Add 'doctest' as a cabal test

We provide a simple wrapper that executes `doctest` on our `src`
directory. This allows us to run all doctests during a regular `cabal
test` run.

flake.nix

@@ -70,12 +70,12 @@
             haskellPackages.haskell-language-server
             haskellPackages.fourmolu
             haskellPackages.hspec-discover
+            haskellPackages.doctest
             cabal-install
             ghcid
             nixfmt
             hpack
             hlint
-            haskellPackages.doctest
           ];
         };
       });

hcat.cabal

@@ -50,6 +50,25 @@ executable hcat
     , hcat
   default-language: GHC2021
 
+test-suite doctest
+  type: exitcode-stdio-1.0
+  main-is: Doctest.hs
+  other-modules:
+      Paths_hcat
+  hs-source-dirs:
+      test/doctest
+  default-extensions:
+      BlockArguments
+      OverloadedStrings
+      ImportQualifiedPost
+  ghc-options: -Wall -fdefer-typed-holes
+  build-tool-depends:
+      doctest:doctest
+  build-depends:
+      base >=4.13 && <5
+    , process
+  default-language: Haskell2010
+
 test-suite spec
   type: exitcode-stdio-1.0
   main-is: Spec.hs

package.yaml

@@ -56,9 +56,10 @@ tests:
     build-tools: hspec-discover
     verbatim:
       default-language: GHC2021
-  # doctest:
-  #   main: Doctest.hs
-  #   source-dirs:
-  #     - test/doctest
-  #   dependencies:
-  #     - process
+  doctest:
+    main: Doctest.hs
+    source-dirs:
+      - test/doctest
+    dependencies:
+      - process
+    build-tools: doctest