And entering the nix-shell works. But then attempting to run cabal repl fails with this:

Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] next goal: mandelbrot (user goal)
[__0] rejecting: mandelbrot-0.1.0.0 (conflict: requires GHC2024 which is not supported)
[__0] fail (backjumping, conflict set: mandelbrot)
After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: mandelbrot

In reply to @mequbic:matrix.org
Does cabal not support GHC2024 yet?

GHC2024 is only supported if you have at least version 9.10 of ghc, according to a warning when initialising projects with cabal. nix installs 9.8.4 by default though.

When I explicitly install a newer version using nix-shell -p cabal-install haskell.compiler.ghc9122, which installs ghc version 9.12.2, it works.

(Well "works": I get a different error, which could be resolved with changing base ^>= 4.19.2.0 to base >= 4.19.2.0 in mandelbrot.cabal, but that's a different topic)

In reply to @le:4d2.org

GHC2024 is only supported if you have at least version 9.10 of ghc, according to a warning when initialising projects with cabal. nix installs 9.8.4 by default though.

When I explicitly install a newer version using nix-shell -p cabal-install haskell.compiler.ghc9122, which installs ghc version 9.12.2, it works.

(Well "works": I get a different error, which could be resolved with changing base ^>= 4.19.2.0 to base >= 4.19.2.0 in mandelbrot.cabal, but that's a different topic)

To have all of this in your shell.nix, change

myPkg = pkgs.haskellPackages.callCabal2nix "mandelbrot" src {};

to

myPkg = pkgs.haskell.packages.ghc9122.callCabal2nix "mandelbrot" src {};