unsupporting GHC < 8.6.5 in the nix-shell

nix/README.md

@@ -23,7 +23,18 @@ $ nix-shell nix/shell.nix --argstr compiler ghcHEAD
 ```
 
 **Possible GHC versions**
--   `ghc822Binary`
--   `ghc865`
+-   `ghc865Binary`
 -   `ghc884`
--   `ghc8102` - default
+-   `ghc8104` - default
+
+### Cabal users
+
+GHC version can be chosen via the nix-shell parameter
+
+`cabal build all`
+
+### Stack version
+
+Since the ghc version is set by the LTS version, it is preferable to use the `ghc8104` version parameter for the nix-shell.
+
+`stack --no-nix --system-ghc <command>`

nix/shell.nix

@@ -1,9 +1,7 @@
-let nixos = fetchTarball { 
-  url = "https://releases.nixos.org/nixos/20.09/nixos-20.09.3505.12d9950bf47/nixexprs.tar.xz";
-  sha256 = "0fsl8bsdb8i536pfs4wrp0826h5l84xqlwx32sbz66jg4ykqp9lr";
+let nixos = fetchTarball { url = "https://github.com/NixOS/nixpkgs/archive/refs/tags/21.05.tar.gz"; 
 }; in
 
-{ compiler ? "ghc8102"
+{ compiler ? "ghc8104"
 , tutorial ? false
 , pkgs ? import nixos { config = {}; }
 }: