Haskell's "stack" failing. Trouble understanding package system

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1

I’m running NixOS in a virtual machine and I’m having this issue running “stack”:

error: attribute ‘ghc902’ missing, at (string):1:43
(use ‘–show-trace’ to show detailed location information)

I presume this is an issue because it can’t find ghc 9.02

I ran the following nix-env -qaPA 'nixos.haskell.compiler' (which I found on StackOverflow) and got the following result:

nixos.haskell.compiler.ghc8107                 ghc-8.10.7
nixos.haskell.compiler.ghc884                  ghc-8.8.4
nixos.haskell.compiler.ghc901                  ghc-9.0.1
nixos.haskell.compiler.ghc921                  ghc-9.2.1
nixos.haskell.compiler.ghcHEAD                 ghc-9.3.20211111
nixos.haskell.compiler.ghc8102Binary           ghc-binary-8.10.2
nixos.haskell.compiler.ghc8102BinaryMinimal    ghc-binary-8.10.2
nixos.haskell.compiler.ghc8107Binary           ghc-binary-8.10.7
nixos.haskell.compiler.ghc8107BinaryMinimal    ghc-binary-8.10.7
nixos.haskell.compiler.ghc865Binary            ghc-binary-8.6.5
nixos.haskell.compiler.integer-simple.ghc8107  ghc-integer-simple-8.10.7
nixos.haskell.compiler.integer-simple.ghc884   ghc-integer-simple-8.8.4
nixos.haskell.compiler.integer-simple.ghc901   ghc-integer-simple-9.0.1
nixos.haskell.compiler.integer-simple.ghc921   ghc-integer-simple-9.2.1
nixos.haskell.compiler.native-bignum.ghcHEAD   ghc-native-bignum-9.3.20211111
nixos.haskell.compiler.ghcjs                   ghcjs-8.10.7

This seems to confirm that NixOS can’t see ghc 9.0.2, just ghc 9.0.1

But what is weird is that I have ran sudo nixos-rebuild switch --upgrade but this makes no difference.

What makes it even more confusing when I search on Nix packages on the website https://search.nixos.org/packages?channel=unstable&from=0&size=50&sort=relevance&type=packages&query=haskell.compiler I only find such packages in the unstable branch, not the stable branch.

I do have nixos-unstable in my nix-channels:

sudo nix-channel --list

nixos https://nixos.org/channels/nixos-21.11
nixos-unstable https://nixos.org/channels/nixos-unstable

But on the website, search, I see ghc 9.0.2, not ghc 9.0.1.

So I’m thoroughly confused. Anyone have any insights here?

2

Here’s two other threads that are at least somewhat related to what you’re trying to do:

If you can’t figure your problem out after reading through these two threads, feel free to ping me and I’ll try to give you a deeper explanation.

If you do figure out your problem, I encourage you to post your solution so that it helps the next person that stumbles on this post.

3

The stable channel takes precedence, there’s no fallback mechanism.

4

I’ll have a look through those threads.

But a point is raised in Can I use Haskell’s stack with nix run for a resolver not in my current nixpkgs? which I don’t think is answered there, which I quote:

What’s going on with https://search.nixos.org/packages having seemingly the wrong package list?

5

To obtain a list of packages, one has to evaluate nixpkgs, i.e. execute a turing complete programming language to yield a non uniformly structured package set. For performance (of normal nix-env usage) reasons a lot of stuff is hidden by default and has to be explicitly included when building the index for nixos-search. Nowadays, the haskell.compiler sub package set is indexed correctly.

TL;DR: A mixture of trade-offs and tech debt, but has been fixed since.

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