Well, I've gotten it to evaluate.

diff --git a/flake.nix b/flake.nix
index a3995c4..837952e 100644
--- a/flake.nix
+++ b/flake.nix
@@ -7,7 +7,7 @@
   };
 
   inputs = {
-    nixpkgs.url = "github:geekygays/nixpkgs";
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
     nixos-hardware.url = "github:nixos/nixos-hardware";
     systems.url = "github:geekygays/systems";
 
diff --git a/modules/nixos/core/default.nix b/modules/nixos/core/default.nix
index 75ef1c9..45fbf7e 100644
--- a/modules/nixos/core/default.nix
+++ b/modules/nixos/core/default.nix
@@ -65,7 +65,7 @@
     '';
   };
   environment.systemPackages = with pkgs; [
-    nvd nixpkgs-fmt nix-output-monitor
+    nvd nixpkgs-fmt
     coreutils mime-types file
     usbutils pciutils gitFull git-crypt
   ];

This works for two reasons:

1. Compiling nix-output-monitor requires GHC.
2. Your custom Nixpkgs incorporates my ghc-cross branch, which prevents the ShellCheck fix from working.

To compile ShellCheck and nix-output-monitor, you must overlay the default GHC builds to replace them with a usable GHC.

Note that my ghc-cross branch doesn't do all of that; it only makes it possible to build a cross-compiler. Something similar to the example module I wrote into the notes is needed to do that.

(If you don't want to make your flake impure, you can use packages.x86_64-linux instead of builtins.storePath and it won't complain about being unable to build x86_64-linux as long as the output is already in the cache.)

tau I've been modernising my RISC-V configuration, so I just so happen to have a pure flake that can evaluate GHC. It's not yet done building so I'm not entirely sure it'll work flawlessly on my VF2, but feel free to use it as a reference for how to overlay GHC.

https://github.com/AlexandreTunstall/nixos-riscv

In reply to @hive:the-apothecary.club
right now it's doing an emulated build, and i'm guessing there's no way to get it to do a cross build ?

Cross-compiling Haskell packages doesn't work.

I think the main issue is probably that Cabal needs to compile Setup.hs for the build platform, so you can't just instruct it to use a GHC that targets the host platform.

In reply to @hive:the-apothecary.club
i don't think nix build has an option for that

Segmentation fault

Check kernel log.

journalctl -ek

If on a systemd distribution (like NixOS)

Example:

ghc_worker[772846]: unhandled signal 11 code 0x1 at 0x0000004e3584b582
CPU: 2 PID: 772846 Comm: ghc_worker Not tainted 6.5.0 #1-NixOS
Hardware name: StarFive VisionFive 2 v1.3B (DT)
epc : 0000004e3584b582 ra : 0000003ff3e06900 sp : 00000004e3ffa730
 gp : 000000000006ea80 tp : 00000004e3fff8e0 t0 : 0000000000004000
 t1 : 0000003ff3e00628 t2 : 0000003ff6f86598 s0 : 00000004f148d652
 s1 : 0000003fedb8afd8 a0 : 0000000000000000 a1 : 0000003fed8dd038
 a2 : 0000000000000002 a3 : 0000000000000002 a4 : 0000000000000032
 a5 : 09e1854e3584b583 a6 : 0000000000000000 a7 : 0000003ff7ffdd50
 s2 : 00000004f311b9f8 s3 : 00000004f16d93c8 s4 : 0000003fed8dc5e0
 s5 : 00000004f31140c0 s6 : 00000004f148d638 s7 : 00000004f36d2870
 s8 : 00000004f2ce1000 s9 : 00000000000003bd s10: 0000003fedb59058
 s11: 00000004f31140c0 t3 : 0000003fed2f0514 t4 : 0000000000000001
 t5 : 0000003fed9c6e2a t6 : 000000000000002e
status: 8000000200006020 badaddr: 0000004e3584b582 cause: 000000000000000c