Add nix support

Author: Fischer Bordwell

.direnv/bin/nix-direnv-reload

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+#!/usr/bin/env bash
+dir="$(realpath $(dirname ${BASH_SOURCE[0]})/../..)"
+_nix_direnv_force_reload=1 direnv exec "$dir" true
+direnv reload
+# direnv reload updates the mtime of .envrc. Also update the timestamp of the
+# profile_rc file to keep track that we actually are up to date.
+touch $dir/.direnv/{nix,flake}-profile-*.rc
+

.direnv/flake-profile-a5d5b61aa8a61b7d9d765e1daf971a9a578f1cfa

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+

.direnv/flake-profile-a5d5b61aa8a61b7d9d765e1daf971a9a578f1cfa.rc

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+use flake

.envrc

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+{
+  description = "learn_physics_with_fp";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/release-23.11";
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = inputs:
+    let
+      overlay = final: prev: {
+        haskell = prev.haskell // {
+          packageOverrides = hfinal: hprev:
+            prev.haskell.packageOverrides hfinal hprev // {
+              learn_physics_with_fp = hfinal.callCabal2nix "learn_physics_with_fp" ./. { };
+            };
+        };
+        learn_physics_with_fp = final.haskell.lib.compose.justStaticExecutables final.haskellPackages.learn_physics_with_fp;
+      };
+      perSystem = system:
+        let
+          pkgs = import inputs.nixpkgs { inherit system; overlays = [ overlay ]; };
+          hspkgs = pkgs.haskellPackages;
+        in
+        {
+          devShell = hspkgs.shellFor {
+            withHoogle = true;
+            packages = p: [ p.learn_physics_with_fp ];
+            buildInputs = [
+              hspkgs.cabal-install
+              hspkgs.haskell-language-server
+              hspkgs.hlint
+              hspkgs.ormolu
+              pkgs.bashInteractive
+            ];
+          };
+          defaultPackage = pkgs.learn_physics_with_fp;
+        };
+    in
+    { inherit overlay; } // inputs.flake-utils.lib.eachDefaultSystem perSystem;
+}