isn't displayed in cat ~/.ghc/x86_64-linux-8.10.7/environments/default nor in ls ~/.cabal/store/ghc-8.10.7/package.db/ even though ghc-pkg list lists Euterpea-2.0.8 and the library also appears in ls .cabal/lib/x86_64-linux-ghc-8.10.7/
• After this steps I followed the same installation procedure using ghc 9.2.4 (by setting it in ghcup tui) to the same results.

What worked

After all this (without clearing the previous steps) I set ghc 8.10.7 back(because most of my installed packages use this version). I then tried the following command:

cabal install Euterpea --lib --allow-newer

Again, without the last flag the command fails. With this the package is downloaded from Hackage instead of this repo.
I haven´t tried this command to install from the repo directory, but it should also work.

Now I was able to import the library and Euterpea appears in:

ls .cabal/store/ghc-8.10.7/package.db/

and

cat ~/.ghc/x86_64-linux-8.10.7/environments/default

Suggestion

I don´t get exactly how the different cabal commands behave and why they differ in results, neither where this issue should be fixed (the Euterpea.cabal? or all the way into the cabal build system or command?).

For the moment I suggest adding the above successful command as an option in the installation instructions.

[Donya Quick]

Thanks for the very detailed info! Interesting...the --lib flag is new to me. A long time ago when just a regular "cabal install" ceased to install things globally, I was looking for something to solve that and never encountered --lib as a suggestion for whatever reason. I just did a quick test and it looks like "cabal install --lib --allow-newer" also works on Windows to install from a local repo copy (I'm also using ghc 8.10.7 currently; haven't tested any other versions). If it's a broader solution across some older versions then I like that a lot better than the v1-install and v2-install things. I've updated the install instructions on euterpea.com for Linux for the moment but it looks like --lib may be the way to go for the other OSs too.

[Donya Quick]

Well, I guess I was too optimistic too quickly. Apparently --lib allows it to import into ghci on Windows but installing another library that depends on it (such as HSoM) will fail because the library isn't found. You may unfortunately find you have the same problem on Linux - if not, please do let me know. I will still keep the flag listed as an option on the website since the most common install scenario is users just wanting Euterpea working in GHCi.

Still a frustrating outcome. The description of --lib says "By default, it is writing to the global environment" but apparently not the way v1-install does.

Two related things I found...
haskell/cabal#7373
haskell/cabal#5559

The 2nd one is an open issue. So I guess there really is still just v1-install as the broadest solution even if --lib helps in some instances where it fails (but then --lib comes with its own problems).

EDIT: I should note that when I say --lib causes the library to not be found for other installs, Euterpea is missing with a "ghc-pkg list" command.

[Donya Quick]

Since --lib does not appear to be a complete solution (i.e. permitting install of HSoM too, not just making Euterpea work with GHCi) without messing around with ghc environments, I have moved mention of the flag for Linux to the Troubleshooting page on the Euterpea website rather than putting it on the main page. I have some concern that if it's on the main page, users who may simply need v1-style commands with --allow-newer might go for --lib and have more problems. I will leave the issue open for now since it seems like there should be a better solution.

[Xavier Góngora]

Thanks for looking into it. I'm going through the book (which is awesome) and still haven´t need the HSoM, but I'll gladly do some further testing in my system and come back.