add subcommand lint_haskell to lint haskell code

Makefile

@@ -14,6 +14,7 @@ usage:
 	@echo "  format_haskell -- Format haskell stuff, including source code and cabal files"
 	@echo "  format_nix -- Format *.nix files only"
 	@echo "  format_check -- Check if all haskell stuff have been formatted correctly"
+	@echo "  lint_haskell -- run hlint on all haskell files"
 
 hoogle:
 	pkill hoogle || true
@@ -37,6 +38,9 @@ format_check:
 	     -not -path './haddock/*' \
 	  | xargs fourmolu $(FORMAT_EXTENSIONS) -m check
 
+lint_haskell:
+	find -name '*.hs' -not -path './dist*/*' -not -path './haddock/*' | xargs hlint
+
 haddock:
 	cabal haddock --haddock-html --haddock-hoogle --builddir=haddock