add hie as language server to dev environment

default.nix

@@ -1,14 +1,26 @@
-{ pkgs ? import (
-  builtins.fetchGit {
-    name = "nixpkgs-pinned";
-    url = https://github.com/NixOS/nixpkgs/;
-    ref = "refs/heads/release-20.03";
-    rev = "da7ddd822e32aeebea00e97ab5aeca9758250a40";
-  }) {},
+{
   compiler ? "ghc865"
 }:
 
 let
+  # pin all-hies for getting the language server
+  all-hies = fetchTarball {
+    url = "https://github.com/infinisil/all-hies/tarball/b8fb659620b99b4a393922abaa03a1695e2ca64d";
+    sha256 = "sha256:0br6wsqpfk1lzz90f7zw439w1ir2p54268qilw9l2pk6yz7ganfx";
+  };
+  pkgs = import (
+    builtins.fetchGit {
+      name = "nixpkgs-pinned";
+      url = https://github.com/NixOS/nixpkgs/;
+      ref = "refs/heads/release-20.03";
+      rev = "076c67fdea6d0529a568c7d0e0a72e6bc161ecf5";
+    }) {
+      # Pass no config for purity
+          config = {};
+              overlays = [
+                      (import all-hies {}).overlay
+                          ];
+    };
   hp = pkgs.haskell.packages."${compiler}";
   src = pkgs.nix-gitignore.gitignoreSource [] ./.;
   drv = hp.callCabal2nix "Hash2Pub" "${src}/Hash2Pub.cabal" {};
@@ -25,6 +37,7 @@ in
         hlint
         stylish-haskell
         pkgs.python3Packages.asn1ate
+        hie
       ];
   };
 }