It looks like you're missing the overlay that replaces the Nixpkgs GHCs with those based off of the cross-compiled GHC.

Do you have a link to a reproducer I can look at?

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.)

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)