Add scripts for linting and styling with all dependencies managed by Nix (#1501)

* Integrate the style and lint dependencies with Nix

* Add documentation on the nix styling scripts.

nix/README.md

@@ -61,6 +61,20 @@ nix-shell --run postgrest-test-spec-postgresql-9.5.21
 
 ```
 
+## Linting and styling code
+
+The nix-shell also contains scripts for linting and styling the PostgREST
+source code:
+
+```bash
+# Linting
+nix-shell --run postgrest-lint
+
+# Styling / auto-formatting code
+nix-shell --run postgrest-style
+
+```
+
 ## Tour
 
 The following is not required for working on PostgREST with Nix, but it will

nix/lint.nix

@@ -0,0 +1,12 @@
+{ writeShellScriptBin, git, silver-searcher, hlint }:
+
+writeShellScriptBin "postgrest-lint"
+  ''
+    set -euo pipefail
+
+    rootdir="$(${git}/bin/git rev-parse --show-toplevel)"
+
+    # Lint Haskell files
+    ${silver-searcher}/bin/ag -l -g '\.l?hs$' "$rootdir" \
+      | xargs ${hlint}/bin/hlint -X QuasiQuotes -X NoPatternSynonyms
+  ''

nix/style.nix

@@ -0,0 +1,37 @@
+{ writeShellScriptBin
+, git
+, nixpkgs-fmt
+, silver-searcher
+, stylish-haskell
+, buildEnv
+}:
+let
+  style =
+    writeShellScriptBin "postgrest-style"
+      ''
+        set -euo pipefail
+
+        rootdir="$(${git}/bin/git rev-parse --show-toplevel)"
+
+        # Format Nix files
+        ${nixpkgs-fmt}/bin/nixpkgs-fmt "$rootdir" > /dev/null 2> /dev/null
+
+        # Format Haskell files
+        ${silver-searcher}/bin/ag -l -g '\.l?hs$' "$rootdir" \
+          | xargs ${stylish-haskell}/bin/stylish-haskell -i
+      '';
+
+  check =
+    writeShellScriptBin "postgrest-style-check"
+      ''
+        set -euo pipefail
+
+        ${style}/bin/${style.name}
+
+        ${git}/bin/git diff-index --exit-code HEAD -- '*.hs' '*.lhs' '*.nix'
+      '';
+in
+buildEnv {
+  name = "postgrest-style";
+  paths = [ style check ];
+}