Use the module system, delegate activation to project.nix core

New
 - Selectively enable tools (#2)
 - Make choice of tool versions overridable
 - User-definable hooks
 - Input validation at the Nix level

Fixes
 - Stop installing files all over the place when doing nix-shell ../some-project to borrow a tool

Cleanup
 - Get rid of installation code

.pre-commit-config.yaml

@@ -1,6 +1,18 @@
-repos:
-   - repo: .pre-commit-hooks/
-     rev: master
-     hooks:
-      -   id: shellcheck
-      -   id: canonix
+# DO NOT MODIFY
+# This file was generated by project.nix
+{
+  "repos": [
+    {
+      "hooks": [
+        {
+          "id": "canonix"
+        },
+        {
+          "id": "shellcheck"
+        }
+      ],
+      "repo": ".pre-commit-hooks/",
+      "rev": "master"
+    }
+  ]
+}

modules/hooks.nix

@@ -0,0 +1,83 @@
+{ config, lib, pkgs, ... }:
+let
+  inherit (config.pre-commit) tools;
+in {
+  config.pre-commit.hooks =
+    {
+      hlint =
+        {
+          name = "hlint";
+          description =
+            "HLint gives suggestions on how to improve your source code.";
+          entry = "${tools.hlint}/bin/hlint";
+          files = "\\.l?hs$";
+        };
+      ormolu =
+        {
+          name = "ormolu";
+          description = "Haskell code prettifier.";
+          entry = "${tools.ormolu}/bin/ormolu --mode inplace";
+          files = "\\.l?hs$";
+        };
+      hindent =
+        {
+          name = "hindent";
+          description = "Haskell code prettifier.";
+          entry = "${tools.hindent}/bin/hindent";
+          files = "\\.l?hs$";
+        };
+      cabal-fmt =
+        {
+          name = "cabal-fmt";
+          description = "Format Cabal files";
+          entry = "${tools.cabal-fmt}/bin/cabal-fmt --inplace";
+          files = "\\.cabal$";
+        };
+      canonix =
+        {
+          name = "canonix";
+          description = "Nix code prettifier.";
+          entry = "${tools.canonix}/bin/canonix";
+          files = "\\.nix$";
+        };
+      nixfmt =
+        {
+          name = "nixfmt";
+          description = "Nix code prettifier.";
+          entry = "${tools.nixfmt}/bin/nixfmt";
+          files = "\\.nix$";
+        };
+      nixpkgs-fmt =
+        {
+          name = "nixpkgs-fmt";
+          description = "Nix code prettifier.";
+          entry = "${tools.nixpkgs-fmt}/bin/nixpkgs-fmt -i";
+          files = "\\.nix$";
+        };
+      elm-format =
+        {
+          name = "elm-format";
+          description = "Format Elm files";
+          entry =
+            "${tools.elm-format}/bin/elm-format --yes --elm-version=0.19";
+          files = "\\.elm$";
+        };
+      shellcheck =
+        {
+          name = "shellcheck";
+          description = "Format shell files";
+          types =
+            [
+              "bash"
+            ];
+          entry = "${tools.shellcheck}/bin/shellcheck";
+        };
+      terraform-format =
+        {
+          name = "terraform-format";
+          description = "Format terraform (.tf) files";
+          entry = "${tools.terraform-fmt}/bin/terraform-fmt";
+          files = "\\.tf$";
+        };
+    };
+}